M.H. Yoo

Application of texture simulation to understanding mechanical behavior of Mg and solid solution alloys containing Li or Y

Abstract The viscoplastic self-consistent model was used to interpret differences in the mechanical behavior of hexagonal close packed magnesium alloys. There are only subtle differences in the compression textures of magnesium and its solid solution alloys containing lithium or yttrium. However, the plane strain compression textures of the alloys showed an increasing tendency for the basal poles to rotate away from the “normal direction” towards the “rolling direction”. Texture simulations enabled these distinctions to be attributed to the increased activity of the non-basal 〈 c + a 〉 slip mode. The alloys had improved compressive ductilities compared to pure magnesium, and the increased c + a slip mode activity provides a satisfying explanation for this improvement, since it can accommodate c-axis compression within individual grains. Accounting for individual deformation mode hardening enabled the flow curves to be simulated and the anisotropic plastic response of textured wrought alloys to be mechanistically understood and predicted.

On the origin of deformation microstructures in austenitic stainless steel: part I—microstructures

Abstract A comprehensive characterization of room temperature deformation microstructures was carried out by transmission electron microscopy for ion irradiated and deformed AISI 316LN austenitic stainless steel. Deformation microstructures were produced by a recently developed disk-bend test method and also by a uniaxial tensile test. Cross-slip was dramatically suppressed by the radiation-induced defects and slip occurred predominantly by planar glide of Shockley partial dislocations. Deformed microstructures consisted of piled-up dislocations, nanotwin layers, stacking faults, and defect-reduced dislocation channel bands. Analyses revealed that all these features were different manifestations of the same type of deformation band, namely a composite of overlapping faulted layers produced by Shockley partial dislocations.

On the origin of deformation microstructures in austenitic stainless steel: Part II—Mechanisms

Abstract Deformation microstructures of austenitic stainless steels consist of profuse pile-up dislocations, stacking faults, nanotwins, and defect-reduced channels as demonstrated in the Part I companion paper of this title [ Acta mater. , 2001, 49 (16), 3269–3276]. Yet the mechanisms of such microstructural evolution are poorly understood. Thus, a comprehensive study was conducted to understand the underlying physics of deformation in metals using radiation damage as a tool. It was found that, for energetic reasons, glide dislocations dissociated into Shockley partials during glide. Consequently, the interaction between a glide dislocation and radiation-induced defects occurs by a two-step reaction, first with the leading partial and then with the trailing partial. With this insight, the origin of deformation microstructures was explained by analyzing Shockley partial dislocations and their interactions with radiation-induced Frank loops.

Yielding and plastic flow behavior of B2-type Fe-39.5 mol.% A1 single crystals in compression

Yielding and plastic flow behavior of B2-type Fe-39.5 mol.% Al were investigated by deforming single crystals in the temperature range from room temperature to 1073 K. Yield stress exhibits a distinct positive temperature dependence followed by a peak for all the orientations examined. The temperatures of the anomalous peak are located between 823 and 873 K for all the orientations except the near-[111] orientation. Only for the near-[111] orientation the peak temperature is located between 773 and 823 K. The slip transition from 〈111〉 direction at intermediate temperatures to 〈100〉 at high temperatures occurs at the peak temperatures. The yield stress at 773 K exhibits a strong orientation dependence and has a good correlation with respect to non-glide stress component. Specimens having compression axes of χ ⪖ 0° exhibit serrations in stress-strain curves below the peak temperatures, whereas the serrations are not observed in those of χ < 0°. In addition, a yield drop is observed around the peak temperatures for all the orientations. Below the peak temperatures, even as low as at room temperature, the yield stress hardly depends on the applied strain rate. This indicates that the motion of 〈111〉-type superdislocations has very small strain-rate sensitivity in the temperature range. On the other hand, there is a strong strain-rate dependence at the peak temperature and above, indicating that the motion of 〈100〉-type dislocations is strongly rate sensitive. The positive temperature dependence of yield stress in B2 FeAl is discussed on the basis of the present results.

Tensile properties of B2-type Fe-39mol%Al single crystals at elevated temperatures

Abstract Tensile properties of B2-type Fe-39mol%Al single crystals have been investigated between room temperature and 923 K. Positive temperature dependence of yield stress was observed in all the orientations examined. Orientation-dependent phenomena in the temperature range where temperature dependence of yield stress is positive, viz., orientation dependences of yield stress and critical resolved shear stress (CRSS), peak stress and peak temperature, and serrated flow behavior, shows trends opposite to those obtained in compression by our previous work. Based on the present results, it is concluded that those orientation-dependent phenomena represent intrinsic characteristics of deformation behavior associated with the positive temperature dependence of yield stress.

Phase stability of intermetallics in the AlTi system: A first-principles total-energy investigation

The phase stability of ordered Ti3Al, TiAl, and TiAl3 alloys is studied in various structures with first-principles total-energy calculations. The observed structures of these alloys are found to have lower energies than other competing structures. The calculated heats of formation are in good agreement with experiment. The most significant finding of these calculations is that for Ti3Al the cubic L12 structure is only slightly higher in energy than the observed hexagonal DO19 structure, i.e. 0.01 eV/atom. Based on the total-energy calculations, it is suggested that additions of ternary elements may be able to stabilize Ti3Al into the cubic L12 structure, which will presumably improve the ductility.

Cleavage fracture of ordered intermetallic alloys

Abstract Fundamental aspects of cleavage fracture behavior of ordered intermetallic alloys are analyzed on the basis of first-principles total energy calculations of cleavage strength (and energy) and anisotropic elasticity modeling of crack tip slip (and twinning). Intrinsic brittleness of Al 3 Sc and NiAl is rationalized from the comparative analyses with Ni 3 Al and FeAl, respectively. The cleavage habit planes of (110) for NiAl and (100) for FeAl are correctly predicted in terms of the anisotropy of cleavage energies and elastic properties of dislocations. Boron strengthening in Ni 3 Al and hydrogen embrittlement in FeAl are discussed in view of the calculated results.

Bonding mechanisms and point defects in TiAl

Abstract The bonding mechanism and point defect structure in TiAl were investigated using a first-principles quantum mechanical calculation. The most remarkable feature found in the calculated binding charge density is the polarization of the p-electron at the aluminum sites, which gives rise to a large bond bending force between the Ti and Al layers. For the point defects, the absence of structural vacancies is predicted in TiAl, and the deviations from stoichiometry are accommodated by the substitutional antisite defects on both sublattices. High vacancy formation energies are obtained, which are closely related to the strong TiAl bonding and the similar atomic radii of Ti and Al.

Yielding and flow behavior of Mo5Si3 single crystals

Abstract Deformation behavior of Mo 5 Si 3 was studied using single crystals under compression. It was found that yielding and flow behavior were strongly dependent on temperature, strain rate and crystal orientation. A stress exponent and an activation enthalpy of lower yield stresses were estimated to be ≈6 and 4.5 eV, respectively.

Interfacial energies in two-phase TiAl-Ti3Al alloy

Abstract The intrinsic values of interfacial energies based on first-principles calculations, including atomic relaxation, were obtained for the three types of γ/γ interfaces and the α 2 /γ lamellar boundary in two- phase TiAl alloy. The pseudo-twin boundary energy is highest, Γ P = 270 mJ/m 2 , and the true-twin boundary energy is lowest, Γ P = 60 mJ/m 2 . Planar fault energies at pseudo-twin and 120°-rotational interfaces are markedly different from those in the bulk of the γ-phase, i.e., approximately, E APB and E CSF decreases by more than 30% and E SISF increases by threefold. Enhanced mobility of ordinary dislocations along the γ/γ and α 2 /α interfaces (except true twin boundaries) is predicted based on the reduced E CSF values at the interfaces.