Yujin Wang

The tensile behavior of Ti36Ni49Hf15 high temperature shape memory alloy

Recently, ternary Ti-Ni-Hf alloys have attracted great interest in the field of high temperature shape memory materials research and development. Extensive studies have been made on its manufacture process, constitutional phases, phase transformation behavior, the structure, substructure and interface structure of martensite and the precipitation behavior during ageing. Yet up to date there is no report about the fundamental mechanical properties of Ti-Ni-Hf alloys, such as the stress-strain data, the variation laws of the yield strength and elongation with the temperature. In the present study, tensile tests at various temperatures are employed to investigate the mechanical behavior of Ti-Ni-Hf alloy with different matrix structures, from full martensite to full parent phase structure, with the corresponding deformation mechanism discussed.

High-resolution electron microscopy study on the substructure of Ti-Ni-Hf B19' Martensite

The substructure of martensite in the Ti,,Ni,,Hf,, alloy has been investigated by high-resolution electron microscopy (HREM) and discussed within the framework of a phenomenological theoretical analysis. The (001) compound twins and (001) stacking faults were confirmed to be the main substructure inside a martensite variant with some (011) stacking faults infrequently observed. The (001) twinning boundary was straight with some blurred regions and (001) faults existing near the interface. The (001) compound twin alone is shown to be unable to produce the lattice invariant shear necessary for the martensitic transformation

Pseudoelasticity, shape memory effect and FCC→ HCP martensitic transformation associated with stacking faults in FCC alloys

Abstract The behavior of stacking faults and the two associated Shockley partial dislocations under an applied stress has been examined. It is shown that the two partials usually experience unequal forces, resulting in further extending or contracting of the stacking fault, and their motion involves crystallographic transformation. In the light of this, pseudoelasticity and the shape memory effect have been discussed associated with stacking faults. It has also been examined how the applied stress interacts with a stacking fault embryo of martensite and assists it to transform into a martensite plate.

Microstructure and High-Temperature Mechanical Properties of ZrO 2 -Al 2 O 3 -SiC Ceramics

In the present work, ZrO2-Al2O3 ceramics incorporated with and without β-SiC were prepared by hot pressing. ZrO2-Al2O3 ceramic powder used in this study is a mixture of 71xa0vol.% YSZ (3xa0mol.% Y2O3 partially stabilized zirconia) and 29xa0vol.% α-Al2O3. β-SiC powders with different volume fractions are added into the ZrO2-Al2O3 powder to form the composite powder. The microstructure and high-temperature mechanical properties of ZrO2-Al2O3-SiC ceramics were investigated by tailoring the compositions and sintering parameters to optimize the strengthening mechanisms. For a comparative study, the TZ3Y20A powder was also hot-pressed under identical sintering condition to form dense bulk ceramic. ZrO2-Al2O3-SiC ceramics consist mainly of t-ZrO2, α-Al2O3, and β-SiC phases. SiC particles in the ZrO2-Al2O3 ceramic restrain the grain growth of the oxide matrix. The incorporation of SiC into ZrO2-Al2O3 ceramic enhances high-temperature flexural strength at 1273xa0K. ZrO2-Al2O3 ceramic incorporated with 15xa0vol.% SiC has a flexural strength of 518xa0MPa at 1273xa0K, much higher than that (201xa0MPa) of unmodified ZrO2-Al2O3 ceramic.

Effect of microstructure of austenite on the shape memory effect in an Fe-Mn-Si-Ni alloy

Since a complete shape memory effect was obtained in an Fe-Mn-Si single crystal, much work has been done to establish the correlation between the concentration of alloying elements and the temperature suitable for obtaining a complete shape memory effect. However, less work has been reported on the effect of the microstructure of the parent phase {gamma} on the shape memory effect. An Fe-Mn-Si-Ni polycrystalline alloy has been developed with good properties of forging, rolling and anti-oxidation at high temperatures in addition to excellent shape memory performance. The purpose of the present work is to report the effect of the austenitic microstructure on the shape memory effect in this alloy.

Crystallography of martensitic transformations with type II twinning as the lattice invariant shear

The phenomenological theories of the crystallography of martensitic transformations have been developed by Wechsler, Lieberman and Read, and Bowles and Mackenzie, independently, in the 1950s. In his comprehensive discussion, christian pointed out that the BM theory and WLR theory are essentially equivalent, although they differ in mathematical formulation. Recently, Otsuka`s systematic work has demonstrated that the martensitic phenomenological crystallographic theory is not only valid for type I twinning, but also for type II twinning, as a lattice invariant shear. The authors have introduced a method to determine {Phi}, which represents a rotation bringing the martensite twin and martensite matrix into coherence in the twinning plane, that can be applied to both type I and type II twinning as the lattice invariant shear in martensitic transformation.