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In this study, we investigate the influence of dislocation strengthening in the metastable parent phase on the deformation-induced martensitic transformation behavior of a face-centered cubic (fcc) Co20Cr20Fe34Mn20Ni6 high-entropy alloy. Annealed and hot-swaged specimens were prepared. In-situ neutron diffraction experiments captured an accelerated transformation kinetics in the hot-swaged specimen due to accumulated dislocations. The phase-specific macrostress development showed that the hexagonal close-packed $ \rvarepsilon $ ε -martensite predominantly accommodated the macroscopic plastic deformation of the annealed counterpart. Conversely, the matrix dislocation strengthening promoted cooperative plasticity of the $ \rgamma $ γ -matrix and the $ \rvarepsilon $ ε -martensitic phase, thus enhancing yield stress while preserving ductility. This paper firstly uncovers the role and significance of matrix dislocation strengthening on the mechanical behavior of metastable high-entropy alloys via in-situ neutron diffraction experiments. [ABSTRACT FROM AUTHOR]

This study demonstrates transformation-induced plasticity (TRIP) in conjunction with twinning-induced plasticity (TWIP) in α + β titanium alloys by introducing metastable retained β-phase. By annealing at 850 °C followed by water quenching, metastable retained β-phase (∼25%) was obtained in Ti–6Al–4V alloy. Stress-induced phase transformation in the retained β-phase produced orthorhombic α′′-martensite associated with (021)α′′ twinning, which significantly increased the work-hardening rate and uniform elongation. The findings reveal that the minor retained β-phase is responsible for macroscopic deformation behavior and could aid in novel alloy design that can increase work hardenability with fewer alloying elements than currently available metastable β-titanium alloys. The minor metastable retained β-phase in an α + β titanium alloy with low alloy content enhanced work hardening because of the combined TRIP/TWIP effect. [ABSTRACT FROM AUTHOR]

Abstract In this study, we examined the effect of pre-existing dislocation structures in a face-centered cubic γ-phase on strain-induced martensitic transformation (SIMT) to produce a hexagonal close-packed ε-phase in a hot-rolled biomedical Co–Cr–Mo alloy. The as-rolled microstructure was characterized by numerous dislocations as well as stacking faults and deformation twins. SIMT occurred just after macroscopic yielding in tensile deformation. Using synchrotron X-ray diffraction line-profile analysis, we successfully captured the nucleation of ε-martensite during tensile deformation in terms of structural evolution in the surrounding γ-matrix: many dislocations that were introduced into the γ-matrix during the hot-rolling process were consumed to produce ε-martensite, together with strong interactions between dislocations in the γ-matrix. As a result, the SIMT behavior during tensile deformation was accelerated through the consumption of these lattice defects, and the nucleation sites for the SIMT ε-phase transformed into intergranular regions upon hot rolling. Consequently, the hot-rolled Co–Cr–Mo alloy simultaneously exhibited an enhanced strain hardening and a high yield strength. The results of this study suggest the possibility of a novel approach for controlling the γ → ε SIMT behavior, and ultimately, the performance of the alloy in service by manipulating the initial dislocation structures. Graphical abstract fx1 [ABSTRACT FROM AUTHOR]

Strengthening of biomedical Co–Cr–Mo alloys has been explored via thermomechanical processing for enhancing the durability of their biomedical applications. However, the effects of cold and hot deformation on the cellular activity continue to be unclear. In this study, we prepared Co–Cr–Mo alloy rods via cold swaging and hot-caliber rolling and studied the relationship between the microstructure and cellular response of pre-osteoblasts. The cold-swaged rod experienced strain-induced martensitic transformation, which increased the volume fraction of the hexagonal close-packed (hcp) ε-martensite to ∼60 vol.% with an increase in area reduction (r) to 30%. The 111 γ fiber texture of the face-centered cubic (fcc) γ-matrix followed the Shoji−Nishiyama orientation relationship with ε-martensite. Cell culture results revealed beneficial effects of cold swaging on the cell response, in terms of adhesion, proliferation and morphology of cells, although increasing r did not significantly affect cellular metabolism levels. The addition of small content of Zr (0.04 wt.%) led to enhanced focal adhesion of cells, which became more significant at higher r. The microstructural evolution during hot-caliber rolling, namely, grain refinement without any phase transformation and strong texture development, did not appreciably affect the cellular activity. These findings are envisaged to facilitate alloy design and microstructural optimization for favorable tuning the osseointegration of biomedical Co–Cr–Mo alloys. [ABSTRACT FROM AUTHOR]

Abstract: Locally developed strains caused by athermal γ face-centered cubic (fcc)→ε hexagonal close-packed (hcp) martensitic transformation were investigated for the γ matrix of Ni-free Co–29Cr–6Mo (wt%) alloys prepared with or without added nitrogen. Electron-backscatter-diffraction-(EBSD)-based strain analysis revealed that in addition to ε-martensite interiors, the N-free alloy that had a duplex microstructure consisting of the γ matrix and athermal ε-martensite plates showed larger magnitudes of both elastic and plastic strains in the γ phase matrix than the N-doped counterpart that did not have a ε-martensite phase. Transmission electron microscopy (TEM) results indicated that the ε-martensite microplates were aggregates of thin ε-layers, which were formed by three different {111}γ〈11 〉γ Shockley partial dislocations in accordance with a previously proposed mechanism (Putaux and Chevalier, 1996) that canceled the shear strains of the individual variants. The plastic strains are believed to have originated from the martensitic transformation itself, and the activity of dislocations is believed to be the origin of the transformation. We have revealed that the elastic strains in the γ matrix originate from interactions among the ε-martensite phase, extended dislocations, and/or thin ε-layers. The dislocations highly dissociated into stacking faults, making stress relaxation at intersections difficult and further introducing local strain evolution. [Copyright &y& Elsevier]

Abstract: Our previous studies indicate that nitrogen addition suppresses the athermal γ (face-centered cubic, fcc)→ε (hexagonal close-packed, hcp) martensitic transformation of biomedical Co–Cr–Mo alloys and ultimately offers large elongation to failure while maintaining high strength. In the present study, structural evolution and dislocation slip as an elementary process in the martensitic transformation in Co–Cr–Mo alloys were investigated to reveal the origin of their enhanced γ phase stability due to nitrogen addition. Alloy specimens with and without nitrogen addition were prepared. The N-doped alloys had a single-phase γ matrix, whereas the N-free alloys had a γ/ε duplex microstructure. Irrespective of the nitrogen content, dislocations frequently dissociated into Shockley partial dislocations with stacking faults. This indicates that nitrogen has little effect on the stability of the γ phase, which is also predicted by thermodynamic calculations. We discovered short-range ordering (SRO) or nanoscale Cr2N precipitates in the γ matrix of the N-containing alloy specimens, and it was revealed that both SRO and nanoprecipitates function as obstacles to the glide of partial dislocations and consequently significantly affect the kinetics of the γ→ε martensitic transformation. Since the formation of ε martensite plays a crucial role in plastic deformation and wear behavior, the developed nanostructural modification associated with nitrogen addition must be a promising strategy for highly durable orthopedic implants. [Copyright &y& Elsevier]

Abstract: The effects of nitrogen doping on microstructural evolution during cold rolling of Ni free Co-Cr-Mo alloys have been investigated. Nitrogen doping improved the cold workability of this alloy system, although initiation of edge cracks was observed for a cold rolling reduction of 30% in a Co-29Cr-6Mo-0.17N (in mass%) alloy, which has the highest nitrogen content of the alloys used in the present study. Nitrogen addition of 0.17% sufficiently stabilizes the γ phase (fcc structure) at room temperature, suppressing the athermal martensitic γ→ɛ transformation during cooling after solution treating, while the primary deformation mechanism is still the strain-induced martensitic transformation (SIMT). The SIMT is responsible for the limited cold workability of Co-Cr-Mo alloys with and without N addition. The development of γ matrix –ɛ martensite lamellae in the initial stages of cold rolling and subsequent shear band (SB) formation in the vicinities of cracks was observed by transmission electron microscopy. Fine grains, which elongate along the shear direction, were observed inside SBs; this is similar to other materials with low stacking fault energies. Such a SB evolution at relatively low strain is thought to originate from the lamellar microstructure that consists of strain-induced ɛ martensites, which leads to crack initiation and propagation at and along γ matrix –ɛ martensite boundaries where stress concentrations readily occur. [Copyright &y& Elsevier]

Abstract: Biomedical Co–Cr–Mo (CCM) alloys have been commonly used for artificial hip and knee joint prostheses, but a need to improve their biomedical inertness and wear resistance has become widely recognized. The mechanical behavior of CCM alloys is dominated by strain-induced martensitic transformation (SIMT), which causes crack initiation during plastic deformation but dramatically enhances the wear resistance in practical use. To develop more reliable CCM alloys it is essential to clarify the factors affecting the occurrence of SIMT. In the present study we have focused on the effect of annealing twin boundaries (ATBs) on SIMT behavior. We have analyzed in detail the substructures near a parallel pair of ATBs after deformation under a stress preferential for slip parallel to the ATBs. Preferential formation of ε-hexagonal close-packed (HCP) phase at ATBs in metastable γ-face-centered cubic (FCC) phase was found by both scanning electron microscopy with electron backscattered diffraction (EBSD) analysis and transmission electron microscopy (TEM). High resolution TEM images indicated that thickening of the ε-HCP phase does not proceed regularly on every second atomic plane, which would form perfect ε-phase HCP structure, but irregularly leaving a high density of stacking faults. Furthermore, the thickness of the ε-HCP phase was found to be different at ATBs on the two sides of the twin. The difference was attributed to the internal stress due to strain incompatibility at the ATBs on the basis of residual stress analysis by the EBSD–Wilkinson method and phase-field simulation of solute segregation at ATBs. [Copyright &y& Elsevier]

The mechanism of extrusion and intrusion formation in Co–Cr–Mo–N alloys during fatigue deformation was investigated. In particular, we focused on the role of the strain-induced martensitic transformation (SIMT), which is the transformation of the metastable γ face-centered cubic (fcc) phase into a stable ε hexagonal close-packed (hcp) phase at room temperature because of the gliding of Shockley partial dislocations in the γ-phase matrix. We found that the SIMT also plays a crucial role in the formation of extrusions and intrusions. Further, the morphology of the extrusions and intrusions formed in the Co–Cr–Mo–N alloy specimens was very different from that seen in other fcc alloys. The extrusions and intrusions were formed by the gliding of perfect dislocations with a Burgers vector of perfect dislocation on the basal plane of the ε-hcp phase. This suggests that the ε-phases introduced by the SIMT can deform readily. [ABSTRACT FROM AUTHOR]