Yuhua Wen

Relationship among grain size, annealing twins and shape memory effect in Fe–Mn–Si based shape memory alloys

In order to clarify the relationship among grain size, annealing twins and the shape memory effect in Fe–Mn–Si based shape memory alloys, the Fe–21.63Mn–5.60Si–9.32Cr–5.38Ni (weight %) alloy with a grain size ranging from 48.9 μm–253.6 μm was obtained by adjusting the heating temperature or heating time after 20% cold-rolling. The densities of grain boundaries and annealing twins increase with a decrease in grain size, whereas the volume fraction and width of stress-induced e martensite after 9% deformation at Ms + 10 K decrease. This result indicates that grain refinement raises the constraint effects of grain boundaries and annealing twins upon martensitic transformation. In this case, the ability to suppress the plastic deformation and facilitate the stress-induced e martensite transformation deteriorates after grain refinement owing to the enhancement of the constraint effects. It is demonstrated by the result that the difference at Ms + 10 K between the critical stress for plastic yielding and that for inducing martensitic transformation is smaller for the specimen with a grain size of 48.9 μm than for the specimen with a grain size of 253.6 μm. Therefore, the shape memory effect declined by decreasing the grain size.

Effect of Manganese on Microstructures and Solidification Modes of Cast Fe-Mn-Si-Cr-Ni Shape Memory Alloys

We investigated microstructures and solidification modes of cast Fe-(13-27)Mn-5.5Si-8.5Cr-5Ni shape memory alloys to clarify whether Mn was an austenite former during solidification. Furthermore, we examined whether the Creq/Nieq equations (Delong, Hull, Hammer and WRC-1992 equations) and Thermo-Calc software® together with database TCFE6 were valid to predict the solidification modes of cast Fe-(13-27)Mn-5.5Si-8.5Cr-5Ni shape memory alloys. The results have shown that the solidification modes of Fe-(13-27)Mn-5.5Si-8.5Cr-5Ni alloys changed from the F mode to the FA mode with increasing the Mn concentration. Mn is an austenite former during the solidification for the cast Fe-Mn-Si-Cr-Ni shape memory alloys. The Delong, Hull, Hammer, and WRC-1992 equations as well as Thermo-Calc software® together with database TCFE6 are invalid to predict the solidification modes of cast Fe-(13-27)Mn-5.5Si-8.5Cr-5Ni SMAs. To predict the solidification modes of cast Fe-Mn-Si-Cr-Ni alloys, a new Creq/Nieq equation should be developed or the thermodynamic database of Thermo-Calc software® should be corrected.

Influence of recovery heating rate on shape memory effect in up-quenched Cu–Al–Mn alloy

Abstract The effect of recovery heating rate on shape memory effect of the up-quenched Cu–8.88Al–10.27Mn (mass fraction, %) alloy was investigated by optical microscopy, electron transmission microscopy (TEM) and electrical resistivity measurement. It is found that the shape recovery rate decreases as the heating rate decreases. It can reach 75% when the heating rate is 20 °C/min, while it is only 8% when the heating rate is 1 °C/min. In situ microstructure observation indicates that the dependence of shape memory effect on recovery heating rate is caused by the stabilization of twinned martensite induced by deformation. The analysis of electrical resistivity shows that the stabilization of twinned martensite may be ascribed to formation of compound defects of vacancies and dislocations at the boundaries of twinned martensite during the slow heating. The compound defects prevent the reverse transformation of twinned martensite.

Role of thermal martensite in shape memory effect of CoAl and CoNi alloys

Abstract To address the role of the HCP martensite in CoAl and CoNi shape memory alloys, the relationship between the shape memory effect (SME) and the content of the thermal and stress-induced HCP martensite was investigated in the solution-treated CoAl and CoNi alloys. In-situ optical observations were employed to investigate the contents of thermal HCP martensite before and after deep cooling and its influence on the stress-induced HCP martensite transformation and SME. The results show that the SME in both the CoAl and the CoNi alloys results from the stress-induced HCP martensite. The role of the thermal HCP martensite in both of them is the strengthening of the matrix. The much higher yield strength in the solution-treated CoAl alloy due to solution strengthening of Al is responsible for its better SME compared with the CoNi alloy.

Effects of Ageing with Electric Pulse Treatment on Shape Memory Effect and Precipitation of Cr23C6 Carbide in a Pre-deformed Fe-Mn-Si-Cr-Ni-C Alloy

Abstract Effects of ageing with electric pulse treatment on shape memory effect and precipitation of Cr23C6 carbide in a pre-deformed Fe-Mn-Si-Cr-Ni-C alloy were investigated. The results showed that electric pulse treatment could accelerate the migration of Cr and C atoms and precipitation of Cr23C6 carbides, lower ageing temperature, shorten ageing time and induce the nucleation of Cr23C6 carbides. Therefore, the shape recovery ratio was improved to the maximum value 89.2% from 32% of a water-quenching alloy within 30 min in a pre-deformed Fe-Mn-Si-Cr-Ni-C alloy aged with electric pulse treatment compared to alloys aged without electric pulse treatment at 973 K.

Evolution of surface structure of bilayer oleic acid-coated Fe3O4 nanoparticles during ethanol washing

The excess amount of oleic acid must be added to synthesize the bilayer oleic acid-coated Fe3O4 nanoparticles by onestep method. To know the evolution of surface structure during removing the residual oleic acid by ethanol washing, the surface structure of bilayer oleic acid-coated Fe3O4 nanoparticles synthesized by one-step method was studied using TEM, FTIR and TGA with washing times. The results showed that after five times washing, the bilayer oleic acid-coated structure still remained; after twenty times washing, the bilayer oleic acid-coated structure evolved into the single layer one.