Abstract: Interfaces in martensites and ferroelastic crystals show internal structures which are not simply the interpolation of the two adjacent domains. These structures can influence solitary front propagation as observed for large depinning forces. They also contribute to local pinning of walls when the applied forces are close to the depinning threshold. Under these conditions, walls propagate in jerks and avalanches. Typical depinning is observed for very small forces in single ferroelastic needle domain. It is shown that jerks occur in elastically driven system both for planar walls (D=2) and for needle tips (which represents a line in the three dimensional crystal, D=1). The experimental power law exponents are ~ -2 for the energy exponent for collective avalanches, -1.3 for the elastic response function and -1.8 for an advancing needle domain in LaAlO3.
Abstract: There is now a great deal known about the atomic mechanisms of solid–state phasetransformations, and this knowledge can be exploited to determine the distribution of crystalorientations. It is possible to estimate accurately, the crystallographic texture, transformationstrains and details of the microstructure, particularly in the context of steels. The concepts havenow been applied to design metallic alloys which compensate automatically for the residualstresses which develop in engineering components when they are cooled heterogeneously fromelevated temperatures. Such materials are now in commercial use and represent an innovationresulting directly from phase transformation theory.
Abstract: A minor, but crucial, modification to the original version of the TM was recently proposed in order to ensure that the model predictions were entirely consistent with those of the phenomenological theory of martensite crystallography (PTMC). In the present paper an analysis based on the principles of the Frank-Bilby equation is used to derive this modified version of the two-dimensional TM equation for the habit plane orientation from first principles. A considerably simpler and more accurate expression for the tilt rotation is also presented. Finally, a simple method for using the TM approach to calculate the shape strain, its magnitude and direction is described. The results of the predictions made with this improved version of the TM are compared with those of the original TM and with the corresponding PTMC predictions.
Abstract: In Fe3Ga single crystals with the D03 structure, three types of pseudoelasticities based on dislocation motion, martensitic transformation and twinning take place depending on the heat treatment, the loading axis and the stress sense. In this paper, we report the detail of the transformation and twinning pseudoelasticities in the crystals focusing on the crystallography and the temperature dependence. In particular, the driving force for the twinning pseudoelasticity was discussed, focusing on the atomic arrangement. In Fe3Ga single crystals homogenized or solutionized in the α disordered region, the martensites with the 14M structure, containing numerous stacking faults were stress-induced during loading, while they disappeared during unloading by the reverse transformation, resulting in the transformation pseudoelasticity with small stress-strain hysteresis. In contrast, twinning pseudoelasticity caused by twinning and untwinning of 2.2T-type pseudo-twins appeared in the well-ordered D03 crystals, accompanied by a serrated flow in the stress-strain curve. The contribution of the twinning pseudoelasticity to strain recovery became significant at low temperatures at which the dislocation motion was difficult. It should be noted that the formation of the pseudo-twins could be regarded as a certain displacive phase transformation since the crystal structure of the twins became orthorhombic due to the twin shear without atomic shuffling. The free energy difference between the D03 matrix and the pseudo-twins resulted in the twinning pseudoelasticity. Moreover, the pseudo-twins were transformed into the perfect twins by annealing at 300 °C where the atomic shuffling could occur. The perfect twins remained even after complete unloading due to their low driving force for the pseudoelasticity.
Abstract: High damping capacity is one of the prominent properties of NiTi shape memory alloy (SMA), having applications in many engineering devices to reduce unwanted vibrations. Recent experiments demonstrated that, the hysteresis loop of the stress-strain curve of a NiTi strip/wire under a tensile loading-unloading cycle changed non-monotonically with the loading rate, i.e., a maximum damping capacity was obtained at an intermediate strain rate (ε.critical). This rate dependence is due to the coupling between the temperature dependence of material’s transformation stresses, latent-heat release/absorption in the forward/reverse phase transition and the associated heat exchange between the specimen and the environment. In this paper, a simple analytical model was developed to quantify these thermo-mechanical coupling effects on the damping capacity of the NiTi strips/wires under the tensile loading-unloading cycle. We found that, besides the material thermal/mechanical properties and specimen geometry, environmental condition also affects the damping capacity; and the critical strain rate ε.critical for achieving a maximum damping capacity can be changed by varying the environmental condition. The theoretical predictions agree quantitatively with the experiments.
Abstract: Among the different systems for high temperature shape memory alloys (SMA’s), equiatomic RuNb and RuTa alloys demonstrate both shape memory effect (SME) and MT temperatures above 800°C. For both systems, it is interesting to find a way to control the transformation temperatures while keeping the shape memory effect. One way to change the transformation temperatures is to change the composition in the binary alloys; another is to add a ternary element like Fe. The eight investigated alloys show two different space groups at room temperature. The monoclinic alloys undergo two successive displacive transformations on cooling, starting from the high temperature β phase field: β (B2) à β’ (tetragonal) à β’’ (monoclinic). The tetragonal alloys exhibit a single transition from cubic to tetragonal. A multiple twinned microstructure can be found in all alloys. Transformation temperatures decrease with lower Ru content and with the addition of Fe. The β’ à β transformation seems to be the main responsible for the SME. Compression tests performed in the martensitic phase give a quantitative result of the shape memory effect. In the binary alloys, the SME decreases with decreasing Ru content, which is in accordance with the evolution of the lattice parameters of martensites. A lower SME in the ternary alloys can also be linked to the lattice parameters and seems to be quite reliable to predict the evolution of the shape memory effect.
Abstract: High temperature shape memory alloys offer numerous potential applications in industrial domains like aeronautics. Even if up to now, none of the studied alloys have found a place in airplane turbines, research in this field is still active. Starting from the well-known “room temperature” shape memory alloy NiTi, it has been demonstrated that the addition of a ternary element such as gold in substitution of nickel greatly enhances the temperatures of the martensitic transformation. In the binary TiAu compound, the martensite start temperature can attain 875 K with satisfying reversibility and cycling stability. From lattice parameters measurements, it has been shown that the maximum transformation strain can reach 10.75 % for Ti47Au53 alloy, which is comparable to that of the NiTi alloy. However, to the best of our knowledge, quantitative measurements of the recoverable strain by shape memory effect are not available in the literature. We present here some quantitative results of shape memory effect associated to this phase transformation in Titanium-Gold alloys measured after compression tests.
Abstract: The deformation process can induce the precipitation of martensite in austenitic stainless steels. When shear stress is applied at temperatures near Ms, displacive transformation (martensitic transformation) mode is activated. When external stresses are applied, the work done contributes to a change in free energy either raising or lowering the Ms-temperature. Orientation relationships during austenite to martensite phase transformation were investigated in an austenitic stainless steel samples deformed by cold rolling and deformed in a tension test. EBSD (electron backscatter diffraction) and X-ray diffraction techniques were used to evaluate parent austenite texture and martensite texture after transformation. The observed orientation relationship between austenite and martensite was compared with the predicted orientation relationship by the phenomenological theory of martensite crystallography (PMTC). Aspects related to variant selection were discussed based on the criterion for the action of applied stress in the martensitic transformation postulated by Patel and Cohen. Results showed a very good agreement between measured and calculated results.
Abstract: Martensite in carbon steels forms in different morphologies, often referred to as lath andplate martensite. The alloy composition has a strong effect on the morphology, for instance in car-bon steels there is a morphological change of the martensite microstructure from lath martensite atlow carbon contents to plate martensite at high carbon contents. In the present work a decarburizedhigh-carbon steel, enabling the isolation of carbons' influence alone, has been studied in order to in-vestigate the changes in morphology and hardness. From the results it is concluded that there is acontinuous change of hardness with increased carbon content. The increasing hardness slows down atabout 0.6 wt%C before decreasing at higher carbon contents. This is in accordance with the change inmorphology since it was found that lath martensite dominates below 0.6 wt%C and the first units ofgrain boundary martensite and plate martensite appear above 0.6 wt%C. At high carbon contents thedominating morphology is plate martensite, but retained austenite is also present.