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Co–28Cr–6Mo–0.23C–0.17N alloy cylindrical rods were fabricated by electron beam melting (EBM) with cylindrical axes along the build direction. The inhomogeneity in microstructures of the as-fabricated rods and heat-treated rods were investigated, along with the creep behavior of the heat-treated rods, focusing on the influence of microstructural inhomogeneity. Although the constituent phase varied along the build direction in the as-EBM-built rod, from single e-hexagonal close-packed (hcp) phase in the bottom to single γ-face-centered cubic (fcc) phase in the top, the γ-fcc phase can be kept in a wide range of build height, i.e. ∼40 mm from the top finishing plane. The as-EBM-built rods consisting of both e phase and γ phase can be transformed into single e-hcp phase by the aging treatment at 800 °C for 24 h. However, the e-hcp grain size in the aged rod was heterogeneous along the build height. The grain size increased along the build height at first, then decreased gradually to the position where the phase transitioned from γ-fcc to e-hcp in the as-EBM-built rod. The grain size was nearly uniform on the top part, which used to be single γ-fcc phase. The values of stress exponent n and apparent activation energy Q were determined to be 5.0 and 365 kJ mol−1, respectively. Intergranular fracture occurred, and fracture nearly always occurred in the homogeneous fine-grain region. The decrease in stacking fault energy with increasing temperature keeps the dislocations expanding and increases the apparent activation energy.

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 e 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 e-hcp phase. This suggests that the e-phases introduced by the SIMT can deform readily.

A Co–Cr–Mo alloy with a single e (hexagonal close-packed, hcp) phase exhibits excellent tensile properties with a 0.2% proof stress of 630 MPa, an ultimate tensile stress of 1072 MPa and an elongation to fracture of 38.3%. The dominant deformation modes are basal 〈a〉 slip and prismatic 〈a〉 slip, and the apparent respective critical resolved shear stresses at room temperature are calculated to be 184 and 211 MPa. This simultaneous activation of both 〈a〉 slips can be explained in terms of the lattice constant ratio c/a of 1.610. There is a tendency for the geometrically necessary dislocations (GNDs) to accumulate at grain boundaries, and the magnitude of this GND accumulation at a particular boundary is dependent on its character. Numerical analysis using a dislocation-model-based strain gradient crystal plasticity calculation makes it possible to characterize the distributions of dislocation density, local stress and local strain in the polycrystalline e Co–Cr–Mo alloy, and the calculation is largely consistent with the experimental results. This simulation reveals that the activity of the prismatic 〈a〉 slip in addition to the basal 〈a〉 slip contributes to the stress relaxation at the boundary. For this reason, excellent tensile ductility is obtained in the polycrystalline e Co–Cr–Mo alloy.

The microstructures and high-temperature tensile properties of a Co–28Cr–6Mo–0.23C–0.17N alloy fabricated by electron beam melting (EBM) with cylindrical axes deviating from the build direction by 0°, 45°, 55° and 90° were investigated. The preferred crystal orientations of the γ phase in the as-EBM-built samples with angles of 0°, 45°, 55° and 90° were near [0 0 1], [1 1 0], [1 1 1] and [1 0 0], respectively. M23C6 precipitates (M = Cr, Mo or Si) were observed to align along the build direction with intervals of around 3 μm. The phase was completely transformed into a single e-hexagonal close-packed (hcp) phase after aging treatment at 800 °C for 24 h, when lamellar colonies of M2N precipitates and the e-hcp phase appeared in the matrix. Among the samples, the one built with 55° deviation had the highest ultimate tensile strength of 806 MPa at 700 °C. The relationship between the microstructure and the build direction dependence of mechanical properties suggested that the conditions of heat treatment to homogenize the microstructure throughout the height of the EBM-built object should be determined by taking into account the thermal history during the post-melt period of the EBM process, especially when the solid–solid transformation is sluggish.

We report the influence of alloying elements (Ni, Al and Mn) on the microstructural evolution of Cu-rich nanoprecipitates and the mechanical properties of Fe–Cu-based ferritic alloys. It was found that individual additions of Ni and Al do not give rise to an obvious strengthening effect, compared with the binary Fe–Cu parent alloy, although Ni segregates at the precipitate/matrix interface and Al partitions into Cu-rich precipitates. In contrast, the co-addition of Ni and Al results in the formation of core–shell nanoprecipitates with a Cu-rich core and a B2 Ni–Al shell, leading to a dramatic improvement in strength. The coarsening rate of the core–shell precipitates is about two orders of magnitude lower than that of monolithic Cu-rich precipitates in the binary and ternary Fe–Cu alloys. Reinforcement of the B2 Ni–Al shells by Mn partitioning further improves the strength of the precipitation-strengthened alloys by forming ultrastable and high number density core–shell nanoprecipitates.

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 e-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 e-HCP phase does not proceed regularly on every second atomic plane, which would form perfect e-phase HCP structure, but irregularly leaving a high density of stacking faults. Furthermore, the thickness of the e-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.

Suzuki segregation in Co–Ni-based superalloys is of longstanding interest. In this study, the development of widely extended stacking fault (SF) ribbons was confirmed in a Co–Ni-based superalloy aged at 973 K after deformation at room temperature, which supports the decrease in stacking fault energy (SFE) due to Suzuki segregation. In addition, the plastic deformation behaviors of Co–Ni-based superalloys with various Nb contents up to 3 wt.% were investigated focusing on the effect of Nb addition on dynamic strain-aging by Suzuki segregation. The negative strain-rate dependence of flow stress due to dynamic strain-aging became more significant with increasing Nb content; however, attempts to detect segregating elements by scanning transmission electron microscopy and energy-dispersive spectroscopy analysis were not successful. A phase-field simulation of Suzuki segregation suggested strong Ni depletion with segregation of Cr and Mo atoms at the SF, and the SFE can become negative as a consequence of the segregation. This agrees with the experimentally observed formation of wide SFs by the aging at 973 K after cold deformation. It is also suggested that Nb atoms are strongly depleted at SFs, and a small amount of Nb addition dramatically enhances Cr segregation, resulting in further decreases in the SFE, which is probably responsible for the observed enhancement of dynamic strain-aging by Nb addition. In addition, the local structural changes, such as short-range ordering and/or an in-plane ordering, accompanying the segregation were discussed as possible additional mechanisms for strain-aging enhancement.