The effect of thermal cycling and stress-assistant ageing two-way shape memory effect in [123]-oriented Co40Ni33Al27 single crystals

The effect of thermal cycling through an interval of B2-L10 martensitic transformation (MT) under action of external stress and tensile stress-assistant ageing on the two-way shape memory effect in [bar 123]-oriented Co40Ni33Al27 (at.%) single crystals are investigated. For the first time it is experimentally established that tensile stress-assistant 100 MPa ageing at 573 K for 1 h along [bar 123]-direction of Co40Ni33Al27 single crystals creates the necessary conditions for two-way shape memory effect (TWSME) with the reversible strain up to epsilon=2.4 (±0.3)% at cooling/heating. The TWSME in quenched [bar 123]-oriented Co40Ni33Al27 single crystals can be induced by thermal cycling through an interval of B2-L10 MT under action of constant external stress 50 MPa with the reversible strain less than 1%

Role of thermally-stable deformation twins on the high-temperature mechanical response of an austenitic stainless steel

In the present study, a two-step thermo-mechanical processing consisting of cold work and heat treatment steps was performed to increase the operating temperature of 316 austenitic stainless steels. A hierarchical microstructure of thermally-stable, nano twin bands was achieved forming into bundles in elongated grains. The mechanical response of the samples with this microstructure was evaluated through uniaxial tension tests at temperatures ranging from 20 °C to 500 °C and compared with those from the fully annealed samples. The results demonstrate that such hierarchical microstructure leads to a significant increase in the elevated temperature yield strengths due to the presence of nano-twin boundaries and resulting decrease in dislocation mean free path and increase in dislocation storage capacity. In fact, the yield strength ratio of the twinned and annealed samples increases with increasing temperature up to 500 °C, indicating the effectiveness of pre-existing thermally-stable twin boundaries as the strengthening source at temperatures as high as 0.46 homologous temperature. The hierarchical microstructure also led to irregular serrations through dynamic strain aging in the stress-strain response at 500 °C, which is attributed to the bi-modal microstructural length-scales present in the structure affecting the diffusion distances during dynamic strain aging. This structure also increases the tensile strength, and without a total loss in ductility, even though the flow stress of the twinned samples surpasses the tensile strength of the annealed samples, especially at elevated temperatures. Total hardening rate is consistently higher in the twinned samples as compared to the annealed samples, indicating the positive role of nano-twin boundaries in the dislocation storage capacity at elevated temperatures. Overall, the present study clearly demonstrate the positive role of thermally stable nano-twins on the elevated temperature mechanical response of austenitic stainless steels

The influence of hydrogen on the mechanical and functional properties of TiNi and TiNiFe single crystals

The effect of hydrogen on temperature dependence of critical stress σcr(T), the shape memory effect and superelasticity for [11]-oriented single crystals of Ti-50.7Ni (at.%) and [23]-oriented single crystals of Ti-49%Ni-1%Fe (at.%) alloys at tensile deformation has been studied. It was shown that the change of temperatures of the martensitic transformation, symmetry of thermal hysteresis and level of σcr(Ms) and σcr(B2) stresses are determined by the phase state of the material (martensite or austenite) in the process of hydrogenation

The effect of hydrogen on shape memory effect and superelasticity in single-phase nickel titanium single crystals

The effect of hydrogen on temperature dependences of axial stresses σcr(T), shape memory effect, and superelasticity of [1¯11]-oriented Ti-50.7% Ni (at %) alloy single crystals has been studied under tensile strain in a single-phase state. It is established that the level of σcr(Ms) and σcr(B2) stresses is determined by the state of hydrogen (in solid solution or titanium hydride particles) in the initial B2 phase. Superelasticity up to 2.0–2.5% occurs when hydrogen is present in the solid solution

Stress-induced thermoelastic martensitic transformations and functional properties in [011]-oriented NiTiHfPd single crystals

The stress-induced martensitic transformation in the [011]-oriented Ni45.3Ti29.7Hf20Pd5 (at. %) single crystals in as-grown, homogenized and aged states were investigated in compression. It is experimentally shown that heat treatments of single crystals result in increase in martensitic transformation temperatures, two-fold decrease in reversible strain and increase in strain-hardening coefficient. As-grown single crystals demonstrate large temperature range of superelasticity (up to 140 K), large reversible strain (up to 4.3%) and large work output in comparison with homogenized and aged crystals

Investigation of the two-way shape memory effect in [001]-oriented Co35Ni35Al30 single crystals

For the first time, a study of the two-way shape memory effect (the TWSME) for the quenched and aged at 673 K for 0.5 h [001]-oriented Co35Ni35Al30 single crystals in compression was carried out. In the quenched CoNiAl crystals, the TWSME with a reversible strain of (-3.1 ± 0.3) % was induced by training procedure in the superelasticity temperature range. The physical reason of the TWSME is the creation of internal stress fields due to the formation of dislocations next to γ-phase particles, and an interface between the B2-matrix and the γ-phase in the quenched crystals. It was experimentally shown that aging at 673 K for 0.5 h under a compressive stress of 100 MPa along the [110] direction creates the conditions necessary for observing the TWSME, without additional training with a reversible strain of (+2.2 ± 0.3) %. The conditions for observing the TWSME are determined by the action of the internal stress fields of 25 MPa created by the oriented arrangement of the dispersed particles in the stress-assisted aged crystals. The TWSME is not observed in the stress-free aged crystals with non-oriented precipitation of particles, both before and after training

Effect of one variant of Ti3Ni4 particles on stress-induced martensitic transformations in <111>-oriented Ti49.2Ni50.8 single crystals

In the present study the effects of stress-assisted aging of the Ti49.2Ni50.8 single crystals oriented along [11] direction on the stress-induced B2-R-B19' thermoelastic martensitic transformations and superelasticity are investigated. It is experimentally established that aging at 823 K for 1h under compression stress of 150 MPa along [11] direction leads to the precipitation of one crystallographic variant of Ti3Ni4 particles of 350(±30) nm in size. Precipitation the single variant of Ti3Ni4 particle results in an appearance of homogeneous long-range internal stress field || ≈ 65 MPa, that defines the main features of stress-induced B2-R-B19' transformation and determines the increase in the characteristic temperatures of martensitic transformation and the existence of two-way shape memory effect

Stress-induced martensitic transformation in high-strength [236]-oriented Ni51Ti36.5Hf12.5 single crystals

The effects of heat treatment on the stress-induced B2-B19' martensitic transformations in the Ni51.0Ti36.5Hf12.5 single crystals oriented along [236] direction are studied. It is shown that in the annealed at 1323K for 4 h crystals, the temperature range of superelasticity increase almost twofold from 75K up to 135K as compared to the as-grown single crystal contained disperse particles of H-phase. The [236]-oriented Ni51.0Ti36.5Hf12.5 single crystals are characterized with high levels of applied compressive stress up to 1700 MPa in the as-grown state and 1900 MPa in annealed crystals for the completely reversible stress- induced B2-B19' martensitic transformation with reversible strain up to |εSE| =1.4%

Orientation dependence of the shape memory effect and superelasticity in ferromagnetic Co49Ni21Ga30 single crystals with γ'-phase particles

This paper reports on the orientation dependence of the shape memory effect and superelasticity in [001] and [overline 123] single-phase and aged at 623K, 1 hour single crystals of ferromagnetic Co49Ni21Ga30 (at.%) alloy with B2-L10 martensitic transformation. It was demonstrated that in single-phase crystals the values of reversible strain and the values of thermal and stress hysteresis depend on the crystal orientation. Precipitation of γ'-phase particles reduces the value of the shape memory effect and superelasticity and reduces their orientation dependence, increases the thermal and stress hysteresis in comparison with singlephase crystals

Superelastic behavior and elastocaloric effect in a Ni51.5Fe21.5Ga27.0 ferromagnetic shape memory single crystal under compression

Ni51.5Fe21.5Ga27.0 single crystals have been subjected to different heat treatments resulting in a different degree of L21 ordering. Superelastic response has been measured at different temperatures in compression mode. The mechanical behavior strongly depends on axis orientation. In the [001] direction, perfect superelasticity over a wide range of temperatures is found. For the [110] orientation, the material fails by brittle fracture short above austenite transformation finish temperature, Af. A linear dependence of the critical stress with temperature has been found in agreement with Clausius-Clapeyron equation. The slope does not significantly change with the degree of order, but it is notably affected by the crystal orientation. The microstructure of the samples after mechanical tests has been studied by transmission electron microscopy. The superelastic cycling produces dislocations with a Burgers vector that suggests local microplastic deformation of the martensitic phase. Finally, the adiabatic temperature change has been used to chacterize the elastocaloric effect in this alloy. The adiabatic cooling is found to be larger in the [110] than in the [001] orientation at 240 K. However, the brittleness of [110] samples avoid testing the adiabatic temperature change at room temperature. The adiabatic cooling in [001] orientation decreases systematically with temperature, which is related to decrease of the strain and entropy change of transformation