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High temperature compression properties of hot-pressed NbMo-matrix composites reinforced with ZrB2 particles
Accepted Manuscript High temperature compression properties of hot-pressed NbMomatrix composites reinforced with ZrB2 particles
Qi Wang, Zongde Liu, Yongtian Wang, Youmei Sun, Yan Gong PII: DOI: Reference:
S0263-4368(18)30382-2 doi:10.1016/j.ijrmhm.2018.08.007 RMHM 4773
To appear in:
International Journal of Refractory Metals and Hard Materials
Received date: Revised date: Accepted date:
4 June 2018 6 August 2018 13 August 2018
Please cite this article as: Qi Wang, Zongde Liu, Yongtian Wang, Youmei Sun, Yan Gong , High temperature compression properties of hot-pressed NbMo-matrix composites reinforced with ZrB2 particles. Rmhm (2018), doi:10.1016/j.ijrmhm.2018.08.007
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ACCEPTED MANUSCRIPT High temperature
compression properties of
hot-pressed
NbMo-matrix composites reinforced with ZrB2 particles Qi Wang 1, a, Zongde Liu
* , Yongtian Wang 1,c, Youmei Sun 1,d , Yan Gong 1,e
Key Laboratory of Condit ion Monitoring and Control for Power Plant Equ ip ment of M inistry of
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1
1,b,
Education, North China Electric Power University, Beijing 102206, China b
email : [email protected]
[email protected] eemail: [email protected]
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*Corresponding author: Zongde Liu
c
email : [email protected]
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email: [email protected]
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a
d
email:
ACCEPTED MANUSCRIPT Abstract The effects of 15, 30, 45, 60 vol.% ZrB2 additions on high temperature compression properties of hot-pressed NbMo-matrix were investigated by the compressio n experiments in the temperature range of 800℃-1300℃. ZrB2 particle additions to
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NbMo matrix could significantly improve the high temperature compressive strength
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of the composites. The best comprehensive property including strength and ductility
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belongs to the composites containing 30 vol.% ZrB2 with refined grains at the temperature below 1100℃. The resistant ability to high temperature was directly
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proportional to ZrB2 content. The compressive strength of composite containing 60
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vol. % ZrB2 (from 1236.56 MPa to 700.46 MPa) was inversely proportional to temperature. The failure planes of composite containing 15 vol. % and 30 vol. % ZrB2
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were always at angle of 45° to the axis of the compression direction, whereas the
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failure pattern of composite containing 45 vol. % and 60 vol. % ZrB2 changed from splitting along the compression direction to cracking at certain angle to the axis of the
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compression direction with temperature rising.
Keywords: Hot-pressing; NbMo; ZrB2 ; high temperature; compression properties
ACCEPTED MANUSCRIPT 1. Introduction The excellent properties of high resistance to corrosion, high melting point, high strength and unique combination of low density and relative high ductility at high temperature make niobium-based alloys a good candidate for high temperature
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structural materials and attractive for the application in the fields of space, nuclear and
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aircraft [1-2]. However, the rapid strength decrease of niobium (Nb) at temperature
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above 1200 K is still a major issue for the use of Nb-based alloys [3]. Adding refractory elements such as molybdenum (Mo) to Nb could greatly improve the high
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temperature strength of Nb by means of solid solution strengthening. Moreover, when
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the Nb/Mo ratio is around 1, the maximum solid solution effects can be achieved. To improve high temperature strength of NbMo-alloys, titanium carbide, zirconium
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carbide et al. particles were added as a reinforcing ceramic phase to Nb and
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NbMo-alloys. For instance, the flow stress of the Nb-ZrC alloys increases with increasing ZrC content at elevated temperature. Nevertheless, with increasing ZrC content, the fracture toughness decrease [4-5]. As one of the ultra-high temperature
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ceramics (UHTCs), zirconium diboride (ZrB2 ) materials have high strength, high melting temperature (3313 K), high hardness, excellent corrosion resistance and high thermal conductivity at high temperature [6-10]. There have been studies on the ZrB2 particles reinforced refractory alloys. Xin Sun et al. investigated the composite of ZrB2 - 25 vol.% Nb sintered by hot-pressing at 1800℃ for 60 min in Argon atmosphere under 30 MPa [11]. Hailong Wang et al. studied the composite of ZrB2 - (0-10) vol.% Mo prepared by hot-pressing at 1950℃ for 60 min
ACCEPTED MANUSCRIPT [12]. In our previous work, (0-60) vol.% ZrB2 -NbMo composites were hot-pressed at 2400℃ for 10 min in Argon atmosphere under 50MPa, and the ratio of Nb/Mo is set at 1 [4-5]. Most of the above mentioned works are about the mechanical properties at room temperature but do not cover the high temperature.
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The main goal of this investigation is to study the high temperature compression
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properties of the (0-60) vol.% ZrB2 -NbMo composites prepared with the process
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mentioned in our published literature [13] at temperature ranged from 800℃-1300℃. 2. Experimental
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Five powder mixtures were prepared (amounts in vol.%):
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Material NM-0: 50% Nb+50% Mo
Material NM-15Z: 85% (50% Nb+50% Mo) + 15% ZrB2
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Material NM-30Z: 70% (50% Nb+50% Mo) + 30% ZrB2
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Material NM-45Z: 55% (50% Nb+50% Mo) + 45% ZrB2 Material NM-60Z: 40% (50% Nb+50% Mo) + 60% ZrB2 The letter N is used as the abbreviation for Nb, M for Mo and Z for ZrB2 in this study.
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The experimental materials were Nb and Mo powders (500-800 mesh, purity > 99.95%) and ZrB2 powders (600 mesh, purity > 99.95%) 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. The powders were mixed and dried at 110℃ for 3 hours, then mixed and pulverized in the planetary milling with a speed of 200 rpm for 10 hours, and finally hot pressed at 2400℃ for 10 min in Argon under the pressure of 50 MPa. The diameter of the final cylindrical sample was 14 mm and the height was
ACCEPTED MANUSCRIPT approximately 70 mm. For high temperature compression examination, samples were cut into specimens that had diameter of 3 mm and height of 5 mm using the method of wire cut electrical discharge machining (WEDM). Then, the specimens were cleaned in ultrasonic cleaner. The surfaces of the specimens for microstructure observation
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and XRD texting were grinded with a series of sandpaper (from #100 to #1500) and
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polished with 4000 mesh diamond paste. The phase composition was detected by
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X-ray diffraction device which was operating at 40 kV and tube current of 100 mA over the 2θ ranging from 10° to 90°, and a scanning speed of 8°/min. FESEM (ZEISS
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SUPRA-55)， FEI Quanta 200F and SU8010 scanning electron microscope were
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operating at an accelerating voltage of 20 kV to observe the morphology. The element composition and distribution were analyzed by Energy Dispersive X-ray spectroscopy
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(EDS). The compression properties were assessed in air using the coupled mechanical
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load-temperature tester (CMT-5204) with a fixture made of SiC at the heating rate of 10℃/min at 800℃, 1000℃, 1100℃, 1200℃, 1300℃, respectively. The samples were hold for 10 min at target temperature to ensure homogeneous heating, and the
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compression speed was 2.5*10-4 s-1 . The direction of compression is parallel to the radial direction of the final cylindrical sample. The stress-strain curves were obtained from the computer recorder connected with tester. After compression text, the samples cooled to room temperature. Results and Discussion 3.1. Stress-strain curve The variations in mechanical properties of the composites were examined by
ACCEPTED MANUSCRIPT compressive tests at different elevated temperatures and the true stress-strain curves are demonstrated in Fig. 1 and Fig. 2. The experimental compressive strength results are summarized in Table 1. Based on our pervious study, the phase composition of these composites are NbMoss, niobium borides and zirconium compounds. The main
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reinforcing phase are niobium borides and zirconium compounds [13]. The melting
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points of Nb (2469℃) and Mo (2620℃) are lower than that of niobium borides (for
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example, NbB: 2903℃) [14-16], and the hardness of niobium borides and zirconium compound is higher than NbMoss, so the NbMoss is the ductile phase. Our previous
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research reveals that samples have different oxidation products in various conditions,
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nevertheless, the oxidation products make little effect o n this compression experiments for the reason that the outermost oxide skin prevents further oxidation of
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is the intimal part.
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the intimal substances. Therefore, the deformation substance discussed in this article
Table 1 Compressive Strength (MPa) of the five composites at five different temperatures. Temperature
800℃
1000℃
1100℃
1200℃
1300℃
264.23
91.11
NM-15Z
1119.51
864.76
511.56
336.35
4.3
NM-30Z
1532.87
1293.18
754.08
523.09
0.5379
NM-45Z
1381.5
795.72
609.77
530.21
202.52
NM-60Z
1236.56
1183.7
982.03
840.71
700.46
Samples
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NM-0
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Fig. 1. Stress-strain curves of five composites at high temperature: (a) NM-0; (b) NM-15Z; (c) NM-30Z; (d) NM-45Z; (e) NM-60Z.
For NM-15Z and NM-30Z, with the temperature rising, the slope decreases, the yielding stage enlarges, the strain increases, and the curve moves towards right. The ductility increases, and these phenomena could be intuitively seen from Fig. 1. For NM-45Z and NM-60Z, though the strain increase cannot be intuitively seen from curves, the residual plastic strain in Table 2 and Fig. 4 calculated by the method
ACCEPTED MANUSCRIPT shown in Fig. 3 increases with the temperature rising, and the slope decreases. These phenomena are attributed to the difference of the thermal expansion coefficient between components. The strain energy stored at the grain boundary leads to void, and the B2 O3 produced in the heating process before compression evaporates. With
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temperature rising, the voids and cracks caused by component expansion and strain
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between grain boundary and evaporation of B2 O3 increase. Additionally, the higher
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temperature aggravates the matrix metal softening, and enhances ductility and increases strain. The first step of the method of computing plastic strain in Fig. 3 is to
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make a straight fitting line (Line 1) of the elastic stage, then Line 2 is made parallel to
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this fitting line through the last point of yield stage to get the abscissa of the intersection of this parallel line and horizontal axis, and the difference value between
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strain finally needed.
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the abscissas of two intersections of line and horizontal axis is the residual plastic
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Fig. 2. Stress-strain curves of composites at the temperature from 800 to 1300℃: (a) 800℃; (b) 1000℃; (c) 1100℃; (d) 1200℃; (e) 1300℃.
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Fig. 3. Example of computing method of residual plastic strain.
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Table 2 Plastic strain (%) of the five composites. Temperature
1000℃
1100℃
1200℃
NM-0
0.32±0.010
1.02±0.009
NM-15Z
0.35±0.011
0.72±0.017
1.83±0.010
3.57±0.060
NM-30Z
0.082±0.002
0.47±0.012
0.56±0.012
2.91±0.066
NM-45Z
0.057±0.001
0.19±0.008
0.49±0.009
0.71±0.010
1.31±0.040
NM-60Z
0.055±0.001
0.14±0.010
0.39±0.004
0.40±0.003
0.61±0.059
1300℃
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800℃
Samples
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As can be seen in Table 2，ductility has a trend of decrease with the increasing ZrB2 content at the same temperature. There are three main reasons for this phenomenon, and the first reason is content. Under the same temperature condition, the composites containing low ZrB2 content have more metal to soften composites. The second reason is the few holes in the composites containing low ZrB2 content caused by evaporation of B2 O3 and MoO 3 , which is supported by the fact that B2 O3 begins to volatilize when temperature exceeds 930℃ and MoO 3 evaporated when temperature exceeds 1155℃ reported in previous researches [17-18]. The third reason is related to
ACCEPTED MANUSCRIPT the Cottrell atmospheres [19]. In the high ZrB2 content composite, the interstitial solute atoms dissolved into NbMoss are more than in low ZrB2 content composite, and high order energy reduces. The more interstitial solute atoms in high ZrB2 content composite could form Cottrell atmospheres for the segregation around dislocation line
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caused by the interaction between atoms and dislocations, and these could increase
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the σ 0 and K in Hall-Petch relationship, hence, the σ s increases, nevertheless the
ductility with rising ZrB2 content.
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3.2 Compressive strength and plastic strain
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ductility of NbMoss decreases. All these reasons are responsible for the decrease of
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Fig. 4. (a) the fitted compressive strength-ZrB2 content curve; (b) the fitted compressive strength-temperature curve.
Fig. 5. (a) the fitted plastic strain- ZrB2 content curve; (b) the fitted plastic stain-temperature curve.
ACCEPTED MANUSCRIPT For all the composites, the compressive strength decreases as temperature rises, implying a sharp sensitivity to the temperature in each composite. At 800℃ and 1000℃, compared to the composite NM-0 (50 vol.% Nb+50 vol.% Mo), the compressive strengths of four ceramic-reinforced composites are much higher for the
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reason that the ceramic particles increase dislocation density, which leads to
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significant strengthening, furthermore, the load can be transferred from the ductile
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NbMoss matrix to the stiffening ceramic particles during the high-temperature deformation. This phenomenon reveals that ZrB2 particles additions to NbMo metal
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matrix can remarkably improve the high temperature compressive strength of the
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composites. It is importantly impressing that the NM-60Z exhibits the extraordinary compressive strength, and its compressive strength is over 700 MPa at 1300℃.
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The high temperature compressive strength in Fig. 4 (a) is directly proportional to
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ZrB2 content at the same temperature, and the compressive strength in Fig. 4 (b) decreases with the increasing temperature for each composite. High ZrB2 content sample has more reinforcing phase to strengthen the composites under the same
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temperature conditions. The compression process is accompanied by three main mechanisms: the softening of NbMoss matrix, hardening of NbMoss matrix and hardening of reinforcing phase. The hardening caused by movement and multiplication of dislocation is beneficial to the strengthening of composites as well as the increase of curve slope, nevertheless, the softening caused by recovery and recrystallization is obviously to the disadvantage of the strengthening of composites. With temperature rising, the negative softening effect become more severe, and the
ACCEPTED MANUSCRIPT effect of metal hardening is too little to be considered, so the metal hardening makes limited functions on deformation and recovery and recrystallization dominate the process. The grain boundary and phase boundary slide because of the softening of the NbMoss matrix with low melting point and the boundary strength greatly decline, so
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the macro bearing capacity and compressive strength decrease. The slopes of fitted
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line in Fig. 4 (a) increase with temperature rising from 800 ℃ to 1300℃, while the
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decline rate of compressive strength in Fig. 4 (b) is slower with the ZrB2 content increasing, and these two phenomena about slope reveal that the resistant ability to
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high temperature is directly proportional to ZrB2 content. The main hardening
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mechanism is reinforcing phase hardening at higher temperature. However, there is an exception that the absolute value of NM-30Z’s slope is too large to be in accordance
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with the slope regulation in Fig. 4 (b). The reason for this exception is the excellent
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compressive strength of NM-30Z below 1200℃ caused by refined grain. Except for the case of excessive oxidation of NM-15Z and NM-30Z at 1300℃, the compressive strength of NM-30Z is always higher than NM-15Z and higher than
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NM-45Z below 1200℃. The excellent property of NM-30Z at temperature below 1200℃ is due to the homogeneous microstructure and small grain size, and the test performance of NM-45Z is not so stable for its holes and big grain size according to our previous study [13]. The compressive strength of NM-60Z begins to exceed that of NM-30Z when temperature exceeds 1100℃. Moreover, the relationship between strength and temperature in Fig. 4 could be applied to predict the compressive strength of the higher temperature which is not covered in this study.
ACCEPTED MANUSCRIPT Table 2 and Fig. 5 shows the plastic strains, which could represent the plastic deformation of composites ruling out recoverable elastic deformation. As shown in Fig. 6, for the four ceramic-reinforced samples, with the increase of temperature, the plastic strain of each composite shows a trend of enhancement, and at the same
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temperature, the composite containing lower ZrB2 content has larger plastic strain in
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general. The explanations for these phenomena could be the more void and cracks in
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higher ZrB2
content composite and the severe softening in higher temperature condition.
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It is need to notice that the NM-30Z has relatively high plastic strain at any
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temperature, which is related to its homogeneous microstructure and small grain size. Therefore, one can conclude that NM-30Z has the best comprehensive property
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3.3 Failure patterns
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including strength and ductility at the temperature below 1100℃.
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Fig .6. Fracture surface of four composites at 800 ℃: (a) NM-15Z; (b) NM-30Z; (c)NM-45Z;
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(d)NM-60Z.
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Fig. 7. Fracture surface of NM-45Z at three different temperature: (a) 800℃; (b) 1100℃; (c) 1300℃.
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In attempt to investigate the failure mechanisms, the microscopic and macroscopic
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morphology of fractures are observed by SEM photographs and photos, respectively. Fig. 6 exhibits the fracture surfaces of the fo ur ZrB2 reinforced composites in 800℃ condition. Only transgranular fracture is found in NM-15Z and NM-30Z, nevertheless,
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both of the intergranular fracture and transgranular fracture are occurred in NM-45Z and NM-60Z, which is in accord with the results of Fig. 1 and Fig. 2 and our previous study. Fig. 7 shows the fracture surfaces of the NM-45Z with same magnification at three different temperatures: 800℃, 1100℃ and 1300℃, and it is evidently that the failure patterns of composites change apparently with the increasing temperature. Under the temperature of 800℃, the brittle deformation is predominant morphology with cracks. While at 1100℃, cleavages and dimples exist, representing for brittle and
ACCEPTED MANUSCRIPT ductile fracture respectively [20], so the deformation is consist of brittle and ductile with tiny cracks. At 1300℃, dimples are pronounced on the whole surface, which indicates the failure pattern is plastic fracture. The fracture surface results are in
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concordance with the flow stress-strain curve in Fig. 1 and Fig. 2.
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Fig. 8. The microstructure after high temperature compression of samples in different experimental environments under high magnification: (a) A1-1100℃ (10000 X); (b) A2-1100℃ (10000 X); (c)
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A3-600℃ (10000 X); (d) A3-1100℃ (10000 X); (e) A4-800℃ (15000 X); (f) A4-1100℃ (10000 X).
The microstructure after high temperature deformation is shown in Fig.8 and Fig.9. The compression direction is along the vertical direction (the white line in Fig.9). During the high-temperature compression process, the general trend of deformation of the zirconium compound is that the large particles decompose into small particles, forming a small particle zirconium compound band. This phenomenon shows that the zirconium compound refines the microstructure and increases the degree of dispersion by decomposing large particles into small particles, thereby strengthening and
ACCEPTED MANUSCRIPT becoming the main strengthening phase. This phenomenon is consistent with the stress-strain curve in Fig.1 and Fig.2. It can be seen from the contrast electron microscope photographs of the A4 sample at two temperatures that the higher the experimental temperature, the finer the grain of the zirconium compound, and the
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stronger the strengthening effect. Fig. 9 shows clearly the small particles spread out at
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certain angle to the axis of the compression directio n (the red line in Fig.9), and the
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angle is approximately 45°.
Fig. 9. The microstructure after high temperature compression of samples in different experimental environments under low magnification: (a) A3-600℃ (250 X); (b) A4-800℃ (250 X).
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Fig. 10 exhibits the failure patterns of composites. For NM, NM-15Z and NM-30Z, the samples not failure or failure in two main blocks, along with crack or failure plane at angle of 45° to the axis of the compression direction. The NM-15Z and NM-30Z samples at temperature above 1100℃ are enfolded by excessive oxide skin. The crumbles of NM-30Z in 1300℃ condition are all oxide skin shed. For NM-45Z and NM-60Z, the failure pattern changes from splitting along the compression direction to cracking at certain angle to the axis of the compression direction with temperature
ACCEPTED MANUSCRIPT rising. These phenomena about failure imply that the ductility increases with temperature rising for each composite, and the ductility decreases with ZrB2 content
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in the same temperature condition.
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Fig. 10. Comparison of failure patterns of five composites in five temperature conditions.
ACCEPTED MANUSCRIPT 4. Conclusions In order to explore a new ingredient of ultra-high temperature materials and improve high temperature properties of ceramic particles reinforced refractory metal matrix composites, the compression properties of the hot-pressed NbMo-matrix composites
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reinforced with ZrB2 particles are studied at the temperature from 800℃ to 1300℃.
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The effect of different ZrB2 additions on the stress-strain curves, compressive strength
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and failure patterns were discussed. The main conclusions can be drawn as follows: (1) ZrB2 particles additions to NbMo metal matrix can significantly improve the high
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temperature compressive strength of the composites. At 1300℃, the strength of the
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composite containing 60 vol.% ZrB2 is over 700 MPa. (2) At the same temperature, the high temperature co mpressive strength is directly
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proportional to ZrB2 content, and the compressive strength decreases with
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temperature rising for each composite. Especially, the composite containing 30 vol.% ZrB2 has the best comprehensive properties including strength and ductility at the temperature below 1100℃ for its refined grains. At high temperatures,
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zirconium compounds are the main strengthening phase. (3) The ductility increases with temperature rising for each composite. The ductility decreases with ZrB2 content in the same temperature condition, which is related to the proportion of ductile metal phase, the number of holes and the Cottrell atmospheres. (4) The failure planes of composite containing 15 vol. % and 30 vol. % ZrB2 were always at angle of 45° to the axis of the compression direction, whereas the failure
ACCEPTED MANUSCRIPT pattern of composite containing 45 vol. % and 60 vol. % ZrB2 changed from splitting along the compression direction to cracking at certain angle to the axis of the compression direction with temperature rising. Although the maximum experimental temperature is 1300 ℃ for the limiting facility
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condition, it is expected that these conclusions could be applied for the further
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reinforced with ZrB2 particles at high temperature.
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structural design and compressive strength prediction of the NbMo-matrix composites
Acknowledgements
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The authors would like to thank the financial support from the National Natural
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Science Foundation of China (No. 11372110) and the Fundamental Research Funds
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for the Central Universities (No. 2017XS053).
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ACCEPTED MANUSCRIPT Highlight At 1300 ℃, the strength of the composite containing 60 vol.% ZrB2 is over 700 MPa. At 1700 ℃, the strength of the composite containing 60 vol.% ZrB2 is over 200 MPa.
30 vol.% ZrB2 -NbMo has the best comprehensive properties at the temperature below 1100 ℃.
At high temperatures, zirconium compounds are the main strengthening phase.
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Qi Wang, Zongde Liu, Yongtian Wang, Youmei Sun, Yan Gong PII: DOI: Reference:
S0263-4368(18)30382-2 doi:10.1016/j.ijrmhm.2018.08.007 RMHM 4773
To appear in:
International Journal of Refractory Metals and Hard Materials
Received date: Revised date: Accepted date:
4 June 2018 6 August 2018 13 August 2018
Please cite this article as: Qi Wang, Zongde Liu, Yongtian Wang, Youmei Sun, Yan Gong , High temperature compression properties of hot-pressed NbMo-matrix composites reinforced with ZrB2 particles. Rmhm (2018), doi:10.1016/j.ijrmhm.2018.08.007
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ACCEPTED MANUSCRIPT High temperature
compression properties of
hot-pressed
NbMo-matrix composites reinforced with ZrB2 particles Qi Wang 1, a, Zongde Liu
* , Yongtian Wang 1,c, Youmei Sun 1,d , Yan Gong 1,e
Key Laboratory of Condit ion Monitoring and Control for Power Plant Equ ip ment of M inistry of
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1
1,b,
Education, North China Electric Power University, Beijing 102206, China b
email : [email protected]
[email protected] eemail: [email protected]
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*Corresponding author: Zongde Liu
c
email : [email protected]
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email: [email protected]
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a
d
email:
ACCEPTED MANUSCRIPT Abstract The effects of 15, 30, 45, 60 vol.% ZrB2 additions on high temperature compression properties of hot-pressed NbMo-matrix were investigated by the compressio n experiments in the temperature range of 800℃-1300℃. ZrB2 particle additions to
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NbMo matrix could significantly improve the high temperature compressive strength
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of the composites. The best comprehensive property including strength and ductility
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belongs to the composites containing 30 vol.% ZrB2 with refined grains at the temperature below 1100℃. The resistant ability to high temperature was directly
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proportional to ZrB2 content. The compressive strength of composite containing 60
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vol. % ZrB2 (from 1236.56 MPa to 700.46 MPa) was inversely proportional to temperature. The failure planes of composite containing 15 vol. % and 30 vol. % ZrB2
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were always at angle of 45° to the axis of the compression direction, whereas the
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failure pattern of composite containing 45 vol. % and 60 vol. % ZrB2 changed from splitting along the compression direction to cracking at certain angle to the axis of the
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compression direction with temperature rising.
Keywords: Hot-pressing; NbMo; ZrB2 ; high temperature; compression properties
ACCEPTED MANUSCRIPT 1. Introduction The excellent properties of high resistance to corrosion, high melting point, high strength and unique combination of low density and relative high ductility at high temperature make niobium-based alloys a good candidate for high temperature
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structural materials and attractive for the application in the fields of space, nuclear and
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aircraft [1-2]. However, the rapid strength decrease of niobium (Nb) at temperature
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above 1200 K is still a major issue for the use of Nb-based alloys [3]. Adding refractory elements such as molybdenum (Mo) to Nb could greatly improve the high
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temperature strength of Nb by means of solid solution strengthening. Moreover, when
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the Nb/Mo ratio is around 1, the maximum solid solution effects can be achieved. To improve high temperature strength of NbMo-alloys, titanium carbide, zirconium
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carbide et al. particles were added as a reinforcing ceramic phase to Nb and
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NbMo-alloys. For instance, the flow stress of the Nb-ZrC alloys increases with increasing ZrC content at elevated temperature. Nevertheless, with increasing ZrC content, the fracture toughness decrease [4-5]. As one of the ultra-high temperature
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ceramics (UHTCs), zirconium diboride (ZrB2 ) materials have high strength, high melting temperature (3313 K), high hardness, excellent corrosion resistance and high thermal conductivity at high temperature [6-10]. There have been studies on the ZrB2 particles reinforced refractory alloys. Xin Sun et al. investigated the composite of ZrB2 - 25 vol.% Nb sintered by hot-pressing at 1800℃ for 60 min in Argon atmosphere under 30 MPa [11]. Hailong Wang et al. studied the composite of ZrB2 - (0-10) vol.% Mo prepared by hot-pressing at 1950℃ for 60 min
ACCEPTED MANUSCRIPT [12]. In our previous work, (0-60) vol.% ZrB2 -NbMo composites were hot-pressed at 2400℃ for 10 min in Argon atmosphere under 50MPa, and the ratio of Nb/Mo is set at 1 [4-5]. Most of the above mentioned works are about the mechanical properties at room temperature but do not cover the high temperature.
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The main goal of this investigation is to study the high temperature compression
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properties of the (0-60) vol.% ZrB2 -NbMo composites prepared with the process
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mentioned in our published literature [13] at temperature ranged from 800℃-1300℃. 2. Experimental
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Five powder mixtures were prepared (amounts in vol.%):
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Material NM-0: 50% Nb+50% Mo
Material NM-15Z: 85% (50% Nb+50% Mo) + 15% ZrB2
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Material NM-30Z: 70% (50% Nb+50% Mo) + 30% ZrB2
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Material NM-45Z: 55% (50% Nb+50% Mo) + 45% ZrB2 Material NM-60Z: 40% (50% Nb+50% Mo) + 60% ZrB2 The letter N is used as the abbreviation for Nb, M for Mo and Z for ZrB2 in this study.
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The experimental materials were Nb and Mo powders (500-800 mesh, purity > 99.95%) and ZrB2 powders (600 mesh, purity > 99.95%) 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. The powders were mixed and dried at 110℃ for 3 hours, then mixed and pulverized in the planetary milling with a speed of 200 rpm for 10 hours, and finally hot pressed at 2400℃ for 10 min in Argon under the pressure of 50 MPa. The diameter of the final cylindrical sample was 14 mm and the height was
ACCEPTED MANUSCRIPT approximately 70 mm. For high temperature compression examination, samples were cut into specimens that had diameter of 3 mm and height of 5 mm using the method of wire cut electrical discharge machining (WEDM). Then, the specimens were cleaned in ultrasonic cleaner. The surfaces of the specimens for microstructure observation
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and XRD texting were grinded with a series of sandpaper (from #100 to #1500) and
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polished with 4000 mesh diamond paste. The phase composition was detected by
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X-ray diffraction device which was operating at 40 kV and tube current of 100 mA over the 2θ ranging from 10° to 90°, and a scanning speed of 8°/min. FESEM (ZEISS
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SUPRA-55)， FEI Quanta 200F and SU8010 scanning electron microscope were
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operating at an accelerating voltage of 20 kV to observe the morphology. The element composition and distribution were analyzed by Energy Dispersive X-ray spectroscopy
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(EDS). The compression properties were assessed in air using the coupled mechanical
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load-temperature tester (CMT-5204) with a fixture made of SiC at the heating rate of 10℃/min at 800℃, 1000℃, 1100℃, 1200℃, 1300℃, respectively. The samples were hold for 10 min at target temperature to ensure homogeneous heating, and the
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compression speed was 2.5*10-4 s-1 . The direction of compression is parallel to the radial direction of the final cylindrical sample. The stress-strain curves were obtained from the computer recorder connected with tester. After compression text, the samples cooled to room temperature. Results and Discussion 3.1. Stress-strain curve The variations in mechanical properties of the composites were examined by
ACCEPTED MANUSCRIPT compressive tests at different elevated temperatures and the true stress-strain curves are demonstrated in Fig. 1 and Fig. 2. The experimental compressive strength results are summarized in Table 1. Based on our pervious study, the phase composition of these composites are NbMoss, niobium borides and zirconium compounds. The main
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reinforcing phase are niobium borides and zirconium compounds [13]. The melting
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points of Nb (2469℃) and Mo (2620℃) are lower than that of niobium borides (for
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example, NbB: 2903℃) [14-16], and the hardness of niobium borides and zirconium compound is higher than NbMoss, so the NbMoss is the ductile phase. Our previous
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research reveals that samples have different oxidation products in various conditions,
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nevertheless, the oxidation products make little effect o n this compression experiments for the reason that the outermost oxide skin prevents further oxidation of
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is the intimal part.
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the intimal substances. Therefore, the deformation substance discussed in this article
Table 1 Compressive Strength (MPa) of the five composites at five different temperatures. Temperature
800℃
1000℃
1100℃
1200℃
1300℃
264.23
91.11
NM-15Z
1119.51
864.76
511.56
336.35
4.3
NM-30Z
1532.87
1293.18
754.08
523.09
0.5379
NM-45Z
1381.5
795.72
609.77
530.21
202.52
NM-60Z
1236.56
1183.7
982.03
840.71
700.46
Samples
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NM-0
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Fig. 1. Stress-strain curves of five composites at high temperature: (a) NM-0; (b) NM-15Z; (c) NM-30Z; (d) NM-45Z; (e) NM-60Z.
For NM-15Z and NM-30Z, with the temperature rising, the slope decreases, the yielding stage enlarges, the strain increases, and the curve moves towards right. The ductility increases, and these phenomena could be intuitively seen from Fig. 1. For NM-45Z and NM-60Z, though the strain increase cannot be intuitively seen from curves, the residual plastic strain in Table 2 and Fig. 4 calculated by the method
ACCEPTED MANUSCRIPT shown in Fig. 3 increases with the temperature rising, and the slope decreases. These phenomena are attributed to the difference of the thermal expansion coefficient between components. The strain energy stored at the grain boundary leads to void, and the B2 O3 produced in the heating process before compression evaporates. With
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temperature rising, the voids and cracks caused by component expansion and strain
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between grain boundary and evaporation of B2 O3 increase. Additionally, the higher
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temperature aggravates the matrix metal softening, and enhances ductility and increases strain. The first step of the method of computing plastic strain in Fig. 3 is to
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make a straight fitting line (Line 1) of the elastic stage, then Line 2 is made parallel to
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this fitting line through the last point of yield stage to get the abscissa of the intersection of this parallel line and horizontal axis, and the difference value between
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strain finally needed.
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the abscissas of two intersections of line and horizontal axis is the residual plastic
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Fig. 2. Stress-strain curves of composites at the temperature from 800 to 1300℃: (a) 800℃; (b) 1000℃; (c) 1100℃; (d) 1200℃; (e) 1300℃.
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Fig. 3. Example of computing method of residual plastic strain.
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Table 2 Plastic strain (%) of the five composites. Temperature
1000℃
1100℃
1200℃
NM-0
0.32±0.010
1.02±0.009
NM-15Z
0.35±0.011
0.72±0.017
1.83±0.010
3.57±0.060
NM-30Z
0.082±0.002
0.47±0.012
0.56±0.012
2.91±0.066
NM-45Z
0.057±0.001
0.19±0.008
0.49±0.009
0.71±0.010
1.31±0.040
NM-60Z
0.055±0.001
0.14±0.010
0.39±0.004
0.40±0.003
0.61±0.059
1300℃
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800℃
Samples
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As can be seen in Table 2，ductility has a trend of decrease with the increasing ZrB2 content at the same temperature. There are three main reasons for this phenomenon, and the first reason is content. Under the same temperature condition, the composites containing low ZrB2 content have more metal to soften composites. The second reason is the few holes in the composites containing low ZrB2 content caused by evaporation of B2 O3 and MoO 3 , which is supported by the fact that B2 O3 begins to volatilize when temperature exceeds 930℃ and MoO 3 evaporated when temperature exceeds 1155℃ reported in previous researches [17-18]. The third reason is related to
ACCEPTED MANUSCRIPT the Cottrell atmospheres [19]. In the high ZrB2 content composite, the interstitial solute atoms dissolved into NbMoss are more than in low ZrB2 content composite, and high order energy reduces. The more interstitial solute atoms in high ZrB2 content composite could form Cottrell atmospheres for the segregation around dislocation line
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caused by the interaction between atoms and dislocations, and these could increase
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the σ 0 and K in Hall-Petch relationship, hence, the σ s increases, nevertheless the
ductility with rising ZrB2 content.
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3.2 Compressive strength and plastic strain
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ductility of NbMoss decreases. All these reasons are responsible for the decrease of
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Fig. 4. (a) the fitted compressive strength-ZrB2 content curve; (b) the fitted compressive strength-temperature curve.
Fig. 5. (a) the fitted plastic strain- ZrB2 content curve; (b) the fitted plastic stain-temperature curve.
ACCEPTED MANUSCRIPT For all the composites, the compressive strength decreases as temperature rises, implying a sharp sensitivity to the temperature in each composite. At 800℃ and 1000℃, compared to the composite NM-0 (50 vol.% Nb+50 vol.% Mo), the compressive strengths of four ceramic-reinforced composites are much higher for the
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reason that the ceramic particles increase dislocation density, which leads to
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significant strengthening, furthermore, the load can be transferred from the ductile
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NbMoss matrix to the stiffening ceramic particles during the high-temperature deformation. This phenomenon reveals that ZrB2 particles additions to NbMo metal
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matrix can remarkably improve the high temperature compressive strength of the
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composites. It is importantly impressing that the NM-60Z exhibits the extraordinary compressive strength, and its compressive strength is over 700 MPa at 1300℃.
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The high temperature compressive strength in Fig. 4 (a) is directly proportional to
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ZrB2 content at the same temperature, and the compressive strength in Fig. 4 (b) decreases with the increasing temperature for each composite. High ZrB2 content sample has more reinforcing phase to strengthen the composites under the same
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temperature conditions. The compression process is accompanied by three main mechanisms: the softening of NbMoss matrix, hardening of NbMoss matrix and hardening of reinforcing phase. The hardening caused by movement and multiplication of dislocation is beneficial to the strengthening of composites as well as the increase of curve slope, nevertheless, the softening caused by recovery and recrystallization is obviously to the disadvantage of the strengthening of composites. With temperature rising, the negative softening effect become more severe, and the
ACCEPTED MANUSCRIPT effect of metal hardening is too little to be considered, so the metal hardening makes limited functions on deformation and recovery and recrystallization dominate the process. The grain boundary and phase boundary slide because of the softening of the NbMoss matrix with low melting point and the boundary strength greatly decline, so
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the macro bearing capacity and compressive strength decrease. The slopes of fitted
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line in Fig. 4 (a) increase with temperature rising from 800 ℃ to 1300℃, while the
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decline rate of compressive strength in Fig. 4 (b) is slower with the ZrB2 content increasing, and these two phenomena about slope reveal that the resistant ability to
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high temperature is directly proportional to ZrB2 content. The main hardening
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mechanism is reinforcing phase hardening at higher temperature. However, there is an exception that the absolute value of NM-30Z’s slope is too large to be in accordance
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with the slope regulation in Fig. 4 (b). The reason for this exception is the excellent
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compressive strength of NM-30Z below 1200℃ caused by refined grain. Except for the case of excessive oxidation of NM-15Z and NM-30Z at 1300℃, the compressive strength of NM-30Z is always higher than NM-15Z and higher than
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NM-45Z below 1200℃. The excellent property of NM-30Z at temperature below 1200℃ is due to the homogeneous microstructure and small grain size, and the test performance of NM-45Z is not so stable for its holes and big grain size according to our previous study [13]. The compressive strength of NM-60Z begins to exceed that of NM-30Z when temperature exceeds 1100℃. Moreover, the relationship between strength and temperature in Fig. 4 could be applied to predict the compressive strength of the higher temperature which is not covered in this study.
ACCEPTED MANUSCRIPT Table 2 and Fig. 5 shows the plastic strains, which could represent the plastic deformation of composites ruling out recoverable elastic deformation. As shown in Fig. 6, for the four ceramic-reinforced samples, with the increase of temperature, the plastic strain of each composite shows a trend of enhancement, and at the same
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temperature, the composite containing lower ZrB2 content has larger plastic strain in
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general. The explanations for these phenomena could be the more void and cracks in
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higher ZrB2
content composite and the severe softening in higher temperature condition.
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It is need to notice that the NM-30Z has relatively high plastic strain at any
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temperature, which is related to its homogeneous microstructure and small grain size. Therefore, one can conclude that NM-30Z has the best comprehensive property
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3.3 Failure patterns
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including strength and ductility at the temperature below 1100℃.
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Fig .6. Fracture surface of four composites at 800 ℃: (a) NM-15Z; (b) NM-30Z; (c)NM-45Z;
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(d)NM-60Z.
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Fig. 7. Fracture surface of NM-45Z at three different temperature: (a) 800℃; (b) 1100℃; (c) 1300℃.
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In attempt to investigate the failure mechanisms, the microscopic and macroscopic
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morphology of fractures are observed by SEM photographs and photos, respectively. Fig. 6 exhibits the fracture surfaces of the fo ur ZrB2 reinforced composites in 800℃ condition. Only transgranular fracture is found in NM-15Z and NM-30Z, nevertheless,
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both of the intergranular fracture and transgranular fracture are occurred in NM-45Z and NM-60Z, which is in accord with the results of Fig. 1 and Fig. 2 and our previous study. Fig. 7 shows the fracture surfaces of the NM-45Z with same magnification at three different temperatures: 800℃, 1100℃ and 1300℃, and it is evidently that the failure patterns of composites change apparently with the increasing temperature. Under the temperature of 800℃, the brittle deformation is predominant morphology with cracks. While at 1100℃, cleavages and dimples exist, representing for brittle and
ACCEPTED MANUSCRIPT ductile fracture respectively [20], so the deformation is consist of brittle and ductile with tiny cracks. At 1300℃, dimples are pronounced on the whole surface, which indicates the failure pattern is plastic fracture. The fracture surface results are in
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concordance with the flow stress-strain curve in Fig. 1 and Fig. 2.
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Fig. 8. The microstructure after high temperature compression of samples in different experimental environments under high magnification: (a) A1-1100℃ (10000 X); (b) A2-1100℃ (10000 X); (c)
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A3-600℃ (10000 X); (d) A3-1100℃ (10000 X); (e) A4-800℃ (15000 X); (f) A4-1100℃ (10000 X).
The microstructure after high temperature deformation is shown in Fig.8 and Fig.9. The compression direction is along the vertical direction (the white line in Fig.9). During the high-temperature compression process, the general trend of deformation of the zirconium compound is that the large particles decompose into small particles, forming a small particle zirconium compound band. This phenomenon shows that the zirconium compound refines the microstructure and increases the degree of dispersion by decomposing large particles into small particles, thereby strengthening and
ACCEPTED MANUSCRIPT becoming the main strengthening phase. This phenomenon is consistent with the stress-strain curve in Fig.1 and Fig.2. It can be seen from the contrast electron microscope photographs of the A4 sample at two temperatures that the higher the experimental temperature, the finer the grain of the zirconium compound, and the
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stronger the strengthening effect. Fig. 9 shows clearly the small particles spread out at
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certain angle to the axis of the compression directio n (the red line in Fig.9), and the
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angle is approximately 45°.
Fig. 9. The microstructure after high temperature compression of samples in different experimental environments under low magnification: (a) A3-600℃ (250 X); (b) A4-800℃ (250 X).
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Fig. 10 exhibits the failure patterns of composites. For NM, NM-15Z and NM-30Z, the samples not failure or failure in two main blocks, along with crack or failure plane at angle of 45° to the axis of the compression direction. The NM-15Z and NM-30Z samples at temperature above 1100℃ are enfolded by excessive oxide skin. The crumbles of NM-30Z in 1300℃ condition are all oxide skin shed. For NM-45Z and NM-60Z, the failure pattern changes from splitting along the compression direction to cracking at certain angle to the axis of the compression direction with temperature
ACCEPTED MANUSCRIPT rising. These phenomena about failure imply that the ductility increases with temperature rising for each composite, and the ductility decreases with ZrB2 content
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in the same temperature condition.
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Fig. 10. Comparison of failure patterns of five composites in five temperature conditions.
ACCEPTED MANUSCRIPT 4. Conclusions In order to explore a new ingredient of ultra-high temperature materials and improve high temperature properties of ceramic particles reinforced refractory metal matrix composites, the compression properties of the hot-pressed NbMo-matrix composites
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reinforced with ZrB2 particles are studied at the temperature from 800℃ to 1300℃.
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The effect of different ZrB2 additions on the stress-strain curves, compressive strength
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and failure patterns were discussed. The main conclusions can be drawn as follows: (1) ZrB2 particles additions to NbMo metal matrix can significantly improve the high
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temperature compressive strength of the composites. At 1300℃, the strength of the
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composite containing 60 vol.% ZrB2 is over 700 MPa. (2) At the same temperature, the high temperature co mpressive strength is directly
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proportional to ZrB2 content, and the compressive strength decreases with
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temperature rising for each composite. Especially, the composite containing 30 vol.% ZrB2 has the best comprehensive properties including strength and ductility at the temperature below 1100℃ for its refined grains. At high temperatures,
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zirconium compounds are the main strengthening phase. (3) The ductility increases with temperature rising for each composite. The ductility decreases with ZrB2 content in the same temperature condition, which is related to the proportion of ductile metal phase, the number of holes and the Cottrell atmospheres. (4) The failure planes of composite containing 15 vol. % and 30 vol. % ZrB2 were always at angle of 45° to the axis of the compression direction, whereas the failure
ACCEPTED MANUSCRIPT pattern of composite containing 45 vol. % and 60 vol. % ZrB2 changed from splitting along the compression direction to cracking at certain angle to the axis of the compression direction with temperature rising. Although the maximum experimental temperature is 1300 ℃ for the limiting facility
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condition, it is expected that these conclusions could be applied for the further
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reinforced with ZrB2 particles at high temperature.
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structural design and compressive strength prediction of the NbMo-matrix composites
Acknowledgements
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The authors would like to thank the financial support from the National Natural
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Science Foundation of China (No. 11372110) and the Fundamental Research Funds
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for the Central Universities (No. 2017XS053).
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ACCEPTED MANUSCRIPT Highlight At 1300 ℃, the strength of the composite containing 60 vol.% ZrB2 is over 700 MPa. At 1700 ℃, the strength of the composite containing 60 vol.% ZrB2 is over 200 MPa.
30 vol.% ZrB2 -NbMo has the best comprehensive properties at the temperature below 1100 ℃.
At high temperatures, zirconium compounds are the main strengthening phase.
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