Papers by Author: Heng Qiang Ye

Abstract: Mg-Zn-Y alloys with long-period stacking ordered (LPSO) phases have superior strength at elevated temperatures. We studied plastic deformation and creep behavior of a Mg₉₇Zn₁Y₂ (at.%) alloy. Deformation kinking of the LPSO phase plays an important role in strengthening the alloy during compression at elevated temperatures. Growth stacking faults with Zn/Y segregation can act as obstacles to non-basal slip and deformation twinning in Mg matrix. The tensile creep strain was only about 0.01% under a tensile stress of 70MPa for 100h at 200 °C, demonstrating excellent creep resistance of this alloy. Generation and motion of basal dislocations led to bending of LPSO phase during tensile creep of the Mg₉₇Zn₁Y₂ (at.%) alloy. Plastic deformation in Mg grains was mostly achieved through basal slip during creep at temperatures below 200 °C, while non-basal slip through the generation and motion of “a + c” dislocations was activated with increasing the temperature to 200 °C and above. Dissociation of dislocations and Suzuki segregation on basal planes occurred widely in Mg matrix, which hindered dislocation motion and thus played an important role in preventing Mg grains from softening during deformation at elevated temperatures. In addition, Cottrell atmospheres were observed along dislocations in plastically deformed LPSO phase, impeding motion of dislocations. The superior strength and creep resistance of the Mg₉₇Zn₁Y₂ (at.%) alloy at elevated temperatures are thus associated with the LPSO phase, stacking faults in Mg grains, formation of Cottrell atmospheres in LPSO and occurrence of Suzuki segregation in Mg.

Abstract: TiAl alloys have great potential because of its low density and the outstanding performance at high temperature. However, the brittleness influences its industrialization process. It is known that the macroscopic nature is greatly influenced by its microscopic structure, and the fault development plays a vital role during the material working process. The paper performed the molecular dynamics (MD) study of the thermodynamic shear deformation in TiAl/Ti₃Al system to promote the understanding in this aspect. Above all, we adopt a special shear deformation model based on the experimental consideration, and conduct the optimal calculation of the related parameters. Then, a series of thermodynamic deformation simulation were carried out using the previous optimized model. The analysis of the potential variation and the structural snapshots showed that the shear deformation is related with the “stick-slip” behavior. The Ti₃Al (TiAl) shows obvious (little) covariant deformation stage before the initiation of the fault transition. For Ti₃Al region near the interface, the final structure is the continued FCC stacking. For TiAl, twin and SISF are observed and the block of twin is the main remnant. The atomic diffusion is locally observed in Ti3Al phase. The interface transits the energy and counterpoises the deformation between the hetero-phases.

Abstract: During long-term thermal exposure, the degeneration of primary MC carbide is a diffusion-controlled process. With the element exchange between the primary MC carbide and the γ matrix, three MC degeneration reactions, that is, γ + MC → γ' + M23C6 (Reaction I), MC + γ →η+ M23C6 (Reaction II) and MC + γ →η+ M6C (Reaction III), sequentially operates. Of them Reaction III has never been reported in the previous references and is kinetically most difficulty. It is also shown that the grain boundary MC decomposes much more rapidly than the grain interior MC and that Reaction III is not as often seen at the grain boundary as in the grain interior.

Abstract: We employ density functional calculations to investigate the doped Al/TiC interfaces. The effects of different segregation atoms are discussed. The results show that the different transition metal atoms have different effects on the adhesion. Results of analysis of atom size and electronic structure have shown that both atom size and activity of the doped atom influence on the adhesion. Our results are consistent with other results of doped metal-oxide interface.

Abstract: The microstructure and tensile mechanical properties of directionally Solidified NiAl-15Cr alloy at various temperatures have been investigated in this paper. The results reveal that the microstructure consists of dendritic β-NiAl phase, interdendritic γ/γ’, γ’ phase transient layer and α-Cr precipitation in β-NiAl phase. With the increase of temperature obvious Brittle-Ductile-Transition (BDT) behavior is observed and the BDT temperature (BDTT) is sensitive to initial strain rate. When the initial strain rate increases by two-order magnitude, the BDTT has an approximate 150K increase. In the temperature range of 1123-1373K, the alloy exhibits good tensile plasticity and poor strength comparing with the NiAl/Cr(Mo,Hf) alloy. Superplastic-like deformation behavior with large elongation (exceed 200%) is achieved at 1323K. The balance between strain hardening (by dislocation glide) and strain softening (dynamic recover and recrystallization) is responsible for the large tensile elongation of this alloy.