Effects of cooling rates on the mechanical properties of a Ti-based bulk metallic glass

Abstract

Mechanical properties of the glassy specimens fabricated at different cooling rates with a composition of Ti 40 Zr 25 Cu 12 Ni 3 Be 20 were systematically investigated. It was confirmed that faster cooling rates caused not only a larger amount of frozen-in free volume but also a higher glass transition temperature in the bulk glassy alloy. Increase in the free volume was found to favor plastic deformation and then to give rise to larger compressive plasticity, whilst the rise in the glass transition temperature seemed to be closely related to the higher yield strength. Moreover, the increase of yield strength and plasticity induced by fast cooling rates may also be associated with the residual stress generated during the fabrication process. Our results suggest that the deformation behavior of bulk metallic glasses is sensitive to various factors and influences from the other factors should be excluded as far as cooling-rate effects on bulk metallic glasses are considered. bulk metallic glasses, mechanical properties, cooling-rate effects, residual stress PACS: 61.43.Dq, 65.60.+a, 81.05.KfBulk metallic glasses (BMGs) are usually fabricated by rapid-quenching techniques in which the cooling rate is a key processing factor. As discussed in literature, glass formation ability (GFA) of BMGs is best described by the critical cooling rate, R c [1-3]. In addition to its role as one of the decisive factors in the vitrification process of BMGs, the cooling rate also influences the final microstructure [4,5] and physical properties [6]. Both theoretic and experimental results have directly shown that mechanical properties of BMGs were closely related to cooling rates applied during the sample preparation process [7,8]. An increase in the cooling rate is likely to give rise to a larger plasticity and lower yield strength [9]. However, further investigations are needed for a full understanding of the cooling-rate effects on mechanical properties of BMGs.Recently, the deformation behavior of amorphous solids was found extremely sensitive to the other factors such as Poison's ratio [10], residual stress [11], and testing specimen sizes [12,13]. As such, effects of the cooling rate are often overlapped with these factors in BMGs. It is thus necessary to exclude influences from the other factors in terms of the cooling-rate effects. In this paper, we attempt to investigate the cooling-rate dependence of mechanical properties in BMGs and the related mechanisms via carefully designed experiments. ExperimentMaster alloys with nominal compositions of Ti 40 Zr 25 -Cu 12 Ni 3 Be 20 (at.%) [14] were prepared by arc-melting a mixture of raw metals including 99.999% Ti, 99.5% Zr, 99.99%Cu, 99.99%Ni and 99.99% Be in a Ti-gettered argon atmosphere. The alloy ingots were melted six times to ensure a compositional homogeneity. Rod-shaped samples were obtained by suction-casting the molten alloys into copper molds with diameters of 2, 5, 8 and 10 mm in helium atmosphere. The amorphous structure of the as-cast samples

Relationship between composite structures and compressive properties in CuZr-based bulk metallic glass system

“…As shown, alloys with a low Al content (i.e., ≤6%) show considerable plasticity, whilst alloys with a large Al content (8%) display a quasi-brittle fracture without any appreciable plasticity, strongly implying a dramatic change in the local atomic structure of these amorphous alloys. As the Al content increases, another distinct feature observed in Figure 1 It has been proposed that yield strength of metallic glasses has a close correlation with their glass transition temperature (T g ) [18,19]. Thermal analysis of the Figure 2 as examples, and the measured thermal parameters are listed in Table 1.…”

Cooling rate effect on Young's modulus and hardness of a Zr-based metallic glass

“…In that case, it was argued that the mechanical properties of MGs could also be affected by the cooling rate [11,13]. The increase of the cooling rate may result in configurationally looser atomic packing and thus, more free-volume zones [14,15], which therefore contributes to larger plasticity [16][17][18]. However, the cooling-rate effect on the yield strength of MGs is yet to be conclusive [16,19,20]; meanwhile, the studies concerning the cooling rate effect on the elastic behavior of MGs are limited.…”

How does the initial free volume distribution affect shear band formation in metallic glass?

“…It would be adverse to the plasticity. Usually, MGs of good plasticity can be obtained by using a high cooling rate, because during the process more randomly distributed free volume or wider FVD is inherited from the liquid [15,16,22,32]. Through compositional changes, Liu et al [17] designed super-plastic bulk metallic glasses which are composed of hard regions surrounded by soft regions.…”