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High thermal cyclic stability is achieved in Ti-50.8at.%Ni alloy by fabricating hierarchical lath colonies along { 112 } B 2 planes using isothermal compression and aging. Transformation temperature shift between the 1st and 20th thermal cycle is 0.03∼0.27°C, which results from boundary strengthening of hierarchical lath colonies. Hierarchical lath colonies are controlled by dislocations. Compression induces b = 1 2 [ 1 ¯ 11 ] screw dislocations on [ 1 ¯ 11 ] (12 1 ¯) slip system with anti-phase boundaries (APBs) located on (12 1 ¯) B 2 plane, while aging promotes dislocation motion and rearrangement, and adjustment of atomic site occupation in local structure near APBs. When distance of APBs is large, one-direction lath-colony boundaries and one-direction lath colony form. When distance of APBs reduces, clover-shaped lath-colony boundaries form, and hierarchical lath colonies are recognized. If area of APB reduces, lath-colony boundaries and lath colony vanish. These findings show the potential for improving properties of NiTi alloy by dislocation engineering. [Display omitted] [ABSTRACT FROM AUTHOR]

R and B2 nano-domains are introduced in NiTi shape memory alloy (SMA) through isothermal compression followed by post aging, and high superelastic stability is achieved. Results show that recovery ratios for as-deformed and as-aged NiTi in 10 loading-unloading cycles are 50.31%±6.28% and 92.19%±0.94%, respectively. A three-step shear stress-induced martensitic transformation (SIMT) in and between (0 1 ¯ 1) B 2 plane is proposed, resulting in transformation of B2→B19′+metastable B2 for as-deformed NiTi, and B2+ R →R→B19′+metastable B2 for as-aged NiTi. During unloading, inverse martensitic transformation-induced stacking faults (SFs) along { 211 } B 2 planes, dislocation multiplication and residual B19′ formation are reasons for plasticity and superelastic instability, indicating B19′+metastable B2→B2+SFs occurs in as-deformed NiTi. Furthermore, B19′+metastable B2→R→B2+ R occurs without SFs because of complex stacking sequence of R phase, rationalizing the superelastic stability of as-aged NiTi. This research opens an avenue to improve superelastic stability of NiTi SMAs by inhibiting the formation of transformation-induced SFs. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: The deformation behavior in isothermal compression of Ti–6Al–4V alloy is investigated in the deformation temperatures ranging from 1093K to 1303K, the strain rates ranging from 0.001s−1 to 10.0s−1 at an interval of an order magnitude and the height reductions ranging from 20% to 60% at an interval of 10%. Based on the experimental results in isothermal compression of Ti–6Al–4V alloy, the effect of processing parameters and grain size of primary α phase on the strain rate sensitivity exponent m and the strain hardening exponent n is in depth analyzed. The strain rate sensitivity exponent m at a strain of 0.7 and strain rate of 0.001s−1 firstly tends to increase with the increasing of deformation temperature, and maximum m value is obtained at deformation temperature close to the beta-transus temperature, while at higher deformation temperature it drops to the smaller values. Moreover, the strain rate sensitivity exponent m decreases with the increasing of strain rate at the deformation temperatures below 1253K, but the m values become maximal at a strain rate of 0.01s−1 and the deformation temperature above 1253K. The strain rate affects the variation of strain rate sensitivity exponent with strain. Those phenomena can be explained reasonably based on the microstructural evolution. On the other hand, the strain hardening exponent n depends strongly on the strain rate at the strains of 0.5 and 0.7. The strain affects significantly the strain hardening exponent n due to the variation of grain size of primary α phase with strain, and the competition between thermal softening and work hardening. [Copyright &y& Elsevier]

Isothermal compression tests, electron backscatter diffraction (EBSD) observations and quantitative analysis are performed to investigate the effect of process parameters on the microstructure variables (i.e. recrystallized fraction, size of austenite grain and grain boundary misorientation) during isothermal compression of NiTi shape memory alloy. And the evolution of dynamic recrystallization and recrystallization texture for this alloy is thoroughly discussed in the present study. The results show that the dynamic recovery is a dominated softening mechanism at less than 0.5 strain. As the strain increases, the discontinuous dynamic recrystallization (original grain boundary bulging and strain-induced grain boundary migration) and the subsequent continuous dynamic recrystallization (subgrain rotation) also participate in the softening of NiTi shape memory alloy, leading to a steady state of flow behavior. In the deformation end stage, the grain refinement of the austenite phase due to the effect of dynamic recrystallization improves the flow stress of NiTi shape memory alloy. The texture intensity increases significantly with increasing strain during isothermal compression of NiTi shape memory alloy, and the occurrence of dynamic recrystallization during deformation promotes the formation of a strong γ-fiber texture {111}//CA. [Display omitted] • The critical strain for DRX is located in the range of 0.5~0.7 at 800 °C and 0.1 s-1. • CDRX mechanism is easily developed under the higher strains at 800 °C and 0.1 s-1. • The texture intensity in ODF maps increases with increasing strain. • A strong γ-fiber recrystallization texture {111} is formed during deformation. [ABSTRACT FROM AUTHOR]

In present study, isothermal compression, solution treatment, electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) observations were carried out to correlate the microstructure and deformation-solution parameters for GH4586 superalloy. And, the recrystallized mechanisms, twin evolution, γ′ precipitation and their interaction were clearly illustrated based on the analysis of the recrystallized fraction, size of γ grains, grain boundary misorientation and length fraction of twin boundaries. The results show that the process of dynamic recrystallization is controlled by the mechanisms of sub-grain rotation and strain-induced boundary migration while the process of static recrystallization is controlled by the mechanism of subgrain boundary migration. The evolution of twin boundaries during deformation is affected by two aspects: (i) the losing of twin's identity due to higher strain; and (ii) the nucleation and growth of new twins within the DRX grains. During deformation of GH4586 superalloy, fine γ′ precipitates have a hindered effect on the recrystallization nucleated from pre-existing subgrains and twin growth. Moreover, the recrystallization process has a noticeable influence on the formation of new twins. Conversely, the existing of these twins is also beneficial for the recrystallization. Image 1 • DRX is controlled by subgrain rotation and strain-induced boundary migration. • Static recrystallization is controlled by subgrain boundary migration. • New twins nucleate and grow within the DRX grains. • The existing of twins is also beneficial for the recrystallization. • Fine γ′ precipitates have a hindered effect on the recrystallization and twin growth. [ABSTRACT FROM AUTHOR]

Abstract: Isothermal compression tests were conducted for Ti–6Al–4V alloy in the α+β two-phase field, and the effect of deformation parameters on the precipitation of secondary α phase in the subsequent air cooling process following isothermal compression was investigated. The experimental results show that the morphology of secondary α phase transforms gradually from lamellar colony to equiaxed morphology with the increase of strain rate and height reduction and with the decrease of deformation temperature. And the precipitation mechanisms of the secondary α phase under different isothermal compression conditions are explored by noting the effect of the crystal defect (i.e., dislocation, grain boundary and subgrain boundary) density in the β matrix. [Copyright &y& Elsevier]