Yield Stress Of Single Crystals Research Articles

Deformation behaviour of nano-twinned single crystals of an Fe–19Cr–16Ni austenitic alloy

The deformation behaviour of an Fe–19Cr–16Ni austenitic alloy was studied by micro-tensile testing of small-scale single crystals. A detailed comparison of specimens with and without nano-twins was conducted. The study revealed that the deformation behaviour depends strongly on the crystallographic orientation of the parent crystal and the spacing between nano-twin bundles. Although the yield stress in single crystals without nano-twins was governed by Schmid's law, the slip transfer inhibition by twin boundaries contributed significantly to the strengthening in nano-twinned samples. With the parent crystal oriented favourably for single-slip, the dislocations in the parent crystal glided by cross-slip onto the primary plane in the twinned crystal. For the hard-oriented parent crystal with dense nano-twins, dislocation accumulated and led to necking at an early stage. For the same orientation but with sparse nano-twins, twin growth and nucleation was promoted by dislocation reactions with the twin boundaries. These findings suggest that control of the nano-twin structure can allow the desired deformation behaviour to be tuned.

Studying the character and mechanisms of thermal hardening for single crystals of alloys with L12 structure

This work discusses mechanisms of temperature dependence of the yield stress of single crystals with L12 superstructure, a feature of which is an anomalous multistage increase in yield stress with temperature. The description of the observed dependences is based on the superpositioning of six different mechanisms of thermal hardening and softening.

Asymmetry of the yield strength in single crystals of titanium nickelide

Based on the results of the experimental studies of the temperature dependence of the yield stress of single crystals of TiNi alloys and of the mechanism of the mechanical twinning of the B2 phase as a martensite-type local reversible phase transformation, the asymmetry of the mechanical behavior of these materials during tension and compression in the [001] direction has been investigated. It is shown that a specific feature of this mechanism is the fact that the critical mode of distortion which determines stresses during mechanical twinning is a homogeneous deformation of transformation of the Bain-deformation type. To analyze the asymmetry of these stresses during tension and compression, a concept of the factor of normal stresses reduced to the principal axes of the tensor of the homogeneous deformation of transformation is introduced. It is shown that this factor can be used for analyzing the asymmetry of the mechanical behavior of single crystals of TiNi alloys.

Contribution of point defects to the temperature anomaly of the yield stress in single crystals of the Ni3Ge alloy

The temperature dependence of the yield stress τ* Ni 3 Ge single crystals is studied. The temperature dependence τ*(T) in the high-temperature region (above 420 K) is found to be conditioned by thermally activated accumulation of the density of non-screw components of superdislocations. Interaction of point defects with edge dislocations and its effect on the temperature anomaly of the yield stress in Ni 3 Ge single crystals are analyzed. The calculated results are found to agree with experimental data.

Effect of orientation on the peak temperature of the yield-stress anomaly in single crystals of the Ni3Ge alloy

Temperature dependence of the yield stress τc(T) and shear stresses, as well as the orientation dependence of the peak temperature Tp of the yield-stress anomaly have been studied on single crystals of the Ni3Ge alloy. The peak temperature Tp corresponds to the maximum in the τc(T) dependence. Profiles of the Tp isolines have been determined based on the equality of the values of the yield stress in the octahedral and cube planes of cross slip. It is shown that in the case of the above equality it is possible to describe the orientation dependence of the peak temperature Tp of the temperature anomaly of the yield stress in single crystals of Ni3Ge.

Dislocation core effects on plasticity

The structure and/or the state of dislocation core often play a significant role in the glide process of the dislocation and hence control the plasticity of the crystal. We show various examples in this paper. (1) The yield stress of single crystals controlled by the Peierls mechanism for screw dislocation does not obey the Schmid law. (2) A narrow dissociation of the core can produce a plateau in the τy- T relation. (3) Duality of the core structure can produce «yield stress anomaly». (4) Multiphonon emission at the core of dislocation in semiconducting crystals by non-radiative recombination of electron-hole pairs enhances drastically the dislocation mobility.

The Effect of Room Temperature Prestrain on the Yield Stress of Single Crystals of γ’ in the Anomalous Regime

AbstractThe yield stress of a specimen prestrained at 294 K increases only slowly or is constant up to a temperature above which the yield stress of a virgin specimen is greater. Above that temperature, the yield stress tends to increase as for a virgin crystal. For sufficiently high prestresses, the yield stress anomaly disappears completely. The results are interpreted in terms of a model in which the operation of Frank-Read sources is rate controlling.

Effects of He + irradiation on the mechanical properties of molybdenum

The effects of 100 keV He ion irradiation on the mechanical properties of molybdenum single crystals were studied by examining the temperature- and dose-dependence of tensile behavior at 77 and 293 K. The yield stress of single crystals increased with increasing dose and the fracture stress decreased with increasing dose at 77 K. A ductility loss occurred in irradiated crystals at 293 K. These results show that the surface damage layer about 200 nm thick causes embrittlement at low temperatures and ductility loss at higher temperatures.

The yield stress of single crystals of α-phase Cu-Ni-Pd and Cu-Pd alloys

Single crystals of Cu- x at. % Ni- x at. % Pd [ x ═ 1 to 6] and Cu- x at. % Pd alloys [ x ═ 1, 2, 4 and 6] were deformed in tension at 293 K and 4.2 K. The crystals had the [331] {͞110} {͞1͞16} orientation which has two equivalent {͞1͞11} <011> and {͞1͞11} <101> primary glide systems. The concentration dependence of the critical resolved shear stress σ at the yield point for both alloy series was given by σ T ═ σ 0 ( T )+ A ( T ) c α , where σ 0 ( T ) and A ( T ) are constants for one alloy series at T K and c is the atomic concentration of the solute. The exponent α is 0.72 or 0.73 for the Cu-Ni-Pd and 0.61 or 0.62 for the Cu-Pd alloy series at 293 or 4.2 K. The data for both alloy series could be fitted to the relation σ 4.2K -σ 293K ═ 1.046+0.526σ 4.2K [MPa]. This establishes the stress equivalence behaviour of single crystals of these alloys.

Ni<SUB>3</SUB>Fe多結晶の機械的性質と規則化過程

Correlations between the tensile mechanical properties and ordering processes have been investigated for Ni3Fe polycrystals, containing iron in the range 20-30 at%. The yield stress first increased gradually, reached a peak, and then decreased to a constant value, being similar to the change of yield stress of single crystals. The most rapid change of the yield stress with ordering time occurred in 27.5 at%Fe alloys which had the highest order-disorder transformation temperature. The stoichiometry effect on the yield stress in the disordered state was interpreted by solid solution strengthening of Fe atoms in Ni while that in the ordered state was explained by defect strengthening of the anti-structure. The phenomenon of the discontinuous yielding accompaying Lüders deformation was found in the ordered state. As ordering proceeds, elongation first made a faint plateau and then decreased to a constant value, the level of which was lower than that of an initial (disordered) state. Correspondingly, the reduction in area of alloys decreased and the intergranular fracturing occurred in some areas. It was suggested that the elongation loss of Ni3Fe polycrystals was caused by the decrease of the grain boundary (cohesive) strength due to the change in atomistic structure and bond nature of the grain boundary plane created by ordering.

The yield stress of single crystals of α-phase copper-aluminium alloys

The composition dependence of the yield stress, σ , of single crystals of α-phase copper-aluminium alloys has been measured at 293 and 4.2 K under precisely defined experimental conditions to determine whether the Friedel relation with σ ∝ c 1/2 or the Mott-Labusch relation with σ ∝ c 2/3 applies. The best statistical fit gives a dependence of σ on c 1/2 for values of c between 0 and 0.15. A curve with σ ∝ c 2/3 can be fitted to the results for 0.003 ≤ c ≤ 0.15 with a small increase in the standard deviation, but a segment with σ ∝ c 1/2 must be introduced for 0 ≤ c ≤ 0.003. On the basis of these measurements it is not possible to decide between the validity of Friedel and of Mott-Labusch statistics for the concentrated alloys.

Deformation of single crystals of the L21 ordered Ag2MgZn

The orientation and temperature dependence of slip geometry and yield stress in single crystals of the L21 ordered Ag2MgZn has been studied in compression in the temperature range 290 to 580 K. The slip direction in Ag2MgZn is exclusively 〈1 1 1〉 in this temperature range, but the slip plane varies with crystal orientations; slip occurs on (¯2 1 1) for orientations near the [0 1 1]-[¯1 1 1] boundary, while for the other orientations in the [0 0 1]-[0 1 1]-[¯1 1 1] unit triangle it occurs on the (¯1 0 1). The critical resolved shear stress (CRSS) for slip on both the (¯1 0 1) [1 1 1] and (¯2 1 1) [1 1 1] systems increases abnormally with increasing temperature and reaches a maximum peak at about 0.92 of the critical temperature Tc, for the L21-type order. The peak temperature and the shape of the CRSS versus temperature curve are independent not only of crystal orientation but also of the operative slip system. It is suggested that the strength anomaly in Ag2MgZn be interpreted in terms of the mechanism based on the transition from unit dislocations to superdislocations.

The inhibition of deformation twinning by precipitates in a magnesium-zinc alloy

Abstract Deformation twinning can be inhibited in a Mg-5·1 wt. % Zn alloy by the presence of effective precipitates. This is true even in single crystals favourably oriented for twinning. This resistance to twinning precludes those premature fractures which frequently nucleate at twins. At 4·2°k twinning is highly favoured but effective precipitation postpones it until twice the glide strain observed in quenched crystals with no precipitates. Twin related fracture at 4·2°k is resisted in the fully hardened condition until three times the glide strain for fracture in the quenched condition. In polycrystals, grain size and texture as well as precipitation have profound effects on the deformation mechanism as a function of temperature. The flow stress in the quenched ease can decrease with decreasing temperature when grain size is large and the texture is appropriate for twinning. This behaviour is similar to that of quenched single crystals when twinning predominates. Conversely, the flow stress increases with decreasing temperature when the grain size is small and the texture is inappropriate for twinning. The latter behaviour is characteristic of slip controlled behaviour. Effective precipitation also encourages slip controlled behaviour. A Petch analysis showed very close correlation between the friction stress resisting twin growth in quenched polyerystals and the normal yield stress of single crystals undergoing twinning.

Temperature Dependence of Plastic Yield Stress of Single Crystals of Magnesium Oxide

The temperature dependence of the plastic yield stress of cleavage slabs of Norton Company magnesium oxide, containing approximately 0.2%, impurity, has been investigated up to 1000° C. for bending under three‐point loading. Previous thermal history has been found to be an important variable. In particular, annealing followed by a rapid quenching produces a marked softening, and at the same time causes a homogenization of the starting material. Such samples have been found to display inflections in strength versus temperature, corresponding to a minimum near 500°C. followed by a maximum near 700°C. At temperatures above those associated with the inflections the samples show a pronounced drop in load after yielding, and subsequent deformation occurs by jerky flow. The various observations have been discussed in terms of the interactions of dislocations with impurities that may be in precipitate or solution form, depending on past history and conditions at the time of testing. Some experimental information has also been obtained on the effect of strain rate on the plastic yield strength.