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Alloy Design for the Development and Production of Titanium Based Bulk Metallic Glasses

Iraz Begüm Demir, Amdulla O. Mekhrabov, M. Vedat Akdeniz

Novel Alloys Design and Development Laboratory (NOVALAB), Department of Metallurgical and Materials Engineering,Middle East Technical University, Ankara 06800, Turkey

Abstract

In order to investigate effect of possible candidate alloying elements on bulk glass forming ability of Ti based bulk metallic glasses, two different multicomponent alloy systems were selected as Ti48Cu30Ni10Al7V5 and Ti47Cu30Ni10Al7V5Si1. Samples of these multicomponent alloy systems were produced by using arc melting water cooled copper mold suction casting under argon atmosphere. The effect of addition of potential alloying elements on the bulk glass forming ability and the phase formation/transformation behavior of Ti based BMG will be presented by structural characterization techniques.

1. Introduction

Bulk metallic glasses (BMGs) are a relatively new class of metallic alloys having many unique properties such as high yield strength (typically above 2 GPa), improved elastic behavior (up to 2% strain), corrosion and wear resistance and lower elastic modulus due to their disordered atomic structure [1]. In the family of BMGs, because of requirement for reduced energy consumption, particular interest is given to the BMGs based on lightweight elements such as Al, Mg, Ca and Ti. Among them, only Ti-based alloy systems currently exhibit the required toughness to be used in structural engineering applications [2]. Therefore, Ti-based BMGs are promising as a new family of lightweight materials to be used in medical applications, defense, automotive and aerospace industries. However most of the Ti based metallic glasses have poor bulk glass forming ability (BGFA) which restrict their potential applications.

From the most general aspect, if the liquid phase is stabilized upon cooling and the competing crystalline phases are difficult to precipitate out, then the glass formation would be obtained. Thus, glass formation involves two components: • Liquid phase stability • Stability of the competing crystalline phases

(resistance to crystallization)

Liquid phase stability is related to the thermodynamic factors and resistance to crystallization depends on kinetic factors. As part of studying the formation of metallic glasses, investigation of BGFA of multicomponent alloy systems has a great importance. However, there is no standard definition for this. Up to now many indicators have been developed such as the amount of negative heat of mixing [3], solidification model [4], deep eutectic rule [5], the degree of atomic size mismatch [6], a reduced transition temperature Trg [7] (= Tg/Tm, where Tm is the liquidus temperature in this case) and supercooled liquid range Txg [8] (= Tx - Tg, where Tx is the onset crystallization temperature) etc.

Many of these approaches to predict the GFA of alloys require a thermophysical data of the potential candidate alloys. For this reason, theoretical modelling and computer simulation studies are much more beneficial. Prediction of the GFA of an alloy without doing excessive number of experiments to produce the glassy alloy can be possible. In this context, some theoreticalmodelling and computer simulations have been developed in recent years [9].

2. Experimental Procedure

Based on solidification model [4] and theoretical calculations [10] proposed by Akdeniz and Mekhrabov two different Ti-based alloy compositions;

• Alloy1: Ti48Cu30Ni10Al7V5 and• Alloy2: Ti47Cu30Ni10Al7V5Si1

were prepared by mixing the appropriate amounts of high purity constituents. Alloy preparation was performed by arc melting under Ti-gettered argon atmosphere. Each composition was remelted several times to improve chemical homogeneity. Then by using suction casting into copper mold in the form of cylindrical rod with diameter of 3 mm and length of 150 mm in argon atmosphere, Figure 1. The cooling rate at the suction part of sample is significantly faster than the cooling rate at the button part of sample.

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Samples were prepared by means of standard metallographical technique and microstructural characterizations of the samples were performed by optical microscope and FEI Quanta 400F field emission scanning electron microscope (FESEM) equipped with energy dispersive spectroscopy (EDS). EDS was used to identify the composition of the alloy and the phases were presented in microstructure. Samples were examined by X-Ray Diffraction (XRD) analysis by D/MAX 2200 Diffractometer. Monochromatic Cu K was used during the analysis in the diffraction angle (2 ) range of 20-100° with a scanning rate of 0.1°/min.

Figure 1. Parts of sample produced by copper mold suction casting.

3. Results and Discussion

The formation of metallic glass is the result of the suppression of the crystallization process during cooling from liquid melt. Accordingly, composition with high BGFA can also be termed as the composition with the minimum driving force for crystallization. As a result, searching for compositions with good BGFA, phase diagrams can be used as a guideline. It is clear that, systems with compositions in the range of phases having growth problems such as intermetallics are good candidates for finding alloys with high BGFA. Also, systems which have a sequential peritectic and eutectic transformations exhibit higher level of complexity of crystallization process [4]. Consequently, Ti-Cu binary system, which includes eutectic point at 960 0C combinedtwo intermetallics and sequence of eutectic and peritectic reaction, seems to be good candidate for high BGFA.

Figure 2. Ti-Cu binary phase diagram However, selection of potential candidate alloying elements which were predicted to increase the BGFA of the Ti-Cu binary system by theoretical calculations and computer simulation was performed by taking into account following criteria [10]:

• An increase in the negative heat of mixing ( Hmix). • A decrease in the critical cooling rate (Rc).

Therefore, based on the theoretical calculations [10] Ni, Al, V and Si are selected for alloying the Ti-Cu binary system. These elements are reported to decrease Rc and calculated Hmix values are given in Table 1.

Table 1. Calculated data of Hmix and RcTi64Cu35X1 Hmix J/mol (104) RcK/s

(105)None (binary) -1.8072 2.2140Si -1.8517 1.5856V -1.8354 1.7114Al -1.8230 1.7880Ni -1.7960 1.9070

The effects of Si on formation and development of solidification structures under conventional cooling conditions, i.e. button part, are given in Figure 3 (a) and (b) for Alloy1 and Alloy2 respectively. Despite only atomic 1% of silicon content, microstructure of the composition with Si is significantly different from that of the composition without Si. In accordance with their comparison of XRD pattern given in Figure 4, the difference originates due to formation and/or volume fraction of TiAlCu2 phase. It is evident Figure 4 that as alloy composition without Si (Alloy1) display TiCu, high intensity Ti2Cu, low intensity TiAlCu2 and (Ti(Ni1-x Cux) peaks, composition with Si shows TiCu, low intensity Ti2Cu and high intensity TiAlCu2 peaks. This would tend to suggest that presence of Si favors the formation of large faceted TiAlCu2 intermetallic particles distributed uniformly within the microstructure as shown in Figure 3(b) and Figure 6. These intermetallic phases can also be observed in Alloy1 as fine interdendritic particles which were further identified by EDS analysis, Figure 5.

(a) (b)

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Figure 3. Optical microscope images: (a) composition without Si; (b) composition with Si.

Figure 4. Comparison of X-ray scattering diffractogram between slow cooled parts of alloy composition without

Si and with Si.

Figure 5. Optical microscope and FESEM images of alloy composition without Si. (1) TiAlCu2 and (2) eutectic

mixture of TiCu and Ti2Cu.

Figure 6. Optical microscope and FESEM images of alloy composition with Si. (1) TiAlCu2 and (2) eutectic

mixture of TiCu and Ti2Cu.

It is interesting to note that increasing solidification rate, i.e under non-equilibrium rapid cooling conditions formation of Ti2Cu and TiAlCu2 intermetallic phases are suppressed and only TiCu phase can be observed for both compositions, Figure 7 (a) and (b) for the composition without Si and for composition with Si respectively.

Figure 7. Comparative XRD pattern button and suction casting part: (a) composition without Si; (b) composition

with Si.

Moreover, contrary to Alloy 1, presence of Si in alloy composition leads to formation of approximately 40μm in thickness amorphous phase at the surface of the rod where solidification rate significantly higher, Figure 8. The cooling rate eventually decreases towards the center of the specimen and thus the variation of the cooling/solidification rate across the cross section of the specimen is accompanied by the structural transition from surface to center. The featureless zone implying the presence of the amorphous phase at the surface undergoes crystallization process at a certain thickness and display planar growth morphology, Figure 9. This plane growth solidification structures survive until the solid liquid interface is subjected to morphological instability where cellular and/or dendritic solidification is observed towards the center of the specimen, Figure 8 (b).

Figure 8. Optical microscope images: (a) composition without Si; (b) composition with Si.

(a) (b)

(a)

(b)

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Figure 9. FESEM images of alloy composition with Si arrow shows where crystallization starts.

4. Conclusion

Effects of alloying element addition (Ni, Al, V and Si) on bulk glass forming ability of Ti-Cu based bulk metallic glasses have been evaluated by using structural characterization techniques. Microstructure of samples considerably changed by the minor addition of Si. Formation of Ti2Cu and TiAlCu2 intermetallic phases are inhibited under non-equilibrium rapid cooling conditions for both compositions. In addition to this, in the alloy composition with Si (Ti47Cu30Ni10Al7V5Si1),approximately 40μm in thickness amorphous structure is obtained at the outer zone of fast cooled part.

It is also worth to note that Si plays an important role for the formation amorphous structure in this Ti-based alloy composition at a certain degree. Therefore, attempts will be made to enlarge the extent of amorphous phase formation by increasing the BGFA of the Ti-Cu based alloy via alloy design.

References

[1] Kim, Y.C.; Kim, W.T.; Kim, D.H., A development of Ti-based bulk metallic glass. Mater. Sci. Eng. A, 2004, 375–377, 127–135.

[2] Jiang, J.Z; Hofmann, D.; Jarvis, D.J., Low-density high strength bulk metallic glasses and their composites: a review, Adv Eng Mater, 2014, 17:761–780.

[3] Takeuchi, A.; Inoue A., Calculation of mixing enthalpy and mismatch entropy for ternary amorphous alloys, [J]. Mater Trans JIM, 2000, 41: 1372-1378.

[4] Akdeniz M.V., Mekhrabov A., Solidification behavior of bulk glass forming alloy systems, Journal of Alloys and Compounds, 2015, 386.

formed?, Journal of Contemporary Physics, 1969, 10(5), 473-488.

[6] Takeuchi, A.; Inoue A., Calculation of mixing enthalpy and mismatch entropy for ternary amorphous alloys [J]. Mater Trans JIM, 2000, 41: 1372-1378.

[7] Mingwei, C., A brief overview of bulk metallic glasses, NPG Asia Materials, 2011, 3, 82-90.

[8] Davies, H.; Lewis, B., A generalized kinetic approach to metallic glass formation Journal of Scripta Metallurgica, 1975, 9(10): 1107-1112.

[9] Shirasawa, N.; Takigawa, T., Calculation of alloying effect on formation enthalpy of TiCu intermetallics from first principles calculations for designing Ti–Cu-system metallic glasses, Philosophical Magazine Letters, 2015.

[10] Suer, S.; Mekhrabov, A., Theoretical prediction of bulk glass forming ability of Ti-Cu based multicomponent allloys, Journal of Non-Crystalline Solids, 2009, 355:373-378.