Texture evolution and dynamic mechanical behavior of cast AZ magnesium alloys under high strain rate compressive loading

In this study, texture and compressive mechanical behavior of three cast magnesium alloys, including AZ31, AZ61 and AZ91, were examined over a range of strain rates between 1000 and 1400 s⁻¹ using Split Hopkinson Pressure Bar. Texture measurements showed that after shock loading, initial weak texture of the cast samples transformed to a relatively strong (00.2) basal texture that can be ascribed to deformation by twinning. Furthermore, increasing the aluminum content in the alloys resulted in increase in the volume fraction of β-Mg₁₇Al₁₂ and Al₄Mn phases, strength and strain hardening but ductility decreased at all strain rates. Besides, it was found for each alloy that the tensile strength and total ductility increased with strain rate. By increasing the strain rate, the maximum value of strain hardening rate occurred at higher strains. Also, it is suggested that a combination of twinning and second phase formation would affect the hardening behavior of the cast AZ magnesium alloys studied in this research.     

高温热变形条件下Ni-Ti形状记忆合金的微结构演变

采用真空感应法炼制了不同氧、碳含量的NiTi和NiTiNb形状记忆合金 .分析研究了合金的微观组织、相结构以及高温热变形对合金微观组织的影响 ,重点研究和探讨了合金微观结构在高温热变形过程中的演变规律 .确定了合金组织中TiNi基体相、Nb相及在晶界形成的氧化物和碳化物的形变特点 .结果表明 ,合金中C、O含量决定其组织中碳化物和氧化物的形态和含量 ,高温热变形时 ,TiNi基体相和Nb相粒子均产生较大塑性变形 .当合金中氧和碳含量较高时 ,氧化物和碳化物在变形时碎裂 ,在组织中的分布比铸态更均匀 .在本文研究的氧、碳含量的范围内 ,氧化物和碳化物的存在对合金的力学性能 ,如拉伸强度、延伸率等影响不大 .     

Kinetic and metallographic study of oxidation at high temperature of cast Ni 25Cr alloy in water vapour rich air

Abstract

The high temperature oxidation behaviour of a Ni–25 wt-%Cr alloy in air enriched with water vapour (180 mbars H₂O) was studied at 1000, 1100, 1200 and 1300°C. The oxidised samples were characterised by X-ray diffraction, electron microscopy and wavelength dispersion spectroscopy. The obtained data were compared to the ones earlier obtained for the same alloy oxidised in dry air. Water vapour globally induced at all temperatures a decrease of the parabolic constant Kp and an increase in the chromia volatilisation constant Kv. The oxide scales do not present morphologic difference between the two atmospheres. After oxidation in humidified air the scale thickness is thinner and the Cr depleted depth is lower than in dry air.     

Uniform corrosion behavior of FeCrAl alloys in borated and lithiated high temperature water

The uniform corrosion behavior of model FeCrAl alloys as well as commercial Zircaloy-4 as reference material were investigated in 360℃borated and lithiated water.The results reveal that FeCrAl alloys exhibited better corrosion resistance than Zircaloy-4.It is found that the oxide films formed on the FeCrAl alloys are composed of outer Fe3O4 layer,inner layer consisting of compact spinel layer and porous spinel layer,and transition layer containing Al-Cr-rich oxides and matrix enriched with Fe.The spinel oxides in the inner layer are FeFe2-x-y-zCrxAlyMozO4.The corrosion mechanism of FeCrAl alloys in high temperature water is discussed.     

Texture development of Ca3Co4O9 thermoelectric oxide by high temperature plastic deformation and its contribution to the improvement in electric conductivity

High temperature uniaxial compression is conducted on Ca₃Co₄O₉ layered cobaltite, in order to achieve a thermoelectric oxide with low resistivity by the development of (0 0 1) texture. It is found that flow stress varies depending on deformation temperature and strain rate. Development of a sharp texture having the maximum (0 0 1) pole density of about 33 times as high as the random level is achieved. It is found that the high temperature compression process is quite effective for the simultaneous achievement of densification and (0 0 1) texture development. It is experimentally confirmed that resistivity decreases drastically by the construction of a sharp (0 0 1) texture.