Z. L. Ning

Deformation behavior of semisolid A356 alloy prepared by low temperature pouring

The deformation behaviors of A356 alloy with various initial grain structures, from a very coarse dendrite to a fine equiaxed structure, prepared at different pouring temperatures, from 625°C to 725°C, were investigated. Cylindrical specimens with 12 mm in diameter and 10 mm in height were compressed in the semisolid state to a height reduction of 6 mm at an initial strain rate of 1.39 × 10−3/s and then jumped to 1.39 × 10−2/s. The instantaneous strain rate sensitivity was calculated under iso-structure conditions. Three very different deformation behaviors have been observed, which are believed to have resulted from their different starting microstructures. The materials produced at low pouring temperatures evolve into a fine globular structure after isothermal holding at semisolid region and shows a very low compression stress, while the materials cast at high pouring temperatures remain as coarse and dendritic in its microstructure and have much higher compression stress. The macrostructural breakage and microstructural segregation of hot compressed samples were analyzed, and the deformation mechanisms were discussed. The low temperature poured material is not sensitive to an instantaneous change in strain rate, while the high temperature poured material is very sensitive.

Tensile deformation of a Mg–2.54Nd–0.26Zn–0.32Zr alloy at elevated temperature

The engineering stress versus engineering strain curves for a Mg–2.54Nd–0.26Zn–0.32Zr cast alloy were measured by Gleeble-1500D thermo-simulation machine in the temperature range of room temperature to 400 °C at initial strain rates of 10−4–10−2 s−1. The effects of strain rate on stress, elongation to facture, and section shrinkage were analyzed. The fractograph morphologies were investigated by using SEM. It was found that strain rate has little effect on engineering stress for the Mg–2.54Nd–0.26Zn–0.32Zr alloy when tested at below 250 °C. When tested at above 250 °C, low strain rate resulted in decreased engineering stress, increased elongation to fracture, and section shrinkage. The fracture mode is cleavage fracture with elongated dimple below 250 °C and changes to typical ductile failure when tested above 250 °C.

In-situ tensile testing of ZrCu-based metallic glass composites

ZrCu-based bulk metallic glass composites (BMGCs) are well known for their plastic deformability, superior to traditional metallic glasses (MGs), which is attributed to a unique dual-phases structure, namely, the glassy matrix and unstable B2 phase. In the present study, in-situ tensile testing is used to trace the deformation process of a ZrCu-based BMGC. Three deformation stages of the BMGC, i.e., the elastic-elastic stage, the elastic-plastic stage, and the plastic-plastic stage are identified. In the elastic-elastic and elastic-plastic stages, the yield strength and elastic limit are major influenced by the volume fraction of the B2 crystals. In the plastic-plastic stage, the B2 phase stimulates the formation of multiple shear bands and deflects the direction of shear bands by disturbing the stress field in front of the crack tip. The deformation-induced martensitic transformation of the metastable B2 phase contributes to the plasticity and work hardening of the composite. This study highlights the formation and propagation of multiple shear bands and reveals the interactions of shear bands with structural heterogeneities in situ. Especially, the blocking of shear bands by crystals and the martensitic transformation of the B2 phase are critical for the mechanistic deformation process and illustrate the function of the B2 phase in the present BMGCs.

Quantitative Analysis on Fatigue Fracture of Spray Formed GH738 Alloy

Low-cycle fatigue (LCF) fracture of the spray formed GH738 alloy was investigated under total strain controlled mode at 650°C. Basic theory for evaluating fatigue lives by fatigue striations has been introduced. The crack length a and the crack propagation rate da/dN were measured and the curve of da/dN in crack propagation zone was obtained with Paris formula and tabulation trapezoidal formula, respectively. The size and symmetry of rapid fracture zone of fatigue were also studied. The results showed that the relationship of a and N was linear with tabulation trapezoidal formula, and the da/dN and a was conic with Paris formula. At last, quantitative analysis of fatigue fracture was also discussed.

Effect of Cooling Condition on Zr-Rich Core Formation and Grain Size in Mg Alloy

The effects of cooling condition on the Zr-rich core formation and grain size in Mg-Zr alloys were investigated in this study. Four moulds with various cooling rates and different Zr additions were used. The results show that when Zr addition was 1.2%, the Zr-rich cores were round and their diameters increased with decreased cooling rates. When Zr addition content was decreased to 0.7%, Zr-rich cores formed only in the sand mould with the lowest cooling rate used in this study, and the morphology changed to rosette-like. The influence of cooling rate on grain size of Mg-Zr alloys was also investigated.