Effects of silicon and chromium additions on glass forming ability and microhardness of Co-based bulk metallic glasses

Transcription

1 Indian Journal of Engineering & Materials Sciences Vol. 21, February 2014, pp Effects of silicon and chromium additions on glass forming ability and microhardness of Co-based bulk metallic glasses Aytekin Hitit a *,Şükrü Talaş b & Rıza Kara b a Department of Materials Science and Engineering, b Department of Metallurgical and Materials Engineering, Afyon Kocatepe University, Afyonkarahisar, Turkey Received 19 December 2011; accepted 5 September 2013 Effects of silicon and chromium additions on glass forming ability (GFA) and microhardness of a Co-Fe-Ta-B bulk metallic glass are investigated by using differential scanning calorimetry (DSC), X-ray diffractometry (XRD) and scanning electron microscopy (SEM). It is found that partial substitution of boron by silicon promotes the GFA of the alloy. Fully amorphous rod of 4 mm is fabricated by suction casting Co 43 Fe 20 Ta 5.5 B 26.5 Si 5 alloy. However, partial replacement of cobalt by chromium decreased the GFA significantly. In fact, critical casting thickness of Co 39 Cr 4 Fe 20 Ta 5.5 B 26.5 Si 5 alloy is determined as 2 mm. It is also determined that microhardness values of the amorphous alloys are about 1200HV 300. This value is lower than the hardness of the base alloy,1455hv 300, and it is believed that decrease in hardness results from the reduction in boron contents of the alloys. Keywords: Bulk metallic glasses, Glass forming ability, Thermal analysis, Microhardness For the last two decades, multicomponent bulk metallic glasses (BMGs) have attracted great attention because of their unusual physical, chemical and mechanical properties. A large number of glassforming alloys with critical cooling rates less than 1000 K/s have been successfully developed in Zr 1-3 -, La 4,5 -, Pd 6,7 -, Mg 6-8 -, Ti 9,10 -, Ni , Cu 14,15 -, Fe and Co based systems, which have significantly broadened the expectation of amorphous alloys for both scientific and engineering applications. When mechanical properties considered, the fracture strength is in the range of MPa for Zrbased alloys 1-3, MPa for Ti-based alloys 9,10, MPa for Cu-based alloys 11,14, MPa for Ni-based alloys 11,12, MPa for Fe-based alloys 17,18 and MPa for Co-based alloys 18,19. The cobalt based alloy Co 43 Fe 20 Ta 5.5 B 31.5 is one of the alloys having the highest fracture strength, but its critical casting thickness is only 2 mm. It is believed that such a low critical casting thickness limits utilization of this alloy as a structural material. Therefore, critical casting thickness of this alloy must be improved. Unfortunately, there is no universal model to predict the alloy compositions which has good glass forming ability (GFA). Based on extensive *Corresponding author ( hitit@aku.edu.tr) experimental results, three empirical rules have been established to favor the formation of bulk metallic glass 21 : (i) multi-component system with more than three components, (ii) significant difference in atomic size ratios above about 12% among the three main constituent elements and (iii) large negative heats of mixing among the three main constituent elements. Although these rules can be useful guidelines for alloy design, development of new alloys with high GFA mainly depends on carrying out a series of experiments where compositions are changed step by step. Improvement of glass forming ability (GFA) is often achieved by partial replacement of a constitute element by another element, selected on the basis of the empirical rules for bulk metallic glasses. The Co-Fe-Ta-B alloy under investigation already satisfies the three empirical rules described above. In order to improve GFA of the Co-Fe-Ta-B alloy, silicon and chromium were selected as candidate elements. Examination of binary phase diagrams of silicon with each of the constitutent elements of the alloy reveals that for all cases, minor silicon additions decrease liquidus temperatures of binary alloys 22. Similarly, binary phase diagrams of chromium and the constitutent elements of the alloy indicate that minor chromium additions also decrease liquidus temperatures of binary alloys. Therefore, proper utilization of these elements as

2 112 INDIAN J ENG. MATER. SCI., FEBRUARY 2014 substitutions for the alloying elements can improve GFA of the alloy by lowering liquidus temperature. Since silicon is a metalloid element, it was subsituted for the metalloid element of the alloy, boron, to keep the total fraction of metalloid elements of the alloy constant. Chromium can be used as substitution for both cobalt and iron for the alloy under investigation due to the fact that chromium, cobalt and iron have very similar atomic radii, which are nm, nm and nm, respectively 23. However, atomic radius of tantalum is nm and replacement of tantalum by chromium violates the second emprical rule described above. In this study, chromium was substituted for cobalt not for iron. There is no particular reason for not choosing iron. In this paper, we report the GFA and microhardness of alloys designed by partial replacements of alloying elements by silicon and chromium in Co 43 Fe 20 Ta 5.5 B 31.5 alloy. Experimental Procedure Multi-component Co-based alloy ingots with composition of Co 43-x Cr x Fe 20 Ta 5.5 B 26.5 Si 5 (where x=0,2,4) were prepared by arc melting the mixtures of pure Co (99.8 wt%), Fe (99.9 wt%) and Ta (99.9 wt%) and Cr (99.7 wt%) metals and pure crystalline B (98 mass%) in a Ti-gettered high purity argon atmosphere. In order to ensure homogenity, master alloys were melted three times. The alloy compositions represent nominal atomic percentages. Bulk glassy alloys in a rod form with diameters up to 5 mm and a length of 50 mm were produced by suction casting method in an arc furnace. The as-cast structures were examined by X-ray diffraction (XRD) (Shimadzu XRD-6000) with Cu-K α radiation and scanning electron microscope (SEM) (Leo 1430 VP ). The glass transition temperature (T g ), crystallization temperature (T x ), solidus temperature (T m ) and liquidus temperatures (T L ) of the alloys were determined by differential scanning calorimetry (DSC) (Netzsch STA 409 PC/PG ) at a heating rate of 0.33 K/s. Microhardness measurements were carried out with a Vickers microhardness tester (Shimadzu HMV 2L ) under a load of 2.94 N. For each alloy, microhardnesses of as-cast samples having casting thickness of 2 mm were measured. Twenty measurements were carried out for each sample and arithmetic mean of measurements were taken as microhardness of the alloy. Results and Discussion XRD patterns of samples are shown in Fig. 1. Co 43 Fe 20 Ta 5.5 B 26.5 Si 5 alloy has critical casting thickness of 4 mm. For the casting thickness of 5 mm, precipitation of (Co,Fe) 2 B phase was observed for this alloy. Critical casting thicknesses of chromium containing alloys Co 41 Cr 2 Fe 20 Ta 5.5 B 26.5 Si 5 and Co 39 Cr 4 Fe 20 Ta 5.5 B 26.5 Si 5 are found to be 3 mm and 2 mm, respectively. For both of these alloys, it was determined that body-centered tetragonal (Co,Fe) 2 B and face-centered cubic (Co,Fe) 23 B 6 type phases precipitate in the samples having diameters larger than critical casting thicknesses. Thermal stability of the alloys were investigated by DSC (Fig. 2). During heating, all the DSC traces showed an endothermic event, characteristics of glass transition and followed by exothermic reactions corresponding to crystallization of the undercooled liquid. T g and T x of the base alloy, Co 43 Fe 20 Ta 5.5 B 31.5, alloy are 910 and 982 K, respectively 24. T g of Co 43 Fe 20 Ta 5.5 B 26.5 Si 5 alloy is 889K, which is about 20 K lower than that of Co 43 Fe 20 Ta 5.5 B 31.5 alloy. Also T x of Co 43 Fe 20 Ta 5.5 B 26.5 Si 5 alloy is 937 K, which is lower than T x of Co 43 Fe 20 Ta 5.5 B 31.5 alloy. It was also determined that T L of Co 43 Fe 20 Ta 5.5 B 26.5 Si 5 alloy is 1450 K, which is about 65 K lower than the base alloy. T g of the chromium containing alloys, Co 41 Cr 2 Fe 20 Ta 5.5 B 26.5 Si 5 and Co 39 Cr 4 Fe 20 Ta 5.5 B 26.5 Si 5, are 903 and 908 K, respectively. These values are very close to T g of the base alloy. Also, T x of Co 41 Cr 2 Fe 20 Ta 5.5 B 26.5 Si 5 and Co 39 Cr 4 Fe 20 Ta 5.5 B 26.5 Si 5 are determined to be 961 K and 979 K, respectively and these values are very close to T x of the base alloy. Fig. 1 XRD patterns of the as-cast Co 43-x Cr x Fe 20 Ta 5.5 B 26.5 Si 5 (x=0,2 and 4) alloys

3 HITIT et al.: Co-BASED BULK METALLIC GLASSES 113 In addition, T L of Co 41 Cr 2 Fe 20 Ta 5.5 B 26.5 Si 5 and Co 39 Cr 4 Fe 20 Ta 5.5 B 26.5 Si 5 alloys are determined to be 1487 and 1500 K, respectively. These values are also very similar to the T L of the base alloy. Thermal properties of the alloys are summarized in Table 1. SEM image obtained from the center region of 4 mm sample of Co 41 Cr 2 Fe 20 Ta 5.5 B 26.5 Si 5 alloy is shown in Fig. 3a. Two types of particles are observed in the SEM image. Because boron content of (Co,Fe) 23 B 6 phase, which is 20 at.%, is lower than that of (Co,Fe) 2 B phase, which is 33 at.%, (Co,Fe) 23 B 6 phase has a higher average atomic number. For this reason, it is concluded that the brighter particles observed in the SEM image are particles of (Co,Fe) 23 B 6 phase and the darker particles are particles of (Co,Fe) 2 B phase. In addition, EDS results show that (Co,Fe) 23 B 6 phase contains some amount of tantalum (Fig. 3b). Also, cubical morphology is observed for particles of (Co,Fe) 23 B 6 phase, which is not unexpected since the particles have face-centered cubic structure. Microhardnesses of the alloys are found to be around 1200 HV 300 (Table 1). During the measurements, it was also observed that for each alloy, measurements were quite consistent and deviations from the average microhardness was less than 5%. Also, microhardness values of the alloys are determined to be quite close to each other and lower than the microhardness of the base alloy. It is believed that significant drop in liquidus temperature of Co 43 Fe 20 Ta 5.5 B 26.5 Si 5 alloy is the reason for higher GFA of this alloy. Lowering T L for a constant T g of the alloy results in a decrease in the temperature difference between T L and T g ; for this reason, a higher cooling rate can be achieved for the same casting diameter. As chromium content is increased there is no significant change for T g of the alloys. However, liquidus temperatures of the alloys increase with chromium content and this must be the reason for having lower GFA for these alloys. Also, having shorter critical nucleation time for an alloy can be another reason for the rapid crystallization of the chromium containing alloys. Nevertheless, for chromium containing alloys, it does not seem to be possible that 2-4 at.% chromium additions cause such a reduction in critical nucleation time of the phases due to the fact that the precipitating phases do not contain noticable amount of chromium. Reduced glass transition temperatures (T g /T l ) of the alloys show very close correlation with the critical casting thickness values (Fig. 4). Indeed, Alloy Table 1 Thermal properties (T g,t x,t l,t m ), parameters for GFA, critical casting thickness and microhardnesses of Co-Fe-Ta-B-Cr-Si alloys T g T x T m T L T x T g /T L γ(t x / (T g +T L )) 18 Co 43 Fe 20 Ta 5.5 B Co 43 Fe 20 Ta 5.5 B 26.5 Si Co 41 Cr 2 Fe 20 Ta 5.5 B 26.5 Si Co 39 Cr 4 Fe 20 Ta 5.5 B 26.5 Si HV 300 D max (mm) Fig. 2 DSC curves of the Co 43-x Cr x Fe 20 Ta 5.5 B 26.5 Si 5 (x=0,2 and 4) alloys: (a) low temperature measurements and (b) melting behaviour

4 114 INDIAN J ENG. MATER. SCI., FEBRUARY 2014 Fig. 4 Relationship between the critical casting thickness (D max ) for the formation of a glassy phase and reduced glass transition temperature (T g /T L ). Fig. 3 Microstructure of the Co 41 Cr 2 Fe 20 Ta 5.5 B 26.5 Si 5 alloy (d=4mm): (a) SEM electron backscattered image, (b) EDX result obtained from particles having lighter contrast and (c) EDX result obtained from particles having darker contrast Co 43 Fe 20 Ta 5.5 B 26.5 Si 5 alloy has the highest T g /T L and critical casting thickness values. Also, DSC measurements show that eutectic temperatures of the alloys are almost the same, which is about 1300 K, and all the alloys have off eutectic compositions. These result implies that if eutectic or near eutectic compositions for these alloys are found, T L of the alloys will be much lower. For this reason, the critical casting thicknesses of these eutectic or near-eutectic alloys are expected to be much higher. In addition to reduced glass transition temperature, some other well-known GFA parameters are also considered. It is found that γ parameter does not show any agreement with critical casting values of the alloys. Also, there is no correlation with T x values and critical casting thicknesses of the alloys either. Microhardnesses of the alloys are determined to be about 1200 HV 300 (~11.7 GPa). Tensile yield strength of the alloys can be estimated by using the equation σ y =Hv/3 24. Based on the microhardness values, the tensile yield strengths of the alloys are determined to be about 3.9 GPa. Microhardness values of the alloys are lower than the microhardness value of the base alloy, which is 1455Hv. This decrease in the hardness is believed to be due to the reduction in number of Co-B, Fe-B and Ta-B pairs in the alloys studied, which resulted from substitution of silicon for boron. Similar results indicating the effect of boron content on hardness were observed in other Co- and Fe-based bulk metallic glasses 18. Based on these results, in Co- and Fe-based bulk metallic glass systems, it is

5 HITIT et al.: Co-BASED BULK METALLIC GLASSES 115 obvious that replacement of boron by other elements for the improvement GFA results in reduction in hardness. For this reason, if the yield strength levels are desired to be higher than 5 GPa, replacement of boron by another element should not be a choice for the improvement of the GFA of Co-based bulk metallic glasses. In other words, partial replacement of cobalt, iron and tantalum by suitable elements should be the strategy for improvement of the GFA of these alloys. It was also observed that, chromium additions do not have any effect on the microhardness values of the silicon modified alloy, which suggests that there is no significant difference in terms of bond strength between Co-B and Cr-B pairs Conclusions The following conclusions can be drawn from this study: (i) Partial replacement of boron by silicon in Co 43 Fe 20 Ta 5.5 B 26.5 enhances the GFA by lowering the liquidus temperature of the alloy. (ii) Partial replacement of cobalt by chromium reduces the GFA because of the fact that chromium substitution increases the liquidus temperatures of the alloys. (iii) Critical casting thicknesses of the alloys show very good correlation with reduced glass transition temperature, T g /T L. (iv) Alloys having eutectic or near eutectic compositions are expected to have much higher critical casting thickness. (v) Replacement of boron by silicon caused reduction in microhardness because of the decrease in the number of (Co,Fe,Ta)- B pairs. Acknowledgements This study was supported by grant no.104m124 from the The Support Programme for Scientific and Technological Research Projects of the Scientific and Technological Research Council of Turkey. References 1 Peker A & Johnson W L, Appl Phys Lett, 63 (1993) Inoue A & Zhang T, Mater Trans JIM, 36 (1995) Fan J T, Wu F F, Zhang Z F, Jiang F & Sun J & Mao S X, J Non-Cryst Solids, 353 (2007) Jiang Q K, Zhang G Q, Chen L Y, Zeng Q S & Jiang J Z, J Alloys Compounds, 424 (2006) Liu W Y, Zhang H F, Wang A M, Lia H & Hua Z Q, Mater Sci Eng A, 459 (2007) Liu L, Zhao X, Ma C, Pang S & Zhang T, J Non-Cryst Solids, 352 (2006) Liu W Y, Zhang H F, Wang A M, Lia H & Hua Z Q, Mater Sci Eng A, 459 (2007) Zheng Q, Cheng J H, Strader E, Ma J & Xu J, Scripta Mater, 56 (2007) Inoue A, Mater Sci Forum, 307 (1999) Kim Y C, Yi S, Kim W T & Kim D H, Mater Res Soc Symp Proc, 644 (2001) L Inoue A, Zhang W, Zhang T & Kurosaka K, Acta Mater, 49 (2001) Zhang T & Inoue A, Mater Trans, 43 (2002) Liang W Z, Shen J & Sun J F, J Alloys Compounds, 420 (2006) Kim Y C, Lee J C, Cha P R, Ahn J P & Fleury E, Mater Sci Eng A, 437 (2006) Fan J T, Zhang Z F, Jiang F, Sun J & Mao S X, Mater Sci Eng A, 487 (2008) Li H X & Yi S, Mater Sci Eng A (2007) Inoue A, Shen B L & Chang C T, Acta Mater, 52 (2004) Inoue A, Shen B L, Chang C T, Intermetallics, 14 (2006) Inoue A., Shen B L, Koshiba H, Kato H & Yavari A R, Acta Mater, 52 (2004) Men H, Pan S J & Zhang T, J Mater Res, 21 (2006) Inoue A, Acta Mater, 48 (2000) Baker H (ed), ASM Handbook: Alloy Phase Diagrams, (1992). 23 International Tables for X-ray Crystallography, (1968). 24 Zhang P, Li S X & Zhang Z F, Mater Sci Eng A, 529 (2011) 62.