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Preparation and properties of hot-pressed NbMo-matrix composites reinforced with ZrB2 particles
Accepted Manuscript Preparation and properties of hot-pressed composites reinforced with ZrB2 particles
NbMo-matrix
Zongde Liu, Qi Wang, Yuan Gao, Yongtian Wang, Youmei Sun, Yan Gong PII: DOI: Reference:
S0263-4368(17)30227-5 doi: 10.1016/j.ijrmhm.2017.06.011 RMHM 4470
To appear in:
International Journal of Refractory Metals and Hard Materials
Received date: Revised date: Accepted date:
12 April 2017 23 June 2017 28 June 2017
Please cite this article as: Zongde Liu, Qi Wang, Yuan Gao, Yongtian Wang, Youmei Sun, Yan Gong , Preparation and properties of hot-pressed NbMo-matrix composites reinforced with ZrB2 particles, International Journal of Refractory Metals and Hard Materials (2017), doi: 10.1016/j.ijrmhm.2017.06.011
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ACCEPTED MANUSCRIPT Preparation and properties of hot-pressed NbMo-matrix composites reinforced with ZrB2 particles
Key Laboratory of Condition Monitoring and Control for Power Plant Equipment of Ministry of
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Zongde Liu 1,a, *, Qi Wang 1,b , Yuan Gao1,c, Yongtian Wang1,d, Youmei Sun1,e, Yan Gong1,f
email: [email protected] bemail : [email protected] cemail :[email protected] demail :
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a
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Education, North China Electric Power University, Beijing 102206, China
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[email protected] eemail: [email protected] femail: [email protected]
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*Corresponding author: Zongde Liu
ACCEPTED MANUSCRIPT Abstract The NbMo-matrix composites reinforced with (0-60 vol.%) ZrB2 were fabricated by hot-pressing at 2400 ℃ for 10 min under a pressure of 50 MPa in dynamic vacuum in the induction heating furnace specially designed in our institute. The optimum
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ZrB2 content in NbMo solid solution was determined to be 30 vol.% for excellent
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comprehensive mechanical property. NbMo-30 vol.% ZrB2 has the highest density of
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99.63%, the most uniform microstructure, high fracture toughness of 5.75 MPa m1/2. The highest ZrB2 concentration that react with NbMo solid solution is at the range of
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30 to 45 vol.%. The types of the formed niobium borides were decided by the
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original ratio of Nb to B. The distribution of Mo and Zr was mutually exclusive in low ZrB2 content composites, however, there was Mo2Zr in high ZrB2 content
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composite. Except for NbMo-45 vol.% ZrB2, the compressive strength increased
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with ZrB2 content (from 927.09 MPa to 1635.91 MPa). The Young’s modulus values were directly proportional to ZrB2 content. The fracture toughness (from 6.34 MPa
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m1/2 to 3.99 MPa m1/2) was inversely proportional to ZrB2 content. The big residual
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ZrB2 particles in high ZrB2 content samples such as NbMo-45 vol.% ZrB2 and NbMo-60 vol.% ZrB2 was the main reason for
nonhomogeneous microstructure,
low density (94.09% and 94.83%, respectively) and low fracture toughness (4.58 MPa m1/2 and 3.99 MPa m1/2, respectively).
Keywords: Hot-pressing; NbMo; ZrB2; microstructure; mechanical properties
ACCEPTED MANUSCRIPT 1. Introduction As one of the most promising refractory niobium-based alloys are of great interest
alloy for use at high temperature, because of their unique combination of
low density, relative high ductility at room temperature, high resistance to corrosion,
a good candidate for space, nuclear and aircraft applications
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niobium-based alloys
These excellent characteristics make
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high melting point and refractory properties.
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[1]. However, the strength of niobium (Nb) decreases substantially at temperature above 1200 K. It was found that the 0.2% flow stress of the Nb-ZrC alloys increases
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with increasing ZrC content from room temperature to elevated temperature [2].
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Adding refractory elements such as molybdenum (Mo) to Nb could greatly improve the high temperature strength of Nb by solid solution strengthening. The introduction
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of Mo to Nb-ZrC plays a part in the improvement of strength by solid solution
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strengthening of NbMo-matirx. The maximum solid solution effects can be achieved when the Nb/Mo ratio is around 1. However, the fracture toughness decrease with
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increasing ZrC content [3].
used as ultra-high temperature materials due to its
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Zirconium diboride (ZrB2) is
extremely high melting temperature (Tmelt > 3000 ℃), high hardness, excellent corrosion resistance and high thermal conductivity [4-6]. The low fracture toughness is an negative to the application of ZrB2. The introduction of metal powders such as Mo can improve the fracture toughness and fracture strength compared to pure ZrB2 [7-8]. ZrB2-based composites toughened by Nb were achieved by hot-pressing [9]. The densification of ZrB2 powders needs very high pressure and temperature
ACCEPTED MANUSCRIPT (2100-2300 ℃) in sintering process [10], however, the final samples have many pores [11]. The densities of the ZrB2-SiC composites are directly proportional to sintering temperature [12]. The characteristic of grain boundary phase and second phase is related to the highest sintering temperature. Hot-pressing (HP) and spark plasma
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sintering (SPS) can be used to synthesis the ultra-high temperature ceramics (UHTC)
because of the shape and large size of components, costs and
reproducibility of properties
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production
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under an external pressure. However, the SPS is not widely used in industrial
are not as excellent as that of samples in the laboratory
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[13]. In early studies, the composite of ZrB2 - 25 vol.% Nb
were only sintered by
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hot-pressing at 1800 ℃ for 60 min in Argon atmosphere under 30 MPa, and the relative density of the ZrB2-25 vol.% Nb is 97.2% [14]. The composite of ZrB2- (0-10) were prepared at 1950 ℃ for 60 min by hot-pressing, and the highest
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vol.% Mo
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density of ZrB2- (0-10) vol.% Mo is 98.9% [15]. It is necessary to explore a hot-pressing process with higher temperature and pressure to improve the density of
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these kinds of composites. Although some researches about the ultra-high temperature
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refractory metal matrix composites reinforced by ceramic particles were conducted, it is necessary to explore the preparation and more feasible ingredients. In this
study, ZrB2 is selected as a single ceramic additive for NbMo, and the ratio
of Nb/Mo is set at 1. A new hot-pressing processing technique was applied to prepare ZrB2-NbMo composites with a content range of ZrB2 from 0 to 60 vol.%. The hardness, relative density, compressive strength, fracture toughness and phase composition
at room temperature was investigated. The effects of ZrB2 on the
ACCEPTED MANUSCRIPT microstructure and mechanical property of the xZrB2-NbMo were analyzed. The optimum content of ZrB2 in NbMo solid solution was discussed. 2. Experimental Procedure 2.1 Preparation
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Five powder mixtures were prepared (amounts in vol.%):
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Material NM-15Z: 85% (50%Nb+50%Mo) + 15%ZrB2
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Material NM-0: 50%Nb + 50%Mo
Material NM-30Z: 70% (50%Nb+50%Mo) + 30%ZrB2
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Material NM-45Z: 55% (50%Nb+50%Mo) + 45%ZrB2
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Material NM-60Z: 40% (50%Nb+50%Mo) + 60%ZrB2 In this article, the letter N is used as the abbreviation for Nb, M for Mo and Z for ZrB2.
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Nb and Mo (500-800 mesh, purity > 99.95%) and ZrB2 (600 mesh, purity > 99.95%)
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were supplied by Beijing gold crown for the new material Technology Co. Ltd. China. The sizes of ZrB2, Nb and Mo are from 15 μm to 30 μm.Commercial powders were 1. The
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used to make five different samples, the compositions are shown in Table
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powders were proportionally mixed, then dried at 110 ℃ for 3 hours, then mixed and pulverized in the planetary milling in stainless steel ball milling containers at the speed of 200 rpm for 10 hours. The mould was filled with the powders and a fire-resistance slab that could facilitate the removal of the specimen and minimize the friction between powders and mould walls. Samples were prepared following the process in Fig.1. First, semi-finished products were prepared in a cylindrical cabon fiber mould at room temperature (25 ℃) and 25 MPa pressure using molding
ACCEPTED MANUSCRIPT powders for 10 min, and dried at 280 ℃ for 50 min. Then the semi-finished products were put in crucible. After that, vacuum pump
was used to control the pressure to
10-1 MPa. Followed by inflating Ar, the final reading of vacuum pump was stable at 0.08 MPa, therefore the absolute pressure was close to 0.02 MPa. The high frequency
furnace was used to measure temperature. When the temperature
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from outside of the
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induction heating equipment (160 kW) was turned on. The temperature thermocouple
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rose to 1800 ℃，the mechanical power presses started at the same time. The holding time was 15 min, this particular time length could prevent the excessive temperature
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difference caused by high heating rate between the inside and the surface of the
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cylinder. The pressure at this holding stage was 25 Mpa in Ar atmosphere. Then the temperature was increased continuously to 2400 ℃. This holding time was 10 min,
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which prevented the overflowing of low-melting phase caused by overheating. The
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pressure at this stage was 50 Mpa. Finally, the sample was cooling to room temperature. The temperature mentioned was recorded by thermocouple, however, the furnace could be 2400 ℃ to 2600 ℃ at this holding
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real temperature inside the
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stage. The entire process was conducted in dynamic vacuum by inflating Ar discontinuously. It was attempted to prepare a sample of 120 mm length but its length couldn’t be controlled due to the reason
of the unevenness distribution of internal
stress and temperature field. Finally, the length of the sample was decided to be 70 mm. The sintering temperature of 1500 ℃ and 1600 ℃ were also tried, but the samples
had low hardness and low relative density. The general furnace could not
reach the high temperature needed. Because of these reasons, the furnace in Fig. 2
ACCEPTED MANUSCRIPT was developed on the purpose of densifying the samples. Table 1 Compositions, densities, relative densities of the five composites. Fig. 1. The sintering process Fig. 2. Internal structure of
furnace
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Fig. 3. The original sample
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2.2 Characteristic
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The final samples with a cylindrical shape (Fig.3) were obtained by hot-pressing. The diameter was 14 mm and the height was approximately 70 mm. Then, samples were
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cut into specimens that had dimensions of 10mm*10mm*8mm. The cutting method After this, the specimens
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was wire cut electrical discharge machining (WEDM).
were cleaned in ultrasonic cleaner. The surfaces of the specimens for microstructure grinded with a series of sandpaper (from #100 to
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observation and XRD texting were
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#1500) and polished with 4000 mesh diamond paste. The phase composition was detected by X-ray diffraction device which was operating at a scanning speed of
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8°/min and 2θ from 10° to 90° at 40 kV and 100 mA. ASEM (ZEISS SUPRA-55)，
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FEI Quanta 200F and SU8010 scanning electron microscope was operated at an accelerating voltage of 20 kV to observe the morphology. The element composition and distribution were analyzed by Energy Dispersive X-ray spectroscopy (EDS). The micro-hardness was measured using the FM-300 Vickers tester under 300g load over an indentation time of 10 s with diamond indenter. The density was measured using the densimeter (FA1104J) following Archimedes’ principle with alcohol as the immersing medium. The reported density was the average of three measurements. The
ACCEPTED MANUSCRIPT final result of micro-hardness was determined as the average value of three points. The compression test was carried on the hydraulic universal tester (SHT 4305) in air at room temperature，and the rate of loading was 600N/m. The Young’s modulus was measured by dynamic Young’s modulus tester (RFDA system 23). The crack for
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observing propagation path was made on the HVS-50P Vickers hardness tester with
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the load of 98 N for 15 s at room temperature. The fracture toughness was determined
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by a three point bending test using the single-edge-notched beam (SENB) specimens by Electromechanical Universal Testing Machine (C45.105). Fracture toughness
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specimens with 3mm*4mm cross-section and 36mm span were prepared and the
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notch was 0.15mm in width and 1.5mm in depth. 3. Results and Discussion
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3.1. Phase Composition
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The Nb-B phase diagram [16] shows the stable phase in this system are BCC (niobium), Nb3B2, NbB, Nb3B4, NbB2, Nb5B6, B (boron) and liquid L.
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Fig. 4. X-ray diffraction (XRD) patterns of the five samples.
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Fig. 4 displays the XRD patterns of five samples. It suggests that the ZrO coexists with Nb3B2 in NM-15Z, NM-30Z, NM-45Z and NM-60Z. The NbMo solid solution exists in the five samples. The peaks of ZrB2 are found in NM-45Z and NM-60Z, and the peaks of NbB and Mo2Zr in NM-60Z are also found. The details are displayed in Table 2, and the “*” is used as the symbol for main phase. Table
2 The compounds in five samples after hot-pressing
According to XRD results, the reactions in four composites are: the eutectic reaction
ACCEPTED MANUSCRIPT (2104 ± 5 ℃) and the peritectoid reaction (1900 - 2100 ℃) [17]. The reaction between Nb and B is easily explained according to the currently accepted Nb-B phase diagram. The types of the formed niobium borides are decided by the ratio of Nb to B [18] and the formation of Mo 2Zr is due to
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the interdiffusion between Mo and Zr [19].
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3.2. Densification and hardness
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Fig. 5. The curves of relative density changed with volume fraction of ZrB 2.
The curves of relative density and hardness of niobium borides are shown in Fig. 5
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and Fig. 6. The theoretical density is obtained by calculations based on the original
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ingredient before hot-pressing, and the effect of oxygen content is assumed to be neglected. The assumed true densities are 6.085 g/cm3 for ZrB2 and 8.57 g/cm3 for
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Nb and 10.2 g/cm3 for Mo. NM-45Z and NM-60Z have low relative density, and this
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may be attributed to that those two ingredients include more ZrB2 particles, and the connection among ZrB2 is more difficult than the connection between ZrB2 and
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niobium boride. Another reason for the low relative density of NM-45Z and NM-60Z
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is the big ZrB2 particles impede phase flowing in the process of solidification. The grain can be seen from Fig. 7, NM-30Z has the most uniform microstructure. The excellent property of NM-30Z should be attributed to its highest relative density and uniform microstructure among the samples. Table 3 Hardness of phase of the five composites. Fig. 6. The curves of hardness of niobium borides changed with volume fraction of ZrB2.
The hardness in Table 3 is the average value of three points. The hardness of NbMoss
ACCEPTED MANUSCRIPT increases with increasing content of ZrB2, the reason is the different degrees of solution strengthening of Zr and B atoms into NbMoss, With increasing content of ZrB2, there are more Zr and B atoms could be introduced in to NbMoss, so the hardness of NbMoss increases for the reason of solution strengthening of Zr and B
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atoms. The niobium borides phase refers to Nb3B2 in NM-15Z, NM-30Z and NM-45Z,
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and Nb3B2+NbB in NM-60Z, because it is not easy to distinguish the two phase in
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NM-60Z. The hardness of the niobium borides in ZrB2-NbMo composites is directly proportional to ZrB2 content. In all the composites, the low hardness district is the
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NbMo-matrix, and the hardness of ZrB2 is around 22.3 GPa. The niobium borides
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have high hardness (16.58 GPa for NbB and 10.08 GPa for Nb3B2), and the hardness of niobium borides significantly increases with the increasing of B [20]. NbB is the
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3.3. Microstructure
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main reason for hardness improvement in NM-60Z.
Microstructure of hot-pressed ZrB2–NbMo composites was observed under
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backscattered electron SEM imaging, and typical examples are shown in Fig. 7.
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Fig. 7. SEM images of polish surface of (a) NM-15Z, (b) NM-30Z, (c) NM-45Z and (d) NM-60Z.
The EDS analysis results show that the white sections in Fig. 8 (a) are NbMo solid solution rich in Mo and the dark grey and light grey sections are NbMo solid solution rich in Nb. The difference of colors between dark grey and light grey is due to the different percentage composition of Nb. The darker section has more Nb. Nb and Mo substitute each other [3]. The solution strengthening of Nb and Mo is beneficial to the improvement of strength and hardness.
ACCEPTED MANUSCRIPT There are Nb and Mo spherical particles powders in NM-0, and the surface of the spherical particles was melted with the increase of temperature. The melt point of Nb (2468 ℃) is lower than Mo (2610 ℃), and the real temperature inside the furnace could be 2400 ℃ to 2600 ℃, so there are more surfaces of Nb particles melted and
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less surfaces of Mo particles melted, and the Mo particles could hold the approximate
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spherical shape in the process of extrusion. So the voids among Mo particles in Fig. 8
solution rich in Mo due to the diffusion of Nb.
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are full of NbMo solid solution rich in Nb. The Mo particles turn to NbMo solid
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Fig. 8. SEM images of NM-0 polish surface at (a) high magnification and (b) low magnification.
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The microstructure of NM-15Z in Fig. 7 is very different from NM-0, but is similar with NM-30Z, NM-45Z and NM-60Z. This phenomenon is result from the
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introduction of ZrB2 particles. The microstructure of NM-15Z has irregular shapes
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and sharp edges. The ZrO is distributed in the NbMo-matrix as a reinforcing phase. The distribution of element Mo and Zr is
mutually exclusive, which might indicates
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that it is less likely to form MoZr solid solution or molybdenum-zirconium
Fig. 9.
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compounds in this sample. Nb3B2 is formed around ZrO in general in Fig. 7 (a) and
Fig. 9. BSE images and EDX maps of NM-15Z.
The elements area profile of NM-30Z is not shown because the elements distribution in NM-30Z is uniform and there is no obvious segregation in this sample. The main phase of NM-30Z is niobium boride, and the area of niobium boride is larger than that of NbMo solid solution. This is because the decomposition of high fraction of ZrB2
ACCEPTED MANUSCRIPT particles. The generated ZrO particles observed in Fig. 7 (b) are distributed in the whole samples with the shape of roundness, which is beneficial to strengthen the material. The relationship of elements is similar to NM-15Z. Fig. 10. BSE images and EDX maps of NM-45Z.
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The elements area profile of NM-45Z is shown in Fig. 10. ZrB2 particles are found
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large and the shape irregular in NM-45Z. The excessive ZrB2 particles are difficult to
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react, and the boundary between the unreacted ZrB2 particles and other phases is not very smooth. All these factors could do harm to the mechanical property. The SEM
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image of NM-60Z is shown in Fig. 7 (d) and the elements area profile of NM-60Z is
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shown in Fig. 11. The microstructures in NM-60Z have several differences from others. First, there is Mo2Zr phase not observed in other samples. The foundation of
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Mo2Zr is in keeping with XRD results. Second, the area of NbMo solid solution is not
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very large, and this may result from the reactions, because the Nb and Mo atoms mostly reacted and less left to form NbMo solid solution. Third, there are redundant
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samples.
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ZrB2 and the sizes of ZrB2 particles in NM-60Z are larger than particles in other three
Fig. 11. BSE images and EDX maps of NM-60Z.
This phenomenon matches the XRD results, and indicates that there is residual ZrB2 in high proportion ZrB2 samples such as NM-45Z and NM-60Z. It is concluded from the phenomenon that the ZrB2 has the highest reaction concentration in NbMo solid solution, and residual ZrB2 in high proportion ZrB2 samples could affect the formed phase and mechanical properties. The highest ZrB2 concentration that react with
ACCEPTED MANUSCRIPT NbMo solid solution is at the range of 30 to 45 vol.%. The dispersion strengthening of ZrB2 particle and solution strengthening of Nb and Mo worked together to strengthen the samples. However, the big ZrB2 particles and unevenness of all particles could affect compression property.
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3.4. Compression and fracture toughness
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The compression results are shown in Fig. 12. The NM-0 and NM-15Z exhibit
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plasticity, and the NM-30Z, NM-45Z and NM-60Z exhibit brittleness. The compressive strength of NM-30Z, NM-45Z and NM-60Z are much higher than NM-0.
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The flow stress increases with increasing ZrB2 content, however, the compressive
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plasticity decreases with increasing ZrB2 content. The reason for the increase is the dispersion strengthening of ZrB2 particles. This confirms the ZrB2 is an effective
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reinforcing phase to NbMo, which work together with the solid solution strengthening
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of NbMo to strengthen the composites. Except for NM-45Z, the compressive strength increases with ZrB2 content. The
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decreasing compressive strength of NM-45Z is related to the reduction of NbMo solid
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solution and the existence of big redundant ZrB2. Overmuch ZrB2 ceramic particle leads to large size particles and uneven size phase in NM-45Z. The large particles result in less boundaries to absorb strain energy than NM-15Z and NM-30Z. The element segregation in NM-45Z is more obvious than other samples, and the reason for this phenomenon is the large particles impede phase flowing in the process of solidifications. And the uneven microstructures in NM-45Z result in stress concentration to form cracks. The low relative density can also reflect much micro
ACCEPTED MANUSCRIPT voids and micro cracks inside NM-45Z. The stress concentration around these micro defects is also harmful to mechanical property. The connection between ZrB2 and other phase in NM-45Z shown in Fig. 7(c) is not so tight as NM-15Z and NM-30Z, and this would also do harm to the compressive strength. All these factors do harm to
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the promotion of compressive strength of NM-45Z. This matches the discussion about
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microstructure above. There are two positive factors that are conducive to the
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improvement of the compressive strength: the solid solution of NbMoss and the dispersion strengthening of ZrB2 particles. There are negative factors that are bad for
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the improvement of compressive strength: reduction of NbMo solid solution and the
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existence of big redundant ZrB2. Comparing NM-45Z to NM-30Z, the increase of negative effects exceed the increase of positive effects, so the compressive strength of
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NM-45Z is lower than the NM-30Z’s. Comparing NM-60Z to NM-30Z, the increase
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of positive effects exceeds the increase of negative effects, so the compressive strength of NM-60Z is higher than the NM-30Z’s. Table 4 shows the Young’s modulus
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of composites. The Young’s modulus values are directly proportional to ZrB2 content.
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Therefore, the addition of ZrB2 could improve the Young’s modulus of composites. Fig. 12. Stress-strain curves at room temperature. Table 4 Compressive strength, Young’s modulus and fracture toughness of the five composites.
The fracture toughness values are inversely proportional to ZrB2 content in the samples added ZrB2. The value of fracture toughness for monolithic ZrB2 is 2.8 MPa m1/2 [21]. Table 4 shows that the fracture toughness values of all composites are higher than monolithic ZrB2, which indicates that the addition of Nb and Mo is in
ACCEPTED MANUSCRIPT favor of enhancing the fracture toughness. This phenomenon could be verified by the crack behavior of these samples on the polished surface of four samples shown in Fig. 13. The crack bridging and crack branch could be found in niobium boride and NbMo solid solution, and the crack deflection is found in niobium boride, NbMo solid
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solution and around a small quantity of ZrB2. The crack propagation of NM-15Z is
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not shown because it is similar with NM-30Z. Comparing NM-0 with NM-30Z, there
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is no crack bridging in NM-0, and the crack in NM-0 propagates along the boundaries of Nb. The crack bridging and crack deflection could be seen around ZrO tiny energy and enhance fracture toughness，
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particles in NM-30Z, which could dissipate
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and the crack branch could be found in Nb3B2. The direction of crack propagation of NM-60Z in Fig. 13 (d) is from the bottom right corner to top left corner. Comparing
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NM-30Z with NM-60Z, the crack deflection of NM-30Z in Fig. 13 (b) is around ZrO.
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However, in NM-60Z, only the small ZrB2 particles in the end of the crack propagation path could have a small role in bridging crack. The reason for the low
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fracture toughness of NM-60Z are the little niobium boride and much big ZrB2 has little effect on improvement of
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particles. An excess of ZrB2 in composite
fracture toughness. Therefore, the lack of niobium boride and existence of big particles in NM-45Z and NM-60Z are the reason for the low fracture toughness. Fig. 13. Typical cracking propagation behavior of (a) NM-0, (b) NM-30Z, (c) NM-45Z and (d) NM-60Z.
Fig.14 shows SEM images of the fracture surface of the composites. Only the transgranular fracture is observed in NM-15Z and NM-30Z. Both of the transgranular
ACCEPTED MANUSCRIPT and intergranular fracture are observed in NM-45Z and NM60Z, and the transgranular and intergranular fracture were respectively progressed in niobium borides and ZrB2. The main fracture mode in all the composites is transgranular fracture of niobium borides, and there is intergranular fracture of ZrB2 particles in NM-45Z and NM-60Z.
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The Nb3B2 is plastically deformed in all the composites，and decohesion is found at
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the interface between Nb3B2 and ZrO in NM-15Z. Ashby et al. [22] have pointed out
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that decohesion at an interface plays an important role in enhancing fracture toughness. The reason for the low fracture toughness of NM-45Z and NM-60Z is the
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little Nb3B2 phase and the existence of big ZrB2 particles. Therefore, the high fracture
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toughness of NM-15Z is attributed to the stretching of Nb3B2 and interface decohesion.
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4. Conclusions
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Fig. 14. Fracture surface of (a) NM-0, (b) NM-30Z, (c) NM-45Z and (d) NM-60Z.
ZrB2-NbMo composites were hot-pressed at 2400 ℃ for 10 min under a pressure of
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50 MPa with ZrB2 content ranging from 0 to 60 vol.%. The object is to determine the
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optimum content of ZrB2 in NbMo solid solution. (1) The optimum ZrB2 content in NbMo solid solution in this paper is determined to be 30 vol.%. The highest ZrB2 concentration that reacts with NbMo solid solution is at the range of 30 to 45 vol.%, and residual ZrB2 in high ZrB2 content samples change phase composition and weaken mechanical properties. (2) The types of the formed niobium borides were decided by the original ratio of Nb to B. The distribution of Mo and Zr was
mutually exclusive in low ZrB2 content
ACCEPTED MANUSCRIPT samples, however, there was Mo2Zr in high ZrB2 content environment. (3) The relative density of the reinforced composites increased with ZrB2 content within 30 vol.% and decreased in 45 vol.% and 60 vol.%. The hardness of the niobium borides in ZrB2-NbMo composites is directly proportional to ZrB2
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content (from 6.48 GPa for NM-15Z to 14.33 GPa for NM-60Z).
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(4) Except for NM-45Z, the compressive strength increases with ZrB2 content. The
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decreasing compressive strength of NM-45Z is related to the reduction of NbMo solid solution and the existence of big residual ZrB2 particles. The Young’s
1/2
for NM-15Z to 3.99 MPa m
1/2
for NM-60Z) is inversely
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(from 6.34 MPa m
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modulus values are directly proportional to ZrB2 content. The fracture toughness
proportional to ZrB2 content. The big ZrB2 particles and little Nb3B2 phase are the
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reason for the low fracture toughness.
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The content of ZrB2 should be controlled to 30 vol.% in order to get excellent comprehensive mechanical property.
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Acknowledgement
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The authors would like to thank the financial support from the National Natural Science Foundation of China (11372110)
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3. Y. Tan, C.L. Ma , A. Kasama , R. Tanaka, Jenn-Ming Yang, High temperature mechanical behavior of Nb-Mo-ZrC alloys, Mater. Sci. Eng. A. 355 (2003) 260-266.
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behavior of hot-pressed ZrB2–SiC–AlN composites, J. Alloys Compd. 475 (2009) 762–765. 5. Eric W. Neuman, Gregory E. Hilmas, William G. Fahrenholtz, Processing, microstructure, and
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10. M. Pastor, Sintering Methods and Properties of Solid Bodies, in Boron and Refractory Borides. Metallic borides: preparation of solid bodies, in: V.I. Matkovich(Ed.), Springer-Verlag, New York, 1977, 457–493. 11. Diletta Sciti, Fr´ed´eric Monteverde, Stefano Guicciardi,Giuseppe Pezzotti, Alida Bellosi,
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Microstructure and mechanical properties of ZrB 2–MoSi2 ceramic composites produced by different
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12. Ipek Akin, Mikinori Hotta, Filiz Cinar Sahin, Onuralp Yucel, Gultekin Goller, Takashi Goto, Microstructure and densification of ZrB2–SiC composites prepared by spark plasma sintering, J. Eur.
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13. F. Monteverde, A. Bellosi, Luigi Scatteia, Processing and properties of ultra-high temperature ceramics for space applications, Mater. Sci. Eng. A. 485 (2008) 415–421.
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14. Xin Sun, Wenbo Han, Ping Hu, Zhi Wang, Xinghong Zhang, Microstructure and mechanical
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properties of ZrB2–Nb composite, Int. J. Refract. Met.Hard Mater. 28 (2010) 472–474. 15. Hailong Wang, Deliang Chen, Chang-An Wang, Rui Zhang, Daining Fang, Preparation and
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characterization of high-toughness ZrB2/Mo composites by hot-pressing process, Int. J. Refract.
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Met.Hard Mater. 27 (2009) 1024–1026. 16. Rudy E, Windisch S. Related binary systems. Part I, Systems V-B, Nb-B and Ta-B. In: Ternary phase equilibria in transition metal-boron-carbon-silicon systems, vol. X. Air Force Materials Laboratory, Wright-Patterson Air Force Base. (1966) 1-104. 17. Zhihong Tang, M.J. Kramer, Mufit Akinc, Evaluation of phase equilibria in the Nb-rich portion of Nb-B system, Intermetallics. 16 (2) (2008) 255-261. 18. A.L. Chamberlain, W.G. Fahrenholtz, G.E, Hilmas, High strength ZrB2-based ceramic,
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J. Am. Chem. Soc. 87 (6) (2004) 1170-1172. 19. A. Paz y Puente, J. Dickson, D.D. Keiser Jr., Y.H. Sohn, Investigation of interdiffusion behavior in the Mo–Zr binary system via diffusion couple studies, Int. J. Refract. Met.Hard Mater. 43 (2014) 317-321.
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20. Zhiping Sun, Xiping Guo, Xiaodong Tian, Liang Zhou, Microhardness and Macrohardness of
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as-Cast Nb-Ti-Si-B Alloys, RARE METAL MAT ENG. 44 (9) (2015) 2148-2151.
composits, J. Eur. Ceram. Soc. 22 (3) (2002) 319-329.
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21. F. Monteverde, A. Bellosi, S. Guicciardi, Processing and properties of zirconium deboride-based
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22. M.F. Ashby, F.L. Blunt, M. Bannister, Flow characteristics of highly constrained metal wires,
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Metall. Mater. 37 (1989) 1847.
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Fig.1. The sintering process.
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Fig. 2. Internal structure of furnace.
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Fig.3 The final sample
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Fig. 4. X-ray diffraction (XRD) patterns of the five samples.
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Fig. 5. The curves of relative density changed with volume fraction of ZrB 2.
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Fig. 6. The curves of hardness of niobium borides changed with volume fraction of ZrB2.
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Fig. 7. SEM images of polish surface of (a) NM-15Z, (b) NM-30Z, (c) NM-45Z and (d) NM-60Z.
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Fig. 9. BSE images and EDX maps of NM-15Z.
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Fig. 10. BSE images and EDX maps of NM-45Z.
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Fig. 11. BSE images and EDX maps of NM-60Z.
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Fig. 12. Stress-strain curves at room temperature.
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Fig. 13. Typical cracking propagation behavior of (a) NM-0, (b) NM-30Z, (c) NM-45Z and (d)
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NM-60Z
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Fig. 14. Fracture surface of (a) NM-0, (b) NM-30Z, (c) NM-45Z and (d) NM-60Z.
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1 Compositions, densities, relative densities of the five composites. Compositions
Test
Theoretical
Relative density
(vol.%)
density(g/cm3)
density(g/cm3)
（%）
94.93 0.77
Mo
ZrB2
NM-0
50
50
0
8.857 0.072
9.330
NM-15Z
42.5
42.5
15
8.674 0.071
8.843
98.09 0.80
NM-30Z
35
35
30
8.325 0.018
99.63 0.21
NM-45Z
27.5
27.5
45
7.405 0.080
7.870
94.09 1.10
NM-60Z
20
20
60
7.001 0.086
7.383
94.83 1.16
8.356
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Nb
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Table
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Table 2 The compounds in five samples after hot-pressing ZrB2
ZrO
Nb3B2
√ (*)
NM-15Z
√ (*)
√
√
NM-30Z
√
√
√ (*)
NM-45Z
√
√
√
√ (*)
NM-60Z
√
√ (*)
√
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NM-0
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NbB
Mo2Zr
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NbMo
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√
√
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Table 3 Hardness of phase of the five composites. Hardness (GPa) of constituent phase Samples Niobium borides
2.77 0.39
NM-15Z
3.75 0.89
6.48 0.46
NM-30Z
3.97 0.49
11.37 0.86
NM-45Z
4.61 0.86
14.12 1.01
NM-60Z
6.58 0.34
14.33 0.40
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NM-0
ZrB2
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NbMoss
22.33 0.87 22.25 1.69
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Table 4 Compressive strength, Young’s modulus and fracture toughness of the five composites. Compressive strength
Young’s modulus E KIC (MPa m 1/2)
Samples （MPa）
（MPa）
NM-0
931.69
226.56 0.50
NM-15Z
927.09
213.84 0.89
NM-30Z
1402.61
250.78 0.01
NM-45Z
1202.23
322.77 2.21
4.58 0.44
NM-60Z
1635.91
345.99 0.86
3.99 0.15
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6.12 0.30 6.34 0.45 5.75 0.58
ACCEPTED MANUSCRIPT Highlight The composites are hot-pressed at 2400 ℃ for 10 min under pressure of 50 MPa The optimum ZrB2 content in NbMo solid solution in this
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paper was 30 vol.%.
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ZrB2 has maximum solubility at the range of 30 to 45 vol.%
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in NbMo solid solution.
The residual ZrB2 in high ZrB2 content samples is the reason
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for poor properties.
NbMo-matrix
Zongde Liu, Qi Wang, Yuan Gao, Yongtian Wang, Youmei Sun, Yan Gong PII: DOI: Reference:
S0263-4368(17)30227-5 doi: 10.1016/j.ijrmhm.2017.06.011 RMHM 4470
To appear in:
International Journal of Refractory Metals and Hard Materials
Received date: Revised date: Accepted date:
12 April 2017 23 June 2017 28 June 2017
Please cite this article as: Zongde Liu, Qi Wang, Yuan Gao, Yongtian Wang, Youmei Sun, Yan Gong , Preparation and properties of hot-pressed NbMo-matrix composites reinforced with ZrB2 particles, International Journal of Refractory Metals and Hard Materials (2017), doi: 10.1016/j.ijrmhm.2017.06.011
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ACCEPTED MANUSCRIPT Preparation and properties of hot-pressed NbMo-matrix composites reinforced with ZrB2 particles
Key Laboratory of Condition Monitoring and Control for Power Plant Equipment of Ministry of
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1
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Zongde Liu 1,a, *, Qi Wang 1,b , Yuan Gao1,c, Yongtian Wang1,d, Youmei Sun1,e, Yan Gong1,f
email: [email protected] bemail : [email protected] cemail :[email protected] demail :
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a
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Education, North China Electric Power University, Beijing 102206, China
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[email protected] eemail: [email protected] femail: [email protected]
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*Corresponding author: Zongde Liu
ACCEPTED MANUSCRIPT Abstract The NbMo-matrix composites reinforced with (0-60 vol.%) ZrB2 were fabricated by hot-pressing at 2400 ℃ for 10 min under a pressure of 50 MPa in dynamic vacuum in the induction heating furnace specially designed in our institute. The optimum
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ZrB2 content in NbMo solid solution was determined to be 30 vol.% for excellent
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comprehensive mechanical property. NbMo-30 vol.% ZrB2 has the highest density of
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99.63%, the most uniform microstructure, high fracture toughness of 5.75 MPa m1/2. The highest ZrB2 concentration that react with NbMo solid solution is at the range of
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30 to 45 vol.%. The types of the formed niobium borides were decided by the
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original ratio of Nb to B. The distribution of Mo and Zr was mutually exclusive in low ZrB2 content composites, however, there was Mo2Zr in high ZrB2 content
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composite. Except for NbMo-45 vol.% ZrB2, the compressive strength increased
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with ZrB2 content (from 927.09 MPa to 1635.91 MPa). The Young’s modulus values were directly proportional to ZrB2 content. The fracture toughness (from 6.34 MPa
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m1/2 to 3.99 MPa m1/2) was inversely proportional to ZrB2 content. The big residual
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ZrB2 particles in high ZrB2 content samples such as NbMo-45 vol.% ZrB2 and NbMo-60 vol.% ZrB2 was the main reason for
nonhomogeneous microstructure,
low density (94.09% and 94.83%, respectively) and low fracture toughness (4.58 MPa m1/2 and 3.99 MPa m1/2, respectively).
Keywords: Hot-pressing; NbMo; ZrB2; microstructure; mechanical properties
ACCEPTED MANUSCRIPT 1. Introduction As one of the most promising refractory niobium-based alloys are of great interest
alloy for use at high temperature, because of their unique combination of
low density, relative high ductility at room temperature, high resistance to corrosion,
a good candidate for space, nuclear and aircraft applications
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niobium-based alloys
These excellent characteristics make
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high melting point and refractory properties.
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[1]. However, the strength of niobium (Nb) decreases substantially at temperature above 1200 K. It was found that the 0.2% flow stress of the Nb-ZrC alloys increases
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with increasing ZrC content from room temperature to elevated temperature [2].
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Adding refractory elements such as molybdenum (Mo) to Nb could greatly improve the high temperature strength of Nb by solid solution strengthening. The introduction
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of Mo to Nb-ZrC plays a part in the improvement of strength by solid solution
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strengthening of NbMo-matirx. The maximum solid solution effects can be achieved when the Nb/Mo ratio is around 1. However, the fracture toughness decrease with
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increasing ZrC content [3].
used as ultra-high temperature materials due to its
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Zirconium diboride (ZrB2) is
extremely high melting temperature (Tmelt > 3000 ℃), high hardness, excellent corrosion resistance and high thermal conductivity [4-6]. The low fracture toughness is an negative to the application of ZrB2. The introduction of metal powders such as Mo can improve the fracture toughness and fracture strength compared to pure ZrB2 [7-8]. ZrB2-based composites toughened by Nb were achieved by hot-pressing [9]. The densification of ZrB2 powders needs very high pressure and temperature
ACCEPTED MANUSCRIPT (2100-2300 ℃) in sintering process [10], however, the final samples have many pores [11]. The densities of the ZrB2-SiC composites are directly proportional to sintering temperature [12]. The characteristic of grain boundary phase and second phase is related to the highest sintering temperature. Hot-pressing (HP) and spark plasma
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sintering (SPS) can be used to synthesis the ultra-high temperature ceramics (UHTC)
because of the shape and large size of components, costs and
reproducibility of properties
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production
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under an external pressure. However, the SPS is not widely used in industrial
are not as excellent as that of samples in the laboratory
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[13]. In early studies, the composite of ZrB2 - 25 vol.% Nb
were only sintered by
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hot-pressing at 1800 ℃ for 60 min in Argon atmosphere under 30 MPa, and the relative density of the ZrB2-25 vol.% Nb is 97.2% [14]. The composite of ZrB2- (0-10) were prepared at 1950 ℃ for 60 min by hot-pressing, and the highest
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vol.% Mo
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density of ZrB2- (0-10) vol.% Mo is 98.9% [15]. It is necessary to explore a hot-pressing process with higher temperature and pressure to improve the density of
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these kinds of composites. Although some researches about the ultra-high temperature
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refractory metal matrix composites reinforced by ceramic particles were conducted, it is necessary to explore the preparation and more feasible ingredients. In this
study, ZrB2 is selected as a single ceramic additive for NbMo, and the ratio
of Nb/Mo is set at 1. A new hot-pressing processing technique was applied to prepare ZrB2-NbMo composites with a content range of ZrB2 from 0 to 60 vol.%. The hardness, relative density, compressive strength, fracture toughness and phase composition
at room temperature was investigated. The effects of ZrB2 on the
ACCEPTED MANUSCRIPT microstructure and mechanical property of the xZrB2-NbMo were analyzed. The optimum content of ZrB2 in NbMo solid solution was discussed. 2. Experimental Procedure 2.1 Preparation
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Five powder mixtures were prepared (amounts in vol.%):
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Material NM-15Z: 85% (50%Nb+50%Mo) + 15%ZrB2
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Material NM-0: 50%Nb + 50%Mo
Material NM-30Z: 70% (50%Nb+50%Mo) + 30%ZrB2
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Material NM-45Z: 55% (50%Nb+50%Mo) + 45%ZrB2
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Material NM-60Z: 40% (50%Nb+50%Mo) + 60%ZrB2 In this article, the letter N is used as the abbreviation for Nb, M for Mo and Z for ZrB2.
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Nb and Mo (500-800 mesh, purity > 99.95%) and ZrB2 (600 mesh, purity > 99.95%)
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were supplied by Beijing gold crown for the new material Technology Co. Ltd. China. The sizes of ZrB2, Nb and Mo are from 15 μm to 30 μm.Commercial powders were 1. The
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used to make five different samples, the compositions are shown in Table
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powders were proportionally mixed, then dried at 110 ℃ for 3 hours, then mixed and pulverized in the planetary milling in stainless steel ball milling containers at the speed of 200 rpm for 10 hours. The mould was filled with the powders and a fire-resistance slab that could facilitate the removal of the specimen and minimize the friction between powders and mould walls. Samples were prepared following the process in Fig.1. First, semi-finished products were prepared in a cylindrical cabon fiber mould at room temperature (25 ℃) and 25 MPa pressure using molding
ACCEPTED MANUSCRIPT powders for 10 min, and dried at 280 ℃ for 50 min. Then the semi-finished products were put in crucible. After that, vacuum pump
was used to control the pressure to
10-1 MPa. Followed by inflating Ar, the final reading of vacuum pump was stable at 0.08 MPa, therefore the absolute pressure was close to 0.02 MPa. The high frequency
furnace was used to measure temperature. When the temperature
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from outside of the
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induction heating equipment (160 kW) was turned on. The temperature thermocouple
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rose to 1800 ℃，the mechanical power presses started at the same time. The holding time was 15 min, this particular time length could prevent the excessive temperature
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difference caused by high heating rate between the inside and the surface of the
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cylinder. The pressure at this holding stage was 25 Mpa in Ar atmosphere. Then the temperature was increased continuously to 2400 ℃. This holding time was 10 min,
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which prevented the overflowing of low-melting phase caused by overheating. The
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pressure at this stage was 50 Mpa. Finally, the sample was cooling to room temperature. The temperature mentioned was recorded by thermocouple, however, the furnace could be 2400 ℃ to 2600 ℃ at this holding
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real temperature inside the
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stage. The entire process was conducted in dynamic vacuum by inflating Ar discontinuously. It was attempted to prepare a sample of 120 mm length but its length couldn’t be controlled due to the reason
of the unevenness distribution of internal
stress and temperature field. Finally, the length of the sample was decided to be 70 mm. The sintering temperature of 1500 ℃ and 1600 ℃ were also tried, but the samples
had low hardness and low relative density. The general furnace could not
reach the high temperature needed. Because of these reasons, the furnace in Fig. 2
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furnace
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Fig. 3. The original sample
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2.2 Characteristic
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The final samples with a cylindrical shape (Fig.3) were obtained by hot-pressing. The diameter was 14 mm and the height was approximately 70 mm. Then, samples were
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cut into specimens that had dimensions of 10mm*10mm*8mm. The cutting method After this, the specimens
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was wire cut electrical discharge machining (WEDM).
were cleaned in ultrasonic cleaner. The surfaces of the specimens for microstructure grinded with a series of sandpaper (from #100 to
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observation and XRD texting were
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#1500) and polished with 4000 mesh diamond paste. The phase composition was detected by X-ray diffraction device which was operating at a scanning speed of
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8°/min and 2θ from 10° to 90° at 40 kV and 100 mA. ASEM (ZEISS SUPRA-55)，
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FEI Quanta 200F and SU8010 scanning electron microscope was operated at an accelerating voltage of 20 kV to observe the morphology. The element composition and distribution were analyzed by Energy Dispersive X-ray spectroscopy (EDS). The micro-hardness was measured using the FM-300 Vickers tester under 300g load over an indentation time of 10 s with diamond indenter. The density was measured using the densimeter (FA1104J) following Archimedes’ principle with alcohol as the immersing medium. The reported density was the average of three measurements. The
ACCEPTED MANUSCRIPT final result of micro-hardness was determined as the average value of three points. The compression test was carried on the hydraulic universal tester (SHT 4305) in air at room temperature，and the rate of loading was 600N/m. The Young’s modulus was measured by dynamic Young’s modulus tester (RFDA system 23). The crack for
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observing propagation path was made on the HVS-50P Vickers hardness tester with
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the load of 98 N for 15 s at room temperature. The fracture toughness was determined
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by a three point bending test using the single-edge-notched beam (SENB) specimens by Electromechanical Universal Testing Machine (C45.105). Fracture toughness
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specimens with 3mm*4mm cross-section and 36mm span were prepared and the
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notch was 0.15mm in width and 1.5mm in depth. 3. Results and Discussion
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3.1. Phase Composition
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The Nb-B phase diagram [16] shows the stable phase in this system are BCC (niobium), Nb3B2, NbB, Nb3B4, NbB2, Nb5B6, B (boron) and liquid L.
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Fig. 4. X-ray diffraction (XRD) patterns of the five samples.
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Fig. 4 displays the XRD patterns of five samples. It suggests that the ZrO coexists with Nb3B2 in NM-15Z, NM-30Z, NM-45Z and NM-60Z. The NbMo solid solution exists in the five samples. The peaks of ZrB2 are found in NM-45Z and NM-60Z, and the peaks of NbB and Mo2Zr in NM-60Z are also found. The details are displayed in Table 2, and the “*” is used as the symbol for main phase. Table
2 The compounds in five samples after hot-pressing
According to XRD results, the reactions in four composites are: the eutectic reaction
ACCEPTED MANUSCRIPT (2104 ± 5 ℃) and the peritectoid reaction (1900 - 2100 ℃) [17]. The reaction between Nb and B is easily explained according to the currently accepted Nb-B phase diagram. The types of the formed niobium borides are decided by the ratio of Nb to B [18] and the formation of Mo 2Zr is due to
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the interdiffusion between Mo and Zr [19].
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3.2. Densification and hardness
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Fig. 5. The curves of relative density changed with volume fraction of ZrB 2.
The curves of relative density and hardness of niobium borides are shown in Fig. 5
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and Fig. 6. The theoretical density is obtained by calculations based on the original
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ingredient before hot-pressing, and the effect of oxygen content is assumed to be neglected. The assumed true densities are 6.085 g/cm3 for ZrB2 and 8.57 g/cm3 for
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Nb and 10.2 g/cm3 for Mo. NM-45Z and NM-60Z have low relative density, and this
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may be attributed to that those two ingredients include more ZrB2 particles, and the connection among ZrB2 is more difficult than the connection between ZrB2 and
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niobium boride. Another reason for the low relative density of NM-45Z and NM-60Z
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is the big ZrB2 particles impede phase flowing in the process of solidification. The grain can be seen from Fig. 7, NM-30Z has the most uniform microstructure. The excellent property of NM-30Z should be attributed to its highest relative density and uniform microstructure among the samples. Table 3 Hardness of phase of the five composites. Fig. 6. The curves of hardness of niobium borides changed with volume fraction of ZrB2.
The hardness in Table 3 is the average value of three points. The hardness of NbMoss
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atoms. The niobium borides phase refers to Nb3B2 in NM-15Z, NM-30Z and NM-45Z,
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and Nb3B2+NbB in NM-60Z, because it is not easy to distinguish the two phase in
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NM-60Z. The hardness of the niobium borides in ZrB2-NbMo composites is directly proportional to ZrB2 content. In all the composites, the low hardness district is the
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NbMo-matrix, and the hardness of ZrB2 is around 22.3 GPa. The niobium borides
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have high hardness (16.58 GPa for NbB and 10.08 GPa for Nb3B2), and the hardness of niobium borides significantly increases with the increasing of B [20]. NbB is the
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3.3. Microstructure
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main reason for hardness improvement in NM-60Z.
Microstructure of hot-pressed ZrB2–NbMo composites was observed under
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backscattered electron SEM imaging, and typical examples are shown in Fig. 7.
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Fig. 7. SEM images of polish surface of (a) NM-15Z, (b) NM-30Z, (c) NM-45Z and (d) NM-60Z.
The EDS analysis results show that the white sections in Fig. 8 (a) are NbMo solid solution rich in Mo and the dark grey and light grey sections are NbMo solid solution rich in Nb. The difference of colors between dark grey and light grey is due to the different percentage composition of Nb. The darker section has more Nb. Nb and Mo substitute each other [3]. The solution strengthening of Nb and Mo is beneficial to the improvement of strength and hardness.
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less surfaces of Mo particles melted, and the Mo particles could hold the approximate
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spherical shape in the process of extrusion. So the voids among Mo particles in Fig. 8
solution rich in Mo due to the diffusion of Nb.
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are full of NbMo solid solution rich in Nb. The Mo particles turn to NbMo solid
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Fig. 8. SEM images of NM-0 polish surface at (a) high magnification and (b) low magnification.
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The microstructure of NM-15Z in Fig. 7 is very different from NM-0, but is similar with NM-30Z, NM-45Z and NM-60Z. This phenomenon is result from the
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introduction of ZrB2 particles. The microstructure of NM-15Z has irregular shapes
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and sharp edges. The ZrO is distributed in the NbMo-matrix as a reinforcing phase. The distribution of element Mo and Zr is
mutually exclusive, which might indicates
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that it is less likely to form MoZr solid solution or molybdenum-zirconium
Fig. 9.
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compounds in this sample. Nb3B2 is formed around ZrO in general in Fig. 7 (a) and
Fig. 9. BSE images and EDX maps of NM-15Z.
The elements area profile of NM-30Z is not shown because the elements distribution in NM-30Z is uniform and there is no obvious segregation in this sample. The main phase of NM-30Z is niobium boride, and the area of niobium boride is larger than that of NbMo solid solution. This is because the decomposition of high fraction of ZrB2
ACCEPTED MANUSCRIPT particles. The generated ZrO particles observed in Fig. 7 (b) are distributed in the whole samples with the shape of roundness, which is beneficial to strengthen the material. The relationship of elements is similar to NM-15Z. Fig. 10. BSE images and EDX maps of NM-45Z.
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The elements area profile of NM-45Z is shown in Fig. 10. ZrB2 particles are found
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large and the shape irregular in NM-45Z. The excessive ZrB2 particles are difficult to
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react, and the boundary between the unreacted ZrB2 particles and other phases is not very smooth. All these factors could do harm to the mechanical property. The SEM
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image of NM-60Z is shown in Fig. 7 (d) and the elements area profile of NM-60Z is
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shown in Fig. 11. The microstructures in NM-60Z have several differences from others. First, there is Mo2Zr phase not observed in other samples. The foundation of
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Mo2Zr is in keeping with XRD results. Second, the area of NbMo solid solution is not
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very large, and this may result from the reactions, because the Nb and Mo atoms mostly reacted and less left to form NbMo solid solution. Third, there are redundant
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samples.
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ZrB2 and the sizes of ZrB2 particles in NM-60Z are larger than particles in other three
Fig. 11. BSE images and EDX maps of NM-60Z.
This phenomenon matches the XRD results, and indicates that there is residual ZrB2 in high proportion ZrB2 samples such as NM-45Z and NM-60Z. It is concluded from the phenomenon that the ZrB2 has the highest reaction concentration in NbMo solid solution, and residual ZrB2 in high proportion ZrB2 samples could affect the formed phase and mechanical properties. The highest ZrB2 concentration that react with
ACCEPTED MANUSCRIPT NbMo solid solution is at the range of 30 to 45 vol.%. The dispersion strengthening of ZrB2 particle and solution strengthening of Nb and Mo worked together to strengthen the samples. However, the big ZrB2 particles and unevenness of all particles could affect compression property.
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3.4. Compression and fracture toughness
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The compression results are shown in Fig. 12. The NM-0 and NM-15Z exhibit
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plasticity, and the NM-30Z, NM-45Z and NM-60Z exhibit brittleness. The compressive strength of NM-30Z, NM-45Z and NM-60Z are much higher than NM-0.
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The flow stress increases with increasing ZrB2 content, however, the compressive
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plasticity decreases with increasing ZrB2 content. The reason for the increase is the dispersion strengthening of ZrB2 particles. This confirms the ZrB2 is an effective
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reinforcing phase to NbMo, which work together with the solid solution strengthening
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of NbMo to strengthen the composites. Except for NM-45Z, the compressive strength increases with ZrB2 content. The
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decreasing compressive strength of NM-45Z is related to the reduction of NbMo solid
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solution and the existence of big redundant ZrB2. Overmuch ZrB2 ceramic particle leads to large size particles and uneven size phase in NM-45Z. The large particles result in less boundaries to absorb strain energy than NM-15Z and NM-30Z. The element segregation in NM-45Z is more obvious than other samples, and the reason for this phenomenon is the large particles impede phase flowing in the process of solidifications. And the uneven microstructures in NM-45Z result in stress concentration to form cracks. The low relative density can also reflect much micro
ACCEPTED MANUSCRIPT voids and micro cracks inside NM-45Z. The stress concentration around these micro defects is also harmful to mechanical property. The connection between ZrB2 and other phase in NM-45Z shown in Fig. 7(c) is not so tight as NM-15Z and NM-30Z, and this would also do harm to the compressive strength. All these factors do harm to
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the promotion of compressive strength of NM-45Z. This matches the discussion about
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microstructure above. There are two positive factors that are conducive to the
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improvement of the compressive strength: the solid solution of NbMoss and the dispersion strengthening of ZrB2 particles. There are negative factors that are bad for
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the improvement of compressive strength: reduction of NbMo solid solution and the
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existence of big redundant ZrB2. Comparing NM-45Z to NM-30Z, the increase of negative effects exceed the increase of positive effects, so the compressive strength of
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NM-45Z is lower than the NM-30Z’s. Comparing NM-60Z to NM-30Z, the increase
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of positive effects exceeds the increase of negative effects, so the compressive strength of NM-60Z is higher than the NM-30Z’s. Table 4 shows the Young’s modulus
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of composites. The Young’s modulus values are directly proportional to ZrB2 content.
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Therefore, the addition of ZrB2 could improve the Young’s modulus of composites. Fig. 12. Stress-strain curves at room temperature. Table 4 Compressive strength, Young’s modulus and fracture toughness of the five composites.
The fracture toughness values are inversely proportional to ZrB2 content in the samples added ZrB2. The value of fracture toughness for monolithic ZrB2 is 2.8 MPa m1/2 [21]. Table 4 shows that the fracture toughness values of all composites are higher than monolithic ZrB2, which indicates that the addition of Nb and Mo is in
ACCEPTED MANUSCRIPT favor of enhancing the fracture toughness. This phenomenon could be verified by the crack behavior of these samples on the polished surface of four samples shown in Fig. 13. The crack bridging and crack branch could be found in niobium boride and NbMo solid solution, and the crack deflection is found in niobium boride, NbMo solid
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solution and around a small quantity of ZrB2. The crack propagation of NM-15Z is
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not shown because it is similar with NM-30Z. Comparing NM-0 with NM-30Z, there
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is no crack bridging in NM-0, and the crack in NM-0 propagates along the boundaries of Nb. The crack bridging and crack deflection could be seen around ZrO tiny energy and enhance fracture toughness，
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particles in NM-30Z, which could dissipate
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and the crack branch could be found in Nb3B2. The direction of crack propagation of NM-60Z in Fig. 13 (d) is from the bottom right corner to top left corner. Comparing
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NM-30Z with NM-60Z, the crack deflection of NM-30Z in Fig. 13 (b) is around ZrO.
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However, in NM-60Z, only the small ZrB2 particles in the end of the crack propagation path could have a small role in bridging crack. The reason for the low
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fracture toughness of NM-60Z are the little niobium boride and much big ZrB2 has little effect on improvement of
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particles. An excess of ZrB2 in composite
fracture toughness. Therefore, the lack of niobium boride and existence of big particles in NM-45Z and NM-60Z are the reason for the low fracture toughness. Fig. 13. Typical cracking propagation behavior of (a) NM-0, (b) NM-30Z, (c) NM-45Z and (d) NM-60Z.
Fig.14 shows SEM images of the fracture surface of the composites. Only the transgranular fracture is observed in NM-15Z and NM-30Z. Both of the transgranular
ACCEPTED MANUSCRIPT and intergranular fracture are observed in NM-45Z and NM60Z, and the transgranular and intergranular fracture were respectively progressed in niobium borides and ZrB2. The main fracture mode in all the composites is transgranular fracture of niobium borides, and there is intergranular fracture of ZrB2 particles in NM-45Z and NM-60Z.
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The Nb3B2 is plastically deformed in all the composites，and decohesion is found at
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the interface between Nb3B2 and ZrO in NM-15Z. Ashby et al. [22] have pointed out
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that decohesion at an interface plays an important role in enhancing fracture toughness. The reason for the low fracture toughness of NM-45Z and NM-60Z is the
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little Nb3B2 phase and the existence of big ZrB2 particles. Therefore, the high fracture
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toughness of NM-15Z is attributed to the stretching of Nb3B2 and interface decohesion.
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4. Conclusions
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Fig. 14. Fracture surface of (a) NM-0, (b) NM-30Z, (c) NM-45Z and (d) NM-60Z.
ZrB2-NbMo composites were hot-pressed at 2400 ℃ for 10 min under a pressure of
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50 MPa with ZrB2 content ranging from 0 to 60 vol.%. The object is to determine the
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optimum content of ZrB2 in NbMo solid solution. (1) The optimum ZrB2 content in NbMo solid solution in this paper is determined to be 30 vol.%. The highest ZrB2 concentration that reacts with NbMo solid solution is at the range of 30 to 45 vol.%, and residual ZrB2 in high ZrB2 content samples change phase composition and weaken mechanical properties. (2) The types of the formed niobium borides were decided by the original ratio of Nb to B. The distribution of Mo and Zr was
mutually exclusive in low ZrB2 content
ACCEPTED MANUSCRIPT samples, however, there was Mo2Zr in high ZrB2 content environment. (3) The relative density of the reinforced composites increased with ZrB2 content within 30 vol.% and decreased in 45 vol.% and 60 vol.%. The hardness of the niobium borides in ZrB2-NbMo composites is directly proportional to ZrB2
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content (from 6.48 GPa for NM-15Z to 14.33 GPa for NM-60Z).
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(4) Except for NM-45Z, the compressive strength increases with ZrB2 content. The
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decreasing compressive strength of NM-45Z is related to the reduction of NbMo solid solution and the existence of big residual ZrB2 particles. The Young’s
1/2
for NM-15Z to 3.99 MPa m
1/2
for NM-60Z) is inversely
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(from 6.34 MPa m
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modulus values are directly proportional to ZrB2 content. The fracture toughness
proportional to ZrB2 content. The big ZrB2 particles and little Nb3B2 phase are the
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reason for the low fracture toughness.
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The content of ZrB2 should be controlled to 30 vol.% in order to get excellent comprehensive mechanical property.
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Acknowledgement
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The authors would like to thank the financial support from the National Natural Science Foundation of China (11372110)
ACCEPTED MANUSCRIPT Reference 1. C. Nico, T. Monteiro, M.P.F. Graça, Niobium oxides and niobates physical properties: Review and prospects, Prog. Mater. Sci. 80 (2016) 1–37. 2. Y. Tan, C.L. Ma, A. Kasama, R. Tanaka, Y. Mishima, S. Hanada, JennMing Yang, Effect of alloy
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3. Y. Tan, C.L. Ma , A. Kasama , R. Tanaka, Jenn-Ming Yang, High temperature mechanical behavior of Nb-Mo-ZrC alloys, Mater. Sci. Eng. A. 355 (2003) 260-266.
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4. YuWang, Jun Liang, Wenbo Han, Xinghong Zhang, Mechanical properties and thermal shock
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behavior of hot-pressed ZrB2–SiC–AlN composites, J. Alloys Compd. 475 (2009) 762–765. 5. Eric W. Neuman, Gregory E. Hilmas, William G. Fahrenholtz, Processing, microstructure, and
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7. Moya JS, Díaz M, Gutiérrez-González CF, Diaz LA, Torrecillas R, Bartolomé JF, Mullite-refractory metal (Mo, Nb) composites, J. Eur. Ceram. Soc. 28 (2) (2008) 479–491. 8. Gu ML, Huang CZ, Zou B, Liu BQ,Effect of (Ni, Mo) and TiN on the microstructure and mechanical properties of TiB2 ceramic tool materials, Mater. Sci. Eng. A. 433 (2006) 39–44. 9. Xin Sun, Wenbo Han, Qiang Liu, Ping Hu, Changqing Hong, ZrB 2-ceramic toughened by refractory metal Nb prepared by hot-pressing, Mater. Des. 3 (2010) 427–4431.
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10. M. Pastor, Sintering Methods and Properties of Solid Bodies, in Boron and Refractory Borides. Metallic borides: preparation of solid bodies, in: V.I. Matkovich(Ed.), Springer-Verlag, New York, 1977, 457–493. 11. Diletta Sciti, Fr´ed´eric Monteverde, Stefano Guicciardi,Giuseppe Pezzotti, Alida Bellosi,
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Microstructure and mechanical properties of ZrB 2–MoSi2 ceramic composites produced by different
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12. Ipek Akin, Mikinori Hotta, Filiz Cinar Sahin, Onuralp Yucel, Gultekin Goller, Takashi Goto, Microstructure and densification of ZrB2–SiC composites prepared by spark plasma sintering, J. Eur.
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Ceram. Soc. 29 (2009) 2379–2385.
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13. F. Monteverde, A. Bellosi, Luigi Scatteia, Processing and properties of ultra-high temperature ceramics for space applications, Mater. Sci. Eng. A. 485 (2008) 415–421.
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14. Xin Sun, Wenbo Han, Ping Hu, Zhi Wang, Xinghong Zhang, Microstructure and mechanical
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properties of ZrB2–Nb composite, Int. J. Refract. Met.Hard Mater. 28 (2010) 472–474. 15. Hailong Wang, Deliang Chen, Chang-An Wang, Rui Zhang, Daining Fang, Preparation and
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characterization of high-toughness ZrB2/Mo composites by hot-pressing process, Int. J. Refract.
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Met.Hard Mater. 27 (2009) 1024–1026. 16. Rudy E, Windisch S. Related binary systems. Part I, Systems V-B, Nb-B and Ta-B. In: Ternary phase equilibria in transition metal-boron-carbon-silicon systems, vol. X. Air Force Materials Laboratory, Wright-Patterson Air Force Base. (1966) 1-104. 17. Zhihong Tang, M.J. Kramer, Mufit Akinc, Evaluation of phase equilibria in the Nb-rich portion of Nb-B system, Intermetallics. 16 (2) (2008) 255-261. 18. A.L. Chamberlain, W.G. Fahrenholtz, G.E, Hilmas, High strength ZrB2-based ceramic,
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J. Am. Chem. Soc. 87 (6) (2004) 1170-1172. 19. A. Paz y Puente, J. Dickson, D.D. Keiser Jr., Y.H. Sohn, Investigation of interdiffusion behavior in the Mo–Zr binary system via diffusion couple studies, Int. J. Refract. Met.Hard Mater. 43 (2014) 317-321.
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20. Zhiping Sun, Xiping Guo, Xiaodong Tian, Liang Zhou, Microhardness and Macrohardness of
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composits, J. Eur. Ceram. Soc. 22 (3) (2002) 319-329.
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21. F. Monteverde, A. Bellosi, S. Guicciardi, Processing and properties of zirconium deboride-based
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22. M.F. Ashby, F.L. Blunt, M. Bannister, Flow characteristics of highly constrained metal wires,
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Fig.1. The sintering process.
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Fig. 2. Internal structure of furnace.
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Fig.3 The final sample
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Fig. 4. X-ray diffraction (XRD) patterns of the five samples.
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Fig. 5. The curves of relative density changed with volume fraction of ZrB 2.
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Fig. 6. The curves of hardness of niobium borides changed with volume fraction of ZrB2.
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Fig. 7. SEM images of polish surface of (a) NM-15Z, (b) NM-30Z, (c) NM-45Z and (d) NM-60Z.
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Fig. 8. SEM images of polish surface at (a) high magnification and (b) low magnification.
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Fig. 9. BSE images and EDX maps of NM-15Z.
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Fig. 10. BSE images and EDX maps of NM-45Z.
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Fig. 11. BSE images and EDX maps of NM-60Z.
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Fig. 12. Stress-strain curves at room temperature.
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Fig. 13. Typical cracking propagation behavior of (a) NM-0, (b) NM-30Z, (c) NM-45Z and (d)
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NM-60Z
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Fig. 14. Fracture surface of (a) NM-0, (b) NM-30Z, (c) NM-45Z and (d) NM-60Z.
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1 Compositions, densities, relative densities of the five composites. Compositions
Test
Theoretical
Relative density
(vol.%)
density(g/cm3)
density(g/cm3)
（%）
94.93 0.77
Mo
ZrB2
NM-0
50
50
0
8.857 0.072
9.330
NM-15Z
42.5
42.5
15
8.674 0.071
8.843
98.09 0.80
NM-30Z
35
35
30
8.325 0.018
99.63 0.21
NM-45Z
27.5
27.5
45
7.405 0.080
7.870
94.09 1.10
NM-60Z
20
20
60
7.001 0.086
7.383
94.83 1.16
8.356
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Table 2 The compounds in five samples after hot-pressing ZrB2
ZrO
Nb3B2
√ (*)
NM-15Z
√ (*)
√
√
NM-30Z
√
√
√ (*)
NM-45Z
√
√
√
√ (*)
NM-60Z
√
√ (*)
√
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NbB
Mo2Zr
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√
√
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Table 3 Hardness of phase of the five composites. Hardness (GPa) of constituent phase Samples Niobium borides
2.77 0.39
NM-15Z
3.75 0.89
6.48 0.46
NM-30Z
3.97 0.49
11.37 0.86
NM-45Z
4.61 0.86
14.12 1.01
NM-60Z
6.58 0.34
14.33 0.40
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NM-0
ZrB2
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22.33 0.87 22.25 1.69
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Table 4 Compressive strength, Young’s modulus and fracture toughness of the five composites. Compressive strength
Young’s modulus E KIC (MPa m 1/2)
Samples （MPa）
（MPa）
NM-0
931.69
226.56 0.50
NM-15Z
927.09
213.84 0.89
NM-30Z
1402.61
250.78 0.01
NM-45Z
1202.23
322.77 2.21
4.58 0.44
NM-60Z
1635.91
345.99 0.86
3.99 0.15
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6.12 0.30 6.34 0.45 5.75 0.58
ACCEPTED MANUSCRIPT Highlight The composites are hot-pressed at 2400 ℃ for 10 min under pressure of 50 MPa The optimum ZrB2 content in NbMo solid solution in this
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paper was 30 vol.%.
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ZrB2 has maximum solubility at the range of 30 to 45 vol.%
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The residual ZrB2 in high ZrB2 content samples is the reason
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