[en] Acta MaterialiaVolume 152, 15 June 2018, Page 300

[en] Acta MaterialiaVolume 152, 15 June 2018, Page 199

[en] Acta MaterialiaVolume 143, 15 January 2018, Page 318

[en] A series of six-component (FeCoNiCrMn)_100−xAl_x (x = 0–20 at.%) high-entropy alloys (HEAs) was synthesized to investigate the alloying effect of Al on the structure and tensile properties. The microstructures of these alloys were examined using transmission electron microscopy, and crystalline phase evolution was characterized and compared with existing models. It was found that the crystalline structure changed from the initial single face-centered cubic (fcc) structure to a duplex fcc plus body-centered cubic (bcc) structure and then a single bcc structure as the Al concentration was increased. Resulting from the structural changes there were also corresponding variations in tensile properties. In the single fcc region, alloys behaved like a solid solution with relatively low strength but extended ductility. In the mixed structure region, alloys behaved like a composite with a sharp increase in strength but reduced ductility. In the single bcc region, alloys became extremely brittle. In this study, close correlation between the microstructure and mechanical properties was also discussed and presented

[en] A mesoscale simulation by shear transformation zone (STZ) dynamics is used to analyze the initial stages of shear localization in amorphous metals, of which many details remain unresolved due to the difficulty in accessing the appropriate time- and length-scales. Examination of a constant strain-rate tensile test of a model amorphous metal reveals four different stages in the microscopic processes that lead to formation of a shear band. These stages are identified as: (i) STZ clustering, where potential nucleation sites form as STZ activations cluster in space; (ii) growth following nucleation, where a nascent shear band exhibits a propagating front that defines the shear band path; (iii) relaxation thickening, where the shear band thickens and relaxes the system down to the flow stress; and (iv) flow thickening, where the shear band exhibits prolonged thickening at the flow stress. The results are consistent with the literature but suggest that a shear band is defined by a front that propagates very quickly with negligible accumulated strain. This propagating front likely occurs below the resolution of current methods that only observe simultaneous slip in a shear band where significant strain can be accumulated at much slower rates. Analysis of a thermodynamic model also suggests a specific critical stress that is required to nucleate a shear band, after which the shear band is allowed to grow unconstrained

[en] Long-range internal back stresses developed during room temperature tensile deformation of titanium aluminide alloys have been determined. Dip tests were implemented during the strain-controlled tensile deformation and the resulting sample deformation was monitored in the relaxation regime of the machine. The internal stress was determined as the critical stress at which the inelastic sample relaxation is reversed, i.e. going from the tensile direction to the compression direction. The investigation involves a wide range of alloy compositions with a corresponding variation in the strength properties. For the alloys investigated, the internal stress is about 80% of the yield stress. The mechanical tests were coupled with electron microscopy examination of the defect structure in order to assess the strain accommodation occurring during deformation. Possible sources for the built up internal stresses are discussed

[en] The large signal macroscopic strain response during an applied bipolar electric field is calculated from field-dependent in situ XRD data using expressions for, first, a lattice strain contribution and, second, a ferroelastic strain contribution. The lattice strain contribution is estimated using a weighted average of the lattice strains for the observed reflections along the field direction. The ferroelastic strain contribution is calculated by integrating the lattice parameter changes weighted with the ferroelastic domain distribution over all orientations relative to the direction of the applied field. Structural parameters are determined by means of both single peak fitting and Rietveld refinements. A large ferroelastic contribution is found for tetragonal (1 − x)Ba(Zr_0.2Ti_0.8)O₃ − x(Ba_0.7Ca_0.3)TiO₃ materials that appears to be the dominant origin for the large signal macroscopic strain. The strong changes in lattice parameters and the decrease in tetragonality as a function of orientation and electric field also indicate a large influence of microstructure constraints on the macroscopic strain response. The total strain calculated from X-ray diffraction using both methods is in good agreement with macroscopic strain measurements

[en] The composite microstructure of two-phase (α/ω) shocked zirconium was studied in situ during heating (constant heating rate and isothermally) with high-energy X-ray diffraction techniques. The volume fraction of the metastable ω phase was monitored as the reverse phase transformation occurred: the start and finish temperatures being 470 and 550 K, respectively, during heating at 3 K min⁻¹. Moreover, isothermal transformation was observed when the shocked material was held at fixed temperatures from 420 to 525 K. Phase strains in each phase were monitored and separated in terms of thermal expansion and mechanical strains due to local phase constraints. Stresses in the α Zr were estimated to be a superposition of a hydrostatic component (of order −50 MPa) and uniaxial component (of order −600 MPa) along the c-axis. These stresses were relaxed during the reverse transformation. A high dislocation density was observed in both the α and ω phases in the as-shocked state. The dislocation density of the ω phase decreased preceding the reverse transformation, suggesting that it is the presence of the high concentration of defects in the ω phase which retarded the reverse transformation to the stable α phase and prevented the system from approaching equilibrium after the completion of the shock

[en] The presence of agglomerates during nanopowder sintering can be problematic and can limit achievable final densities. Typically, the practical solution is to use high pressures to overcome agglomerate breakdown strengths to reach higher packing fractions. The strength of agglomerates is often difficult to determine and makes processing parameters challenging to optimize. In this work, we used in situ transmission electron microscopy nanoindentation experiments to assess the mechanical properties of individual MgAl₂O₄ agglomerates under constant indenter head displacement rates. Electron microscopy revealed highly porous agglomerates with pores on both the micron and nanometric length scales. Individual agglomerate strength, at fracture, was calculated from compression tests with deformation behavior correlating well with previously reported modeling results. Macroscopic powder properties were also investigated using green-pressed pellets consolidated at pressures up to 910 MPa. The unexpectedly high strength is indicative of the role agglomerates play in MgAl₂O₄ nanopowder densification

[en] The mechanisms governing the reverse martensite (α′) to austenite (γ) transformation (α′ → γ) and the effect of prior precipitation on the austenite reversion are investigated for martensitic Fe–Mn alloys containing 5 and 10 wt.% Mn and their age-hardenable variants with the addition of 1 wt.% Pd, respectively. Dilatometric experiments employing heating rates between 0.5 and 200 K min⁻¹, atom-probe tomography measurements on continuously heated specimens and thermo-kinetic simulations were performed. On fast heating (200 K min⁻¹), the α′ → γ transformation appeared in a single stage and can be regarded as a partitionless and interface-controlled reaction. In comparison to the binary alloys, the transformation temperatures of the Pd-containing steels are considerably increased, due to precipitates which act as obstacles to migrating austenite/martensite interfaces. For low heating rates of 0.5 and 2 K min⁻¹, splitting of the α′ → γ transformation into two consecutive stages is observed for both the binary and the ternary alloys. With the assistance of thermo-kinetic simulations, a consistent description of this phenomenon is obtained. The first transformation stage is associated with the decomposition of the martensite matrix into Mn-rich and Mn-deficient regions, and the austenite formation is dominated by long-range diffusion. In the second stage, the austenite reversion mechanism changes and the Mn-depleted regions transform in a predominantly interface-controlled mode. This is corroborated by the results for the ternary alloys. The precipitates mainly impede the austenite formation in the second stage, which occurs over a considerably wider temperature range compared to the binary alloys