On the transformation-induced work-hardening behavior of Zr_47.5Co_47.5Al₅ ultrafine-grained alloy

Abstract

The work-hardening behavior of Zr_47.5Co_47.5Al₅ ultrafine-grained alloy, obtained by water-cooled copper mold casting, has been investigated. The Cu- and Ni-free alloy shows high compressive strength combined with large plasticity. The work-hardening behavior and the remarkable enhancement of the ductility and strength origin from the B2 to B33 deformation-induced martensitic transformation associated with the high strength ultra-fine grains. The fracture mode of the alloy is both intergranular and transgranular fracture. Our current finding may offer useful guidelines to design materials with improved strength and ductility as practical engineering and biomedical applications.

Introduction

The deformation of bulk metallic glass (BMG) at ambient temperature is highly localized into shear bands due to strain softening and/or thermal softening [1], [2], [3]. Finally, they develop towards catastrophic failure along one dominant shear plane, restricting wide applications of BMGs. Nano-grained (NG) materials usually exhibit a very high strength but a very limited ductility and almost no work-hardening before catastrophic failure because dislocation slip is substantially suppressed by the extremely small grains and grain boundary (GB) sliding or diffusional creep is not active enough to accommodate plastic straining at ambient temperature [4]. However, ultra-fine grained (UFG) materials are much more attractive for many structural applications because of their excellent mechanical properties as compared with BMGs, NG materials and even conventional crystalline materials.

The phenomenon of transformation-induced plasticity (TRIP) has been investigated in steel for several decades both experimentally and theoretically [5]. TRIP has also been found in Zr–Cu [6], [7], Ti–Ni [8], Zr–Co [9], [10], [11], [12], [13], [14] et al. based alloys and are verified to be an effective way to enhance the performance of materials and components [1]. B2 (CsCl structure) ZrCo alloys is inherently ductile and is convinced the existence of B2 to B33 martensitic transformation based on the present research [10], [11], [12], [13], [14]. However, monolithic B2-ZrCo phases are often very soft in both compressive and tensile tests, which restrict their applications as engineering materials [10], [11], [12], [13], [14]. A possible way is to synthesize UFG alloy through rapid solidification method to balance the high strength and large plasticity. Nevertheless, most of the Zr- even Ti-based alloys often require the addition of copper, nickel or beryllium, which makes these alloys unfavorable to be used as biomedical implants [15]. Aluminum, one hand, is the most abundant metal in the earth's crust and so chemically reactive that native specimens are rare and limited to extreme reducing environments [16]. On the other hand, Al is remarkable for the metal's low density and for its ability to resist corrosion due to the phenomenon of passivation [17]. What's more, the addition of Al disturbs inevitably the chemical and topological structure of Zr–Co–Al alloy (shown in Table 1). Recently, the discovery of Zr–Co–Al ternary system with a superior corrosion resistance and good bioactivity renewed the research interests [18], [19], [20], [21], [22]. ZrCo alloys together with low cost aluminum are a good candidate to verify this conjecture.

In this work, Zr_47.5Co_47.5Al₅ ultrafine-grained alloy has been obtained by water-cooled copper mold casting. The transformation-induced work-hardening behavior has been investigated by compression test, micro-indentation test, X-ray diffraction (XRD) and high-resolution scanning electron microscopy (SEM). The implications on the deformation mechanism are discussed as well.