Deformation induced FCC to HCP transformation in a Co–27Cr–5Mo–0.05C alloy

Abstract

In this work, the role played by plastic straining on the FCC(γ) → HCP(ɛ) strain induced transformation (SIT) ɛ-martensite in a Co–Cr–Mo–0.05C alloy was investigated. It was found that alloy plastic deformation at room temperature promotes the development of ɛ-martensite giving rise to the formation of intragranular striations. Moreover, aging at 800 °C of pre-strained specimens initially promotes SIT ɛ-martensite. This is followed by a time plateau (1 h) delay after which isothermal ɛ-martensite develops. Apparently, prior plastic straining leads to increasing incubation times for the onset of the isothermal ɛ-martensite transformation. In addition, tensile specimens containing various amounts of isothermal ɛ-martensite were tested all the way to fracture and their respective stress–strain properties were experimentally determined. Comparisons between experimental and simulated flow curves using the rule of mixtures indicated some deviations in the simulations at volume fractions above 0.25 ɛ-martensite. In contrast, predictions of yield and tensile strength were in good agreement with the experimental outcome. In particular, linear trends were exhibited by both, the yield and tensile strength as a function of the volume fraction of ɛ-martensite. Finally, the alloy elongation remained nearly constant with the volume fraction of ɛ-martensite in the range of the 0.15 < HCP < 0.55. Higher or lower values lead to a rapid decrease in the exhibited elongation.

Introduction

Cobalt base (Co–Cr–Mo–C) alloys exhibit two crystal structures, HCP and FCC, with HCP being thermodynamically stable at room temperature. In these alloys, the FCC to HCP transformation is difficult to achieve under normal cooling conditions and usually retain the high temperature FCC structure [1], [2], [3]. The FCC (γ) to HCP (ɛ) transformation can be induced athermally [3], [4], [5], [6], isothermally [5], [6], [7], [8], [9], [10], [11], [12] or through plastic straining [13], [14], [15], [16] with the resultant HCP phase known as ɛ-martensite. In addition, recent work by Kurosu et al. [17], [18] have found the development of a massive ɛ-martensite transformation in C and N-free Co–Cr–Mo alloys. The kinetic aspects of the athermal and isothermal transformation kinetics of ɛ-martensite have been investigated in some detail [6], [10]. Yet, in these alloys there is limited information on the SIT ɛ-martensite transformation.

From the published literature [13], [14], [15], [16] it is apparent that these alloys exhibit appreciable work hardening probably as a result of SIT. Recent work on the SIT ɛ-martensite indicates that it is detrimental for the cold workability of these alloys [19] due to shear band formation. In contrast, SIT ɛ-martensite can lead to improved wear properties due to a decrease in the number of slip systems [20], [21]. In a high carbon Co–Cr–Mo alloy, it has been found that pre-straining combined with isothermal aging accelerates the ɛ-martensite transformation [22]. Apparently, aging at 850 °C leads to reduced transformation times (8 h) for a complete ɛ-transformation after 10–20% alloy pre-straining [22].

Olson and Cohen in an early work [23] proposed a model for the nucleation of ɛ-martensite induced by plastic straining. In their model, shear band intersections in the form of twins, or stacking fault bundles were considered as the active nucleation sites for ɛ-martensite. In a related work, Rajan and Vander Sande [1] using transmission electron microscopy were able to identify stacking fault intersections and twins as the dominant defects contributing to the work hardening behavior in a cast Co–Cr–Mo–C alloy. Accordingly, plastic straining is expected to lead to an increase in the number of shear band intersections and thus, to the generation of ɛ-martensite embryos.

In addition, it has been found that the effect of grain size on the FCC to HCP transformation follows the Hall–Petch relation [13] for the yield strength but the alloy ductility drops at decreasing grain sizes. From the published reports, it is evident that the γ → ɛ transformation induced by plastic straining is somewhat limited as it usually does not proceed beyond a volume fraction of 0.4–0.5 [24]. In contrast, isothermal aging after annealing at 1150 °C can lead to a complete γ → ɛ transformation when aging temperatures of the order of 650–950 °C are employed. Moreover, there are no reports on the effect of alloy pre-straining on the isothermal γ → ɛ transformation in low carbon wrought alloys, nor on the effect of alloy pre-straining. Hence, in this work an attempt is made to further elucidate the role played by plastic straining on the γ → ɛ transformation in a low carbon Co-based alloy, as well as on the exhibited alloy strength.