| The shape memory effect of traditional shape memory alloys (SMAs), such as NiTi andCuZnAl alloys, is attributed to the thermoelastic martensite transformation induced bytemperature changes. The frequency of shape memory response in such alloys is significantlyaffected by their thermal conductivities and heat dissipation conditions. Therefore, the lowworking frequency of traditional SMAs hampers their further applications in themanufacturing of intelligent sensors and actuators where high responsive rate is required. Inrecent years, a new kind of ferromagnetic shape memory alloy (FSMA), e.g., Ni2MnGa alloy,was developed sucessfully. The shape memory behavior of FSMAs is produced by means ofthe reorientation of martensite twin variants induced by external magnetic fields, exhibitingsuperior characteristics such as high responsive rate and large strain output achieved byproper material design. Up to the present, studies on Ni2MnGa alloy mainly focused onthermodynamics, transformation kinetics, mechanical and magnetic properties of the materials,while limited attention has been paid to the crystallography of martensite transformation. Adeep understanding of the transformation mechanism is essential, since one can predict andcontrol the microscopic structures and hence optimize the macroscopic properties andperformance of the materials. It is shown that the shape memory effect of SMAs is stronglyrelated to their crystallographic features of martensite transformation, therefore, a systematicand thorough study on the martensite transformation crystallography in Ni2MnGa alloy isdesired.The topological model of martensite transformation describes the interface separating theparent and martensite phases based on interfacial dislocation theory, in addition to predict notonly the typical crystallographic features of the transformation, i.e., habit plane index,orientation relationship and shape strain, but also the interfacial dislocation networkparameters. In this thesis study, expressions of the strain fields of a set of edge, screw andmixed dislocations in various configurations are deduced firstly, according to the linear elasticstrain field formulations of a single edge, screw and mixed dislocation, so that the long-rangeeffect of a dislocation network can be obtained. Therefore, the assertion that the coherentstrains can be fully relieved by a network of interfacial dislocations derived by theFrank-Bilby equations is verified analytically. Secondly, the crystallographic constraint ofcomplete relief of coherent strains by a network of disconnections and lattice invariantdeformation (LID) dislocations is derived under the framework of the topological model, so that a constraint equation associated with the twist angle ω for determining the optimum valueωois established, referred to as the optimum twist criterion. The novel criterion is applied tothe study of the crystallography of the classical FCC to BCC martensite transformation in theferrous alloys. The predictions show good agreement with the results obtained byexperimental approaches and the phenomenological theory of martensite crystallography(PTMC). Finally, martensite transformation from the Heusler parent phase to non-modulatedmartensite product in Ni2MnGa alloy is investigated by using the improved topologicalmodel, where the dislocation structure of the interface separating the parent and martensitephases is obtained, as well as the typical crystallographic features of martensitetransformation, namely the habit plane indices, orientation relationship and macroscopicshape strain. The predictions so obtained are compared with that determined by thephenomenological theory.In previous studies, the determination of the twist value invokes thehabit-plane-matching method, where the calculated habit plane needs to be compared withexperimental measurements so that one can select the twist value by assessing the extent ofmatching of the habit plane data, which is relying on experimental results strongly. This maybecome infeasible when the alloy of interest lacks relevent experimental measurements.Moreover, the twist determined by the habit-plane-matching method might not guarantee thatthe coherent strain is accommodated completely by the interfacial dislocation network. Inother words, short-range diffusion at the interface is required when the LID dislocation needsreorientation to accommodate the coherent strain completely. On the other hand, theconfiguration of the interfacial dislocation network for fully relieving the coherent strain canbe identified in the first place, i.e., an equation is formulated to describe the coincidence ofthe LID dislocation line direction and the intersection between the dislocation glide plane andhabit plane, so that the optimum twist can be determined quantitatively. Hence, it not onlyallows the twist value to be determined independently without selecting by comparisons, butalso ensures that the coherent strain can be accommodated fully without long-or short-rangediffusion invoked for interfacial dislocation to reorient.In Ni2MnGa alloy exhibiting an non-modulated martensite phase, the optimum twist isidentified to be ωo=-0.023o when choosing the N-W relationship as initial orientation, andthe habit plane index is determined as (0.690,-0.090,0.718)P; the orientation relationship isrefined as(111)P0.548o away from(101)Mand[211]P0.023o away from[101]M. Besides,the plastic strains produced by the motion of disconnections, LID dislocations and coherency dislocations are deduced according to the formulations describing the distortion raised when aset of dislocations sweeps out of the material. Combining the low-angle rotation by theout-of-habit-plane components of dislocations, the total shape strain is obtained, where themacroscopic shear direction is calculated as [-0.658,0.118,0.744]Pwith its shear magnitudeof0.125. Moreover, the parameters of network of disconnections and LID dislocations areobtained as well, including the Burgers vectors, line directions and spacings of interfacialdislocations, where the spacings are0.512nm and0.631nm, respectively. |
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