Mechanical properties of as-forged Ni-free Co–29Cr–6Mo alloys with ultrafine-grained microstructure

Abstract

We have succeeded the fabrication of Ni-free Co–29Cr–6Mo alloys with ultrafine-grained (UFG) microstructures via conventional hot forging process with the reduction up to 83.4% (true strain of 1.8). Grain refinement to submicron order is an effective method for improvement of strength and athermal martensite ɛ phase formed during quenching seems to determine the ductility of this alloy system.

Introduction

It has been recognized that ultrafine-grained (UFG) materials having grain sizes smaller than 1 μm exhibit superior mechanical properties, such as high strength, good fracture toughness and high fatigue strength, compared with conventional coarse-grained materials. The UFG metallic materials are generally obtained by so-called severe plastic deformation (SPD) techniques [1], [2], [3], [4], [5], [6], [7], [8], [9]. However, SPD processes, including equal channel angular extrusion (ECAE) [1], [3], [4], high pressure torsion (HPT) [1], accumulative roll-bonding (ARB) [5], [6] and multi-directional forging (MDF) [7], [8], require significantly large equivalent strain of 4–5 [10], leading to the difficulties in the practical applications of UFG materials. The alternative approaches to produce UFG microstructures without SPD processes, therefore, have been intensively studied especially for steels [11], [12]. Recently, the present authors [13] have revealed that the grain refinement to submicrometer order can be achieved employing conventional hot forging process with extremely low compressive strain of 60% (true strain; ɛ_t = 0.92) in the Co–Cr–Mo alloys, which are used for biomedical implant applications such as artificial hip and knee joints. The proposed process utilizes dynamic recrystallization (DRX) during hot deformation. It has been also pointed out that planar dislocation slip and a lot of stacking faults, which are likely to result from the formation of Shockley partial dislocations, were found in hot deformation microstructures, suggesting the stacking fault energy (SFE) of this alloy system is extremely low even at elevated temperatures. The low SFE suppresses the dynamic recovery of accumulated dislocations and enhances the heterogeneous deformation, i.e., introduction of geometrically necessary (GN) dislocation boundaries into matrix (grain subdivision) [14]. Thus, grain refinement proceeds in such a manner that a localized form of “continuous” DRX, as described in [13]. Grain refinement by above-mentioned process is though to be a promising way to improve the mechanical properties resulting in the reliability in biomedical use of these alloys. However, there are only a few studies concerning the effect of the grain refinement on mechanical properties in Co–Cr–Mo alloys [15]. The aim in this article is to evaluate the tensile properties of UFG structured Co–Cr–Mo alloy fabricated by hot forging process.