K. Hagihara

Plastic deformation behaviour and operative slip systems in Ni3Nb single crystals

Temperature and orientation dependence of yield stress and operative slip system in Ni{sub 3}Nb single crystals were examined. Four slip systems of (010)[100], (001)[100], {l_brace}201{r_brace}(10{bar 2}) and (010)[001] and three twinning systems of {l_brace}011{r_brace}{l_angle}0{bar 1}1{r_angle}, {l_brace}211{r_brace}{l_angle}{bar 1}{bar 0} 7 13{r_angle} and {l_brace}012{r_brace}{l_angle}0{bar 2}1{r_angle} were activated depending on the crystal orientation and tested temperature. Anomalous strengthening was observed for some orientations where (010)[100] or {l_brace}201{r_brace}{r_angle}10{bar 2}{r_angle} slip was operative. A remarkable increase in the critical resolved shear stress (CRSS) for the (010)[100] slip occurred over a wide temperature range from {minus}196 to 800 C. the CRSS did not satisfy Schmids law and the strain rate sensitivity of flow stress at a small plastic strain was negligible between room temperature and 700 C where an anomalous stress increase occurred. The anomalous strengthening mechanism in Ni{sub 3}Nb crystals deformed by [100] dislocations was discussed on the basis of anisotropy of APB energy on (010) and (001) in analogy with that in Ni{sub 3}Al with the L1{sub 2} structure.

Plastic deformation behaviour in Ni3Ti single crystals with D024 structure

Abstract The plastic deformation behaviour of Ni 3 Ti single crystals with the D0 24 structure was investigated in compression tests. Two slip planes of the type {1 1 00} and (0001) were selected depending on crystal orientation. Temperature dependences of the critical resolved shear stress (CRSS), strain rate sensitivity of flow stress, activation volume and dislocation structure were examined. Anomalous strengthening slightly appeared for the {1 1 00} 2 0> prism slip system in a narrow temperature range between 300 and 500 °C. Accompanying the yield stress anomaly (YSA) were small jogs formed on 1/3 2 0> screw dislocations, acting as pinning points for motion of the dislocations and resulting in a zigzag shape of dislocation lines inclined to the Burgers vector by ±15~20°. In contrast, a remarkable YSA was found for the (0001) basal slip system over a wide temperature range between −196 and 800 °C. At low temperatures at and below 300 °C, 1/3 1 1 0> superlattice dislocations were dissociated into two 1/3 1 0> super-Shockley partials bounded by an SISF after unloading. Above 300 °C, the dissociation mode of 1/3 1 1 0> superlattice dislocations changed to APB-type splitting accompanied by a large increase in CRSS. The strong YSA was caused by the Kear-Wilsdorf locking of 1/3 1 1 0> screw dislocations from the (0001) slip plane onto the {01 1 0} planes.

Plastic deformation behavior of Mg97Zn1Y2 extruded alloys

Abstract The mechanical properties of the Mg97Zn1Y2 extruded alloy containing the long-period stacking ordered phase, the so-called LPSO-phase, with a volume fraction of 24%-25%, were examined by compression tests and cyclic tension-compression deformation tests. The plastic behavior of the extruded alloys with compositions of Mg99.2Zn0.2Y0.6 and Mg89Zn4Y7 (molar fraction, %), which were almost the same compositions of Mg matrix phase and LPSO phase in Mg97Zn1Y2 Mg/LPSO two-phase alloy, respectively, were also prepared. By comparing their mechanical properties, the strengthening mechanisms operating in the Mg97Zn1Y2 extruded alloy were discussed. Existence of the LPSO-phase strongly enhanced the refinement of Mg matrix grain size during extrusion, which led to a large increment of the strength of alloy. In addition, the LPSO-phases, which were aligned along the extrusion direction in Mg97Zn1Y2 extruded alloy, acted as hardening phases, just like reinforced fibers.

Plastic deformation behavior of oriented Ni3Sn crystals with D019 structure

Abstract Plastic deformation behavior of oriented Ni3Sn crystals with the D019 structure was investigated in a temperature range from −196 to 900 °C. 1/3〈2 1 1 0〉 dislocations operate on (0 0 0 1) basal and {1 1 0 0} prism planes depending on temperature. Anomalous strengthening caused by the Kear–Wilsdorf locking on {0 1 1 0} clearly appears in condition of basal slip.

Mechanical Properties of C40-based Ternary Mo(Si,Al)2 and Quaternary (Mo,Zr)(Si,Al)2 Silicides

Refractory silicides with transition metals are of interest as structural materials operating at very high temperatures to improve energy efficiency. MoSi{sub 2} is particularly attractive because of its high melting point (2,030 C), relatively low density (6.24 g/cm{sup 3}), superior oxidation resistance and high thermal conductivity. Nevertheless, MoSi{sub 2} still has several problems which must be overcome before structural application. In this paper an attempt to improve the ductility, toughness and high-temperature strength of C40-based MoSi{sub 2} silicides was made by controlling additional Al and Zr contents in order to change the ductility, species of the constituent phase and the volume fraction of each phase.

Operative slip systems and anomalous strengthening in Ni3Nb single crystals with the D0a structure

Abstract The plastic deformation behavior of Ni 3 Nb single crystals was examined in tension and compression to determine the operative slip and twinning systems, and to explore the anomalous strengthening behavior. A strong temperature dependence of the CRSS for both the slip and twinning systems was observed, which was dependent on the sample orientation. Anomalous flow behavior was also observed in Ni 3 Nb crystals deformed by (010)[100] and (001)[100] slip. The anomalous strengthening mechanism is discussed on the basis of both the anisotropy of APB energy and the formation of dragging atmosphere around moving dislocations.

Effects of the anisotropy of the anti-phase boundary energy on the yield-stress anomaly in Ni3 X compounds with close-packed crystal structures

This paper is aimed at clarifying the effects of the anisotropy of the anti-phase boundary (APB) energy on the yield-stress anomaly (YSA) of five distinct Ni3(Ti, Nb) single crystals with various long-period ordered structures, in which the APB energy on the non-close-packed (non-CP) plane varies with changes in the stacking sequence of the close-packed planes. The APB energies of interest are estimated, based on a pair-wise interaction parameter model to the second neighbour. The YSA can be categorized in two groups of two and three compounds, within which the mechanical properties are nearly independent of composition. The microstructures resulting from deformation at 700°C were compared. It is verified that the YSA is dominantly controlled by differences in the probability of locking by a Kear–Wilsdorf mechanism, which itself is governed by the anisotropy of APB energy in the non-CP planes relative to the primary slip plane.

Plastic deformation behavior and operative slip systems of ZrSi2 single crystals with C49 type of structure

Plastic deformation behavior of ZrSi2 single crystals with C49 type of structure was investigated at temperatures up to 1500 °C in comparison with other silicides with C11b, C40 and C54 types of structures. [1 0 0](0 1 0), [0 0 1](0 1 0), [1 0 1](0 1 0), and [0 0 1](1 0 0) slip systems were determined to operate and their CRSSs monotonously decreased with increasing temperature.

Cyclic deformation behavior of Ni3Nb single crystals deforming by slip on (010)[100]

Abstract Cyclic deformation behavior in Ni 3 Nb single crystals deforming by slip on (010)[100] was investigated. Fatigue tests were performed in a symmetrical push/pull mode at a fixed total strain amplitude (Δ e ) from ±0.06 to ±0.30% at ambient temperature. The cyclic hardening behavior varied below and above Δ e =±0.15%. In the low strain amplitude group, the maximum stress amplitude reached a saturated value after initial strong hardening, while the cyclic hardening behavior in the high strain amplitude group were divided into three stages including the initial strong hardening. This initial strong hardening is related to the exhaustion hardening caused by the Kear–Wilsdorf locking. The hardening mechanism in each stage is discussed focusing on the cyclic stress-strain behavior, slip trace morphology and dislocation structure.

First-principles investigation of phase stability and electronic structure of tetragonal (P4/m) Ga3− x Al x Ti2 (x = 0–3) compounds

We report a first-principles study of the phase stability and electronic structure of tetragonal (P4/m) Ga3− x Al x Ti2 (x = 0–3) compounds to investigate the conspicuous absence of Al3Ti2 in the Al–Ti phase diagram while isostructural Ga3Ti2 is stable in the Ga–Ti phase diagram. It is found that Ga3Ti2 is the most stable phase compared to its ternary derivatives (Ga3− x Al x )Ti2, with the stability of Al3Ti2 being the lowest. Furthermore, the equimolecular phase mixture of γ-TiAl and r-Al2Ti is found to be more stable than Al3Ti2. By analysing the total and partial density of states, the valence charge transfer and the valence charge density distributions, we substantiate our conclusions from the perspective of the electronic structure.