Particle-Stimulated Nucleation (PSN) in the Co–28Cr–5Mo–0.3C Alloy

Hamid Reza Lashgari,Shahab Zangeneh

doi:10.3390/met10050671

Abstract

The present work is aimed at refining the grain size in the Co–28Cr–5Mo–0.3C (wt%) cast alloy using particle-stimulated nucleation (PSN) of recrystallization. It is pointed out that PSN resulted in considerable grain refinement (≈80%) of the as-cast structure, leading to an increased yield and tensile strength (around 30%). Partial solutionizing is associated with the formation of γfcc and athermal martensite. During PSN, the intensity of the hexagonal close-packed (hcp) phase increases due to the formation of isothermal martensite. It appears that new dynamic recrystallized (DRX) grains are formed around coarse undissolved particles (≈10 μm in size), especially where these particles are present in large clusters. The high-resolution TEM image shows the formation of heavily faulted regions and subgrains, with maximum misorientation near the carbides providing the driving force for the nucleation of new grains.

Highlights

Co–Cr–Mo alloys have been widely used in biomedical applications, such as artificial hip and knee joints, due to their excellent corrosion and wear resistance [1].CCM alloys are frequently used in the as-cast condition, but they show intrinsic defects such as coarse dendritic microstructures and microsegregation
The microstructures of CCM alloys in the as-cast condition are the combination of dendritic matrixes and secondary phases, mainly identified as M23 C6 carbides (M = Cr, Mo, and Co) located at interdendritic regions and grain boundaries
The dissolution of M23 C6 carbides in γfcc -austenite is a diffusion-controlled slow process, which means that after partial solution treatment, some round-like carbide particles still exist and remain embedded in the matrix

Summary

Introduction

Co–Cr–Mo alloys (hereafter referred to as CCM alloys) have been widely used in biomedical applications, such as artificial hip and knee joints, due to their excellent corrosion and wear resistance [1]. Yamanaka et al [4,5] showed that dynamic recrystallization (DRX) in CCM alloys could refine grain size (from 40 to 1.6 μm after hot deformation) and improve mechanical properties. During strain deformation of materials containing coarse nondeformable particles, these particles can induce a high degree of local lattice curvature in their vicinities due to particle-matrix strain incompatibility, and, more complex dislocation structures are formed. These regions are referred to as deformation zones. During subsequent annealing, such particle-related deformation zones are favorable nucleation sites for primary recrystallization This mechanism is, referred to as PSN [6]. We believe that this finding can be employed for further microstructural control and material processing

Material and Experimental Procedures

Results and Discussion

Conclusions