Abstracts:A micromechanical study has been conducted on low temperature dwell fatigue resistance in multiphase polycrystalline titanium alloys. The origin of the observed peak in strain rate sensitivity (SRS) over temperature has been explained by the transition from high-stress/low-temperature to low-stress/high-temperature thermally activated dislocation escape. The SRS peak is found to depend considerably on the rate sensitive slip systems in hexagonal close packed (HCP) α phase and body centered cubic (BCC) β phase in Ti alloys. This motivates the study of structural rate dependence, using the SRS peak, in commercially important textured multiphase Ti alloys.

Abstracts:A new model for hydrogen-assisted fatigue crack growth (HAFCG) in BCC iron under a gaseous hydrogen environment has been established based on various methods of observation, i.e., electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM), to elucidate the precise mechanism of HAFCG. The FCG in gaseous hydrogen showed two distinguishing regimes corresponding to the unaccelerated regime at a relatively low stress intensity factor range, ΔK, and the accelerated regime at a relatively high ΔK. The fracture surface in the unaccelerated regime was covered by ductile transgranular and intergranular features, while mainly quasi-cleavage features were observed in the accelerated regime. The EBSD and ECCI results demonstrated considerably lower amounts of plastic deformation, i.e., less plasticity, around the crack path in the accelerated regime. The TEM results confirmed that the dislocation structure immediately beneath the crack in the accelerated regime showed significantly lower development and that the fracture surface in the quasi-cleavage regions was parallel to the {100} plane. These observations suggest that the HAFCG in pure iron may be attributed to “less plasticity” rather than “localized plasticity” around the crack tip.

Abstracts:We investigate the atomic structures, chemical bonding, stability and fracture mechanism of B- and Ti-terminated incoherent TiB2 (0001)/Fe (111) and semi-coherent TiB2 (0001)/Fe (100) interfaces using first-principles calculations. It is found that all Ti-terminated interfaces (Ti-HCP, Ti-MT and Ti-OT) as well as B-HCP type TiB2 (0001)/Fe (100) interface are non-diffusive type. Meanwhile, B-HCP, B-MT and B-OT configurations of TiB2 (0001)/Fe (111) interface are diffusive type due to the formation of additional Fe x B intermetallic compound at the original Fe/TiB2 interface. The calculated works of adhesion and interfacial energies indicate that Ti-HCP and B-HCP are the most stable structures for both incoherent and semi-coherent interfaces. We find that the magnitude of interfacial elastic energy is comparable to that of the chemical energy for the semi-coherent TiB2 (0001)/Fe (100) interface. The electronic structures of TiB2/Fe interfaces reveal the formation of Fe-B, Ti-B and Fe-Ti bonds at or next to the interface. Using Griffith's theory, it is predicted that the mechanical failure of TiB2/Fe composite would initiate at the interface between TiB2 and Fe. The first principles tensile experiment performed on all Ti-HCP interfaces agrees with the prediction. In the case of B-HCP interfaces, due to the formation of either Fe x B diffusive layer or strong covalent Fe-B bonds, the mechanical failure eventually occurs in the Fe slab rather than that predicted by Griffith's theory. We also find the formation of diffusive Fe x B layer could significantly suppress the local magnetic moment of Fe atom at TiB2/Fe interface due the formation of strong covalent Fe-B bond.

Abstracts:Plastic deformation largely determines the properties of tribological contacts. To modify such contacts, it is thus necessary to understand the underlying mechanisms of dislocation motion and multiplication. In a sliding contact, dislocations do not only multiply, but also follow the moving tip — and are therefore ”transported” by the tip. Three dimensional Discrete Dislocation Dynamics simulations of sliding with a spherical tip are conducted to analyse dislocation transport in an fcc crystal, at stresses below those typically required to nucleate dislocations at a free surface. Sliding direction, glide plane orientation and Burgers vector orientation determine whether a dislocation can be transported or not, and how far the transport leads. A model is proposed to identify the glide systems, on which dislocations are transported.

Abstracts:Nanocrystalline metals contain a large fraction of high-energy grain boundaries, which may be considered as glassy phases. Consequently, with decreasing grain size, a crossover in the deformation behaviour of nanocrystals to that of metallic glasses has been proposed. Here, we study this crossover using molecular dynamics simulations on bulk glasses, glass–crystal nanocomposites, and nanocrystals of Cu64Zr36 with varying crystalline volume fractions induced by long-time thermal annealing. We find that the grain boundary phase behaves like a metallic glass under constraint from the abutting crystallites. The transition from glass-like to grain-boundary-mediated plasticity can be classified into three regimes: (1) For low crystalline volume fractions, the system resembles a glass–crystal composite and plastic flow is localised in the amorphous phase; (2) with increasing crystalline volume fraction, clusters of crystallites become jammed and the mechanical response depends critically on the relaxation state of the glassy grain boundaries; (3) at grain sizes $\ge 10\text{nm}$, the system is jammed completely, prohibiting pure grain-boundary plasticity and instead leading to co-deformation. We observe an inverse Hall–Petch effect only in the second regime when the grain boundary is not deeply relaxed. Experimental results with different grain boundary states are therefore not directly comparable in this regime.

Abstracts:The anisotropic coefficient of thermal expansions (CTEs) of SAC305 grain are measured using a full-field in-plane displacement measurement technique. Theoretical relationships among (1) the transversely-isotropic CTEs, (2) the surface strains of a specimen containing a single grain with arbitrary orientation, and (3) the direction of a grain orientation are derived first. Cube shape specimens that contain a single SAC305 grain are fabricated by controlling cooling rates. Thermally-induced displacements fields with a sub-micron resolution are documented on two perpendicular surfaces of the specimen as a function temperature, and the engineering strains are calculated from the displacement fields. Two directional CTE values are determined from the theoretical relationships. The direction of the c-axis is also obtained during CTE calculations. The validity of the measurement is corroborated by comparing the c-axis direction obtained from the experiment with a grain orientation measured by the electron backscatter diffraction (EBSD) method.

Abstracts:Nanocrystalline metals are distinct from traditional engineering materials due to their high concentration of grain boundaries and corresponding structural disorder at grain boundaries. The effect of local disorder in nanocrystalline materials manifests in ways reminiscent of fully amorphous materials, such as mesoscale shear localization and pressure-dependent yielding, owing to the high concentration of grain boundaries and their predominance in governing plasticity. Relaxation processes in nanocrystalline materials that facilitate reconfigurations of grain boundaries and lower their energy, such as low temperature annealing, have been shown to enhance mechanical strength. However, processes that raise the energy of a nanocrystalline metal have not been observed, limiting the tunability of properties and the prospect for suppressing shear localization. Here, we use femtosecond laser processing as a unique non-equilibrium process that can generate complex stress states due to ultrafast electronic excitation and subsequent relaxation events. Experiments on nanocrystalline Al-O and Cu-Zr alloys indicate that sub-ablation femtosecond laser pulses cause up to an 87% reduction in hardness with no change in grain size, which can be ascribed to grain boundary-mediated processes. Parallels between our results and rejuvenation processes in glassy systems will be discussed in the context of controlling metastable structural configurations through novel processing routes.

Abstracts:Electron back-scattering diffraction was used to track the microstructure evolution of a fully annealed Fe-24Mn-3Al-2Si-1Ni-0.06C TWinning Induced Plasticity (TWIP) steel during interrupted reverse (tension-compression) loading. Direct observation of the same selected area revealed that all deformation twins formed during forward tension loading (0.128 true strain) were removed upon subsequent reverse compression loading (0.031 true strain). Consequently, the present study provides the first unambiguous experimental evidence of de-twinning during the reverse loading of a polycrystalline TWIP steel. The reverse loading behaviour was simulated by a dislocation-based hardening model embedded in the Visco-Plastic Self-Consistent (VPSC) polycrystal framework taking into account the accumulation and annihilation of dislocations and back-stress effects. The model has been extended to account for the processes of twinning and de-twinning, as well as the twin barrier effect under load reversal. A new formulation based on the changes in the dislocation mean free path is proposed to track twin lamellae generation/annihilation throughout deformation along with its associated effect on hardening.

Abstracts:Precipitates play significant roles in materials design but in-depth understanding of the evolution of nano-scale precipitates in complex martensitic steels is still limited. In this study, the evolution of nano-precipitates and their effects on the mechanical properties of 17-4 precipitation hardening stainless steel (17-4 PH SS) aged at 450 °C were investigated by high-resolution transmission electron microscopy (HRTEM) and atom probe tomography (APT). The results from APT revealed that the abundant nucleation of Cu-rich clusters was responsible for the initial significant hardening effect during aging. Core-shell structured Cu-rich precipitates (CRPs) were formed at peak aging condition. As aging was prolonged, co-precipitation of Ni, Mn, Si and Nb-rich precipitates (NMSN) and CRPs occurred at the expense of core-shell CRPs precursors with untwinned 9R structure. Ni, Si and (Mn, Nb) atoms were preferentially segregated at the (009)9R plane of twinned or W-shaped twinned CRPs, with atom ratio of 16:7:6. Furthermore, as CRPs became coarser, Cr atoms were rejected from CRPs into the matrix enabling nucleation of Cr-rich α′domains. The density and size of Cr-rich α′ domains increased as aging was prolonged to 200 h. Finally, the individual contributions of CRPs and Cr-rich α′ domains were calculated. Overall, these findings indicated the evolution of multiple precipitates and their interactive effects, which can be helpful for the design and fabrication of next generation PH steels for wide applications.

Abstracts:We investigated the microstructure of the Dy-free high coercivity sintered magnet that was developed by optimizing the chemical composition of Nd-Fe-B sintered magnet with 0.1 at.%Ga doping in order to understand the reasons for the high coercivity of μ 0 H c  = 2.0 T and the improved squareness of 0.95 compared to a recently developed 0.5 at. %Ga doped magnet. The as-sintered 0.1Ga sample shows a relatively high coercivity of 1.5 T owing to the finer grain size of ∼3.3 μm and the higher Al concentration up to 1.3 at.% compared to post-sinter annealed 0.5Ga magnet that has a grain size of ∼5.5 μm and Al concentration of 0.9 at. %. The post-sinter annealing leads to a substantial coercivity increase from 1.5 to 2.0 T due to the formation of a thick and crystalline intergranular grain boundary (GB) phase containing 30-60 at.% of Nd. The trace Ga addition of 0.1 at.% improved the wettability of the liquids and facilitated the formation of thick GB phase during the post-sinter annealing. The good squareness is mainly attributed to the ferromagnetic nature of the intergranular GB phase as well as the strong crystal alignment to the easy axis. Finite element micromagnetic simulations of the demagnetization processes of the models incorporating experimentally determined GB thickness and chemistry well explain the simultaneous achievement of the high coercivity and good squareness observed in this magnet in contrast to the very poor squareness observed in the post-sinter annealed 0.5Ga sample.