Jianfeng Wan

In situ atomic force microscope study of high-temperature untwinning surface relief in Mn-Fe-Cu antiferromagnetic shape memory alloy

The N-type untwinning surface relief associated with the fcc ↔ fct martensitic transformation (MT) was observed in the Mn81.5Fe14.0Cu4.5 antiferromagnetic high-temperature shape memory alloy (SMA) by in situ atomic force microscopy. The measured untwinning relief angles (θα|θβ) at the ridge and at the valley were different, and both angles were less than the conventional values. The surface relief exhibited good reversibility during heating and cooling because of the crystallographic reversibility of thermal-elastic SMAs. Untwinning shear was proposed as the main mechanism of the N-type surface relief. The order of the reverse MT was discussed based on the experimental measurements.

A chemical-structural model for coherent martensite/parent interface in Mn-based antiferromagnetic shape memory alloys

The martensite/parent coherent interface of Mn-based shape memory alloys (SMAs) is a significant part in the research of their martensitic transformation, reversible shape memory effect and magnetic shape memory effect. In the present work, a chemical-structural model was proposed to calculate the martensite/parent coherent interfacial energy of Mn-X (X = Cu, Fe) alloys. In this model, the coherent heterophase interfacial energy consists of chemical and structural parts. Resulting from the formation process of the heterophase interface, the chemical interfacial energy is expressed as the incremental value of bond energy, while the structural part is obtained by calculating the interfacial strain energy. The results show that the structural interfacial energy plays the chief role in the total interfacial energy, and the total interfacial energy decreases as the temperature rises when the alloy composition is fixed. In addition, the preferred orientation has noteworthy influence on the total interfacial energy. Using the proposed model, interfacial energy, interfacial entropy, interfacial enthalpy and interfacial heat capacity are found to be correlated with temperature and interface preferred orientation. Furthermore, the influences of alloy composition, modulus softening, and the index of the habit plane on the results were discussed.

Tunable elastic modulus in Mn-based antiferromagnetic shape memory alloys

Compared with the normal relation between temperature (T) and elastic modulus (E) in most materials, martensitic transformation (MT) and magnetic transition could result in the softening of elastic modulus (dE/dT > 0) within a narrow range of T ( 0. The present results may enrich approaches to designing new functional materials, e.g. the elastic and Elinvar alloys.

Strain-Induced Phase Transition in Martensitic Alloys: Phase-Field Simulation

Besides stress- and thermal-induced phase transition, strain can also be used to induce the structural transformation in martensitic alloy. Phase-field method was performed to simulate the nucleation and growth of martensite under the strain. During the martensitic transformation (MT) under the deformation, Md as a specific point represents the maximum temperature of stress- or strain- induced phase transition, which was confirmed by using Phase-field simulation in our work. The simulated results showed that the normal strain and the shear strain can induce MT between Ms and Md. The critical normal strain and the critical shear strain related to the temperature were calculated and compared. The mechanism of strain-induced MT was discussed and compared with that of thermal-induced MT at last.

Phase-Field Study of Microstructure and Plasticity in Polycrystalline MnNi Shape Memory Alloys

AbstractThe evolution of microstructure and plasticity in polycrystalline MnNi shape memory alloys was simulated by a phase-field method considering plastic deformation. Plastic strain was found to occur around grain boundaries and intersections between martensitic bands. In addition, the accumulation of plastic strain would keep on continuously occurring during the thermal or stress cycling process. The occurring of plastic strain could be caused by the lattice incompatibility between austenite and martensite near the tip of martensite plate.

Interfacial Modulus Mapping during Structural Transformation in Shape Memory Alloys

Through the modified phase-field model the local soft mode mechanism of nucleation during martensitic transformation was confirmed in shape memory alloys. It was discovered that the modulus loss (8 pct) depended on the martensitic nucleation exceeding the loss (1 pct) during the martensitic growth. The elastic modulus and the stress across the martensite/parent interface differed from those across the martensitic twin boundary. The modulus losses in systems with three variants, two variants, and one variant were compared.

Intrinsic origin of rubber-like behavior in thermoelastic alloys

The evolution of polytwinned martensitic variants, which accompanies changes in various energies under applied stress, is investigated using a 3-dimensional phase-field simulation. The intrinsic origin of the rubber-like behavior (RLB) in a thermoelastic alloy is revealed as the disappearance of one variant during loading and its reappearance during unloading, whereas long “stress aging” enhances martensite stabilization. The applied stress and elastic strain energies drive the respective microstructural evolution and its reversal. This intrinsic origin of the RLB cannot be excluded in thermoelastic alloys for which the RLB can be explained by “symmetry conforming-short range order” principle.