Effect of temperature on mechanical properties of Ti-based metallic glass matrix composite

Intermetallics 67 (2015) 121e126

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Effect of temperature on mechanical properties of Ti-based metallic glass matrix composite Y.S. Wang a, *, G.J. Hao b, Y. Zhang b, J.W. Qiao a, J.P. Lin b, ** a

Laboratory of Applied Physics and Mechanics of Advanced Materials, College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China b State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 April 2015 Received in revised form 22 June 2015 Accepted 13 August 2015 Available online xxx

Systematic mechanical behaviors were investigated in a Ti-based metallic glass matrix (MGM) composite containing the in-situ b-dendrite phase at 100 Ke298 K. We found that the yielding strength increased but the plastic strain decreased with a decrease temperature. The sharp ductile to brittle transition occurred at 100 K. The MGM composite exhibits the large work-hardening behavior at 298 K, but all sample display the work-softening behavior below 298 K. The nominal work-hardening parameter was employed to express the dependence of mechanical properties on temperatures including the brittle failure, the work-hardening and work softening behaviors. It may provide a useful way to evaluate the dependence of mechanical properties on temperatures of MGM composite. © 2015 Published by Elsevier Ltd.

Keywords: Metallic glasses Brittleness and ductility Fracture Mechanical properties Yield stress Mechanical testing

1. Introduction Ductile-to-brittle transition (DBT), as one of universal issues of the effect of mechanical properties on the loading temperature, has attracted extensive scientific and technological interest [1,2]. Generally, for ductile alloys with body-centered cubic structure (B.C.C.), the strength of crystalline alloys increases, but plasticity declines with the decrease of the deformed temperature. During the loading temperature below a critical value, the materials exhibit the brittle failure (named as the DBT temperature), caused by the absence of dislocation activity [1,2]. Due to lack of the longrange structural periodicity, metallic glasses (MGs) possess many excellent mechanical properties, such as large elastic strain, high fracture strength and fracture toughness, etc. [3e5]. Basically, although MGs always take place brittle failure just after linearlyelastic deformation by shear bands at room temperature, they exhibit an enhanced compressive strength with the dropped temperature at cryogenic ranges [3e10]. Moreover, many results

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y.S. Wang), [email protected] (J.P. Lin). http://dx.doi.org/10.1016/j.intermet.2015.08.005 0966-9795/© 2015 Published by Elsevier Ltd.

show that the plasticity can be improved at cryogenic temperature [7e11]. In-situ metallic glasses matrix (MGM) composites, as an efficient way to enhance plasticity, have been fabricated by introducing ductile dendrites into MGs [12,13]. Large room temperature plasticity and high strength can be achieved simultaneously because of the motion of dislocations within dendrites and the multiplication of shear bands in the MG phase [14]. However, no agreement has been accepted for the dependence of mechanical properties on temperatures in MGM composites, though several investigations have been conducted about this topic [15e18]. Recently, Li et al. [16] found that a Ti-based MGM composite with a nominal composition of Ti48Zr20Nb12Cu5Be15 exhibits the excellent cryogenic plasticity of 18.4% and final fracture of 2760 MPa at 77 K. However, Qiao et al. [17] have reported that MGM composites with composition of Ti44Zr20V12Cu5Be19 and Zr38.5Ti32.5Nb7.3Cu6.2Be15.3 failed catastrophically at 77 K, stemming from DTB in dendrites. Charpy impact testing at 100 Ke300 K [18] indicated that the toughness of these composites decreased linearly with temperature without sharp transition. On the other hand, although an empirical equation in MGs and several MGM composites has been proposed to calculate the dependence of the maximum strength on the cryogenic temperatures [17,19,20], it is not enough to express the details of

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temperature-dependent mechanical properties for only at 77 K and 300 K data points, such as the work-hardening behavior or the plastic strain. Therefore, the cryogenic mechanical properties and deformation behaviors of in-situ MGM composites are still far from be clarified, which motivates further investigations. In this paper, we conduct compressive experiments in Ti-based MGM composites over a wide range of the cryogenic temperature to investigate the basic dependence of mechanical properties on the temperatures. A relationship of the normalized parameter (Hc) versus temperatures is obtained, which may provide a way to understand the dependence of mechanical properties on the loading temperature in MGs and MGM composites. 2. Experimental procedures Ingots with nominal composition of Ti47Zr19Be15V12Cu7 (atomic percentage) were prepared from a mixture high purity elements (purity > 99. 9 wt %) by arc-melting under a Ti-gettered argon atmosphere. Bridgman solidification was employed to fabricate the MGM composites [21]. The structure of the composite sample was characterized by X-ray diffraction (XRD) in a PHILIPS APD-10 diffractoneter with Cu Ka radiation. The cylindrical specimens with a diameter/length ratio of 1:2 were used for compressive tests. The typical fracture characteristics were examined by scanning electron microscopy (SEM, SUPRA-55). The compression tests at a strain rate of 2
104 s1 were conducted by using an MTS testing machine. The different temperatures can be obtained by controlling the quantity ratio of the liquid nitrogen and alcohol. A mercury thermometer clung on the structural support of the specimen to test the temperature. The detailed can be found elsewhere [20,22]. Fig. 1. Representative microstructure a Ti-based MGM composite (a) and XRD pattern (b).

3. Results and discussion

3000

Fig. 1(a) shows the representative SEM image of the Ti-based MGM composite. The b-Ti crystal with the B.C.C. structure is homogeneously distributed in the amorphous matrix, which is identified by XRD pattern in Fig. 1(b). The typical characteristic of the MGM composite is the sharp diffraction peaks of dendrite phase adding to the broad diffuse scattering maxima of glass phase. The bTi crystal volume fraction is approximately ~45% by analyzing the contrast of the dendrite phase and glass matrix from SEM image using Photoshop software.

2500

Stress (MPa)

3.1. Microstructure

Engineering stress-strain

2000 1500

True stress-strain

1000 500

3.2. Mechanical properties

0

Test temperature: 298 K 0

10

20

30

40

Strain (%)

True Stress (MPa)

3.2.1. Stressestrain curves Fig. 2 shows the stressestrain curves of the composite at different temperatures. Fig. 2(a) displays the stressestrain curve at 298 K. It yielded at 1490 MPa, followed by a large work-hardening stage to 1730 MPa at the total fracture strain of 37% of the true stressestrain curve, which is consistent with the previous results [21]. Fig. 2(b) shows the true stressestrain curves of the composite at 100 Ke236 K. Before yielding, all samples display the lineallyelastic deformation. After that, the curves has deviated line y caused by the dendrite yielded. The yield strength sT , increases with decreasing of the loading temperature. However, with dropping of temperatures, the fracture strain decreases but without a sharp transition before 104 K. The composite samples exhibit the work-softening behavior above the 100 K. However, the sample shows brittle fracture with the fracture strength of 2485 MPa and total strain of 2.51% at 100 K, named as the DBT transition point of the current Ti-based MGM composites. It is consistent with previous results that the Ti44Zr20V12Cu5Be19 MGM composite exhibits

(a)

(b)

2000 1500 1000 500

2%

100 K 104 K 112 K 143 K 213 K 236 K

0

True Strain Fig. 2. The stressestrain curves of the composite at different temperatures: (a) 298 K; (b) 100 Ke236 K.

Y.S. Wang et al. / Intermetallics 67 (2015) 121e126

brittle failure at 77 K caused by the DTB in dendrites [17].

3.2.2. Temperature effects on the strength and plasticity Fig. 3 shows the dependence of mechanical properties on temperatures. For the work-softening behavior, the yielding strength is f y f higher than that of the fracture strength (sT ), sT > sT ; however, for the brittle failure of the composite deformed at 100 K, syT zsfT , as shown in Fig. 3(a). At 298 K, the large work-hardening behaviors exists for the plastic deformation of the dendrite phase, resulting in y f the sT < sT . Fig. 3(b) shows the temperature effect on the macroplasticity of the composite. Clearly, the fracture strain decreases linearly with the decreasing of loading temperatures. It is consistent with the results of the Charpy impact testing at 100 Ke300 K [18]. When the loading temperature dropped, the toughness of the composites containing the in-situ b-crystal decreased linearly without sharp transition. Considering the composites including the dendrites and the glass phase, the temperature effects on mechanical properties of in-situ MGM composites related with the deformation of the dendrite phase as well as the glass phase. For the current in-situ Ti-based MGM composites, the DBT behavior occurred at 100 K, as shown in Fig. 2. At low temperatures (e.g. 77 K), the atoms bonds become stiffer; the thermodynamic mobility of atoms decrease; and the local heating can be delivered and relieved more easily at the cryogenic surroundings [19]. For the glass matrix (like monolithic MGs), the harder creation and accumulation of free volume at cryogenic temperatures leads to an enhanced compressive strength. Typically, the relationship between the strength and the temperature has been established as y [19]:sG ¼ aEðb  TÞ=Tg . Here, T, Tg and E represent the testing

Strength (MPa)

(a)

Fracture strength Yielding strength

2400 2200 2000 1800 1600 1400 50

100

150

200

250

300

Total fracture strain (%)

Temperature (K) 35

temperature, the glass-transition temperature and Young's modulus, respectively; a and b are constants. As previous reported that the yielding strength of the Ti40Zr25Ni3Cu12Be20 MG increased 8% from 300 K to 123 K [20]. However, for the b-Ti dendrite phase (like crystalline alloys with B.C.C. microstructure), the higher frictional-resistance stress of dislocations motion raises an increased strength of the dendrite phase during the temperature decreased, leading to the increased yield strength but the decreased macro-plasticity [1,2]. Generally, for crystalline alloys with B.C.C. microstructure, the compressive strength can be y expressed as [23]: DsD ¼ B expðCTÞ. Here T is the test temperature; B and C are positive constants. Ti-based MGM composite failed with the fracture strength of 2485 MPa at 77 K [24], which increased ~ 46%. However, for b titanium alloy, the yielding strength is of 900 MPa at 300 K but 1760 MPa at 77 K [24]. On the other hand, monolithic MGs always exhibit an enhanced compressive plasticity with the temperature decreased, e.g. Ni-[9], Ti-[20], Au-[25], and Zr-based [4,8,26,27]. The propagation of shear bands became more difficult than that of branched or multiplication at the cryogenic temperature, resulting in the improvement plasticity. However, it is different with the cryogenic fracture strain decreases of current MGM composites, as shown in Fig. 3(b). At cryogenic temperature, the activity of the dislocation emission and motion decreased [1,2,23,24]. Therefore, limited slip bands formed within the dendrite nearby the interface when the applied stress reached at its yielding strength, resulting in the micro-instabilities. Moreover, effects of the dendrite to prohibit the single shear band and to promote the formation of multiplication shear bands are also weakening during loading at cryogenic environment. According to Dundur's parameter [28], a:

a ¼ ðGdendrite  Gmatrix Þ=ðGdendrite þ Gmatrix Þ

2600

40

123

(b)

30 25 20 15 10 0 50

where G is the shear modulus. When a test temperature satisfies that: 298 K  T ＞ 100 K, a penetrating shear band or micro-cracks is vulnerable to be arrested by the dendrites for the negative a which caused by a lower modulus of the dendrite phase than that of glass matrix; however, when T  100 K, dendrites are inefficient prohibit a propagating shear band, which stemmed from the increased a.

3.2.3. Work-hardening to softening transition So far, although several studies have investigated the effect of the cryogenic temperature on mechanical properties in MGs and MGM composites [4,7e9,25e27,30], the results are inconsistent. Process variation and slight compositional variation will lead to the different cryogenic mechanical properties for these materials [4,26,27,30]. Here, the strength and strain have the strong dependence on the applied temperatures MGM composites, as shown in Figs. 2 and 3. Interestingly, true stressestrain curves exhibit the transition from the work-hardening to work-softening with decreasing of the loading temperature. Therefore, for BMGs and MGM composites, the normalized parameter, Hc, was employed to evaluate the dependence of the work-hardening or work-softening ability on the temperature [29].

.

.  syT ¼ sfT syT  1 Hc ¼ sfT  syT

5 100

150

200

250

300

Temperature (K) Fig. 3. The dependence of mechanical properties on the applied temperatures: (a) the strength versus temperatures; (b) the plasticity versus temperatures.

(1)

y

f

(2)

where sT and sT are the true yield strength and true fracture strength. Basically, a critical normalized work-hardening parameter, H0 ¼ 0, exists to clarify the dependence of mechanical properties on the loading temperatures, which separates the illustration into two parts, as shown in Fig. 4. Above the H0 line, the in-situ

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0.15

MGM composite

Work-hardening

Ti47Zr19Be15V12Cu7-Present

0.10

Hc

Ti48Zr20Nb12Cu5Be15-[16]

Brittle failure

0.05

Zr38..5Ti32.5Nb7.3Cu6.2Be15.5-[17] Ti44Zr20Be16V12Cu8-[17]

0.00

BMG

-0.05

Zr57.4Ni13.4Cu17.9Al10.3Nb1-[4]

Work-softening

-0.10

Ti40Zr25Ni3Cu12Be20-[20] Zr55Ni5Cu30Al10-[26]

-0.15 50

Zr52.5Ni14.6Cu17.9Al10Ti5-[27]

100

150

200

250

300

Zr52.5Ni14.6Cu17.9Al10Ti5(annealed)-[30]

Temperature (K) Fig. 4. The dependence of work-hardening ability on the applied temperature.

MGM composites exhibit the work-hardening behavior, such as all composites deformed at 298 K. Here, the work-hardening behavior of the current MGM composite lessens with the reducing temperatures. Not only for the MGs but also for MGM composites, when Hc is near to H0, sf zsy , materials take place brittle failure. It is agreement with the previous results of Ti44Zr20V12Cu5Be19 and Zr38.5Ti32.5Nb7.3Cu6.2Be15.3 MGM composites [17], but different with the composite Ti48Zr20Nb12Cu5Be15 [16]. It is believed that the high content Nb element played the positive role in the high strength and plasticity at 77 K. However, below the H0 line, the MGs and MGM composites show the work-softening behavior, such as Zr52.5Ni14.6Al10Cu17.9Ti5 MG deformed below 298 K [17]. The current MGM composite exhibits work-softening behavior at cryogenic temperatures for the increment of the yielding strength is higher than that of the fracture strength, which is different with the MGM composites with high melting point particles such as (Zr55Ni10Cu20Ta3Al12)96Ta4 and Zr57Nb5Al10Cu15.4Ni12.6 containing 60% W particle [31,32]. Basically, the ductility dendrite degraded gradually with a decrease temperature for the harder and harder motion of dislocations [1,2,23,24]. As the temperature dropped, the activity of the dislocation decreased, leading to losing of the fracture strain and work-hardening ability. Therefore, the normalized parameter, Hc, can be quasi-empirical and statistical used to express the dependence of mechanical properties on the loading temperature in MGs and in-situ MGM composites. 3.3. Fracture surface morphologies Fig. 5 shows the typical fracture surfaces of the MGM composite under various temperatures. Clearly, the composite sample deformed at 298 K shows a shear failure with a fracture angle of 44.5 , as shown in Fig. 5(a). Profuse shear bands and slip bands are formed on the whole surface of the sample, which means that the whole sample participated in the plastic deformation, as shown in Fig. 5(b). These bands show the chaos propagation direction, caused by the intersection between slip bands and shear bands including primary and secondary ones, corresponding to the excellent plasticity. The coarse fracture surface including the shear slip steps, vein-like and melting droplets can be seen in Fig. 5(c), which is related with the interaction between initiation of critical shear bands and ductile dendrites [15,16]. The sample failed with a fracture angle of 44.8 at 236 K, as shown in Fig. 5(d). Comparatively, shear bands propagated along the same orientation with the fracture angle, as shown in Fig. 5(e). On the other hand, the profuse shear bands distributed on the lateral surface near to the fracture instead of the whole sample surface. Basically, the fracture surface is similar with the fracture feature at 300 K,

but the region of shear slip steps enlarged, as shown in Fig. 5(f) and (g). However, the composite deformed at 143 K failed with a fracture angle of 42 , as shown in Fig. 5(h). Compared with the Fig. 5(d) to (e), the number of primary shear bands decreased but the interval spacing increased, as illustrated in Fig. 5(i), suggesting the decreasing plastic deformation of the alloy. Many fine cracks can be observed in the shear fractured dendrite. Meanwhile, the melting layer covers the fracture surface instead of slip steps, as illustrated in Fig. 5(j), revealing that the strong adiabatic shear deformation induced by temperature rising in local shear bands. The sample failed as a catastrophic pattern with a fracture angle of 54 at 100 K, resulting from the main individual crack on the sample surface, as shown in Fig. 5(k). The traces of remelted fracture feature fracture includes the vein-like pattern and melting regions cover the whole fracture surface, as illustrated in Fig. 5(l) and (m). It suggests that primary crack dominates the instantaneous fracture on a localized shear zone accompanying by the high elastic energy release. According to the previous reports [12e14], the improvement of plasticity for in-situ MGM composites with b-crystal phase at room temperature is contributed by the plastic deformation via dislocation mechanism within dendrite phase, the formation of profuse slip bands, the initiation of the multiplication shear bands by dendrites instead of the unlimited propagation of the individual one, and the alternative deflection of propagation direction including the shear band and slip bands. However, the amount of shear bands decrease with the reducing temperature but cracks are formed; the chaos extension direction of these bands is replaced by the consistent one; the melting layer becomes the main feature on the fracture surface instead of the shear slip steps. According to Fig. 5, we can deduce that the amount of shear bands and slip bands determine the macro-plasticity, but the extension direction of these bands related with work-hardening behaviors. Therefore, these mutual effects including the number, propagation direction and local hearting release play a key role in improving the whole plasticity and the strength of the materials. 4. Conclusions In summary, the cryogenic mechanical behaviors of a Ti-based MGM composite were investigated systematically at low temperatures. We found that the yielding strength increased, but the plastic strain decreased with a decrease temperature without sharp transitions before the DBT transition point at 100 K. Although the MGM composite exhibits the large work-hardening behavior at 298 K, all sample display the work-softening behavior below 298 K. The nominal work-hardening parameter was employed to express the dependence of mechanical properties on temperatures. Present

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Fig. 5. SEM micrographs of the fractured specimens: (a) the overall fracture at 298 K, (b) the sample lateral surface, and (c) the fracture morphology; (d) the overall fracture at 236 K, (e) the lateral surface, (f) and (g) the fracture morphologies; (h) the overall fracture at 143 K, (i) the sample lateral surface, and (j) fracture morphology; (k) the overall fracture at 100 K, (l) and (m) the fracture morphology and its enlarge image.

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