The corrosion behaviours of die-cast AZ91D magnesium alloys were investigated in 0.1 mol/L sodium sulphate （Na 2 SO 4 ） solutions with different pH values. The corrosion rates, morphologies, and compositions of the corrosion products were studied by means of scanning electron microscopy （SEM）, Fourier transform infrared spectroscopy （FTIR）, and X-ray diffractometry （XRD）. The results indicate that the order of corrosion rates in Na 2 SO 4 solutions with various pH values is pH 2pH 4pH 7pH 9pH 12. The corrosion rates in acidic solutions are higher than those in alkaline solutions, and the corrosion products are mainly magnesium hydroxide （Mg（OH） 2 ） and hydrated sulphate pickeringite （MgAl 2 （SO 4 ） 4 ·22H 2 O）. The results also indicate that the solution pH can influence the corrosion rate and morphology of corrosion products. Chloride ions and sulphate ions have different pitting initiation time.
A nanocrystalline layer was synthesized on the surface of TWIP steel samples by surface mechanical attri- tion treatment （SMAT） under varying durations. Microhardness variation was examined along the depth of the de- formation layer. Microstructural characteristics of the surface at the TWIP steel SMATed for 90 min were observed and analyzed by optical microscope, x-ray diffraction, transmission and high-resolution electron microscope. The re- sults show that the orientation of austenite grains weakens, and a-martensite transformation occurs during SMAT. During the process of SMAT, the deformation twins generate and divide the austenite grains firstly~ then a-martens- ite transformation occurs beside and between the twin bundles~ after that the martensite and austenite grains rotate to accommodate deformation, and the orientations of martensite and between martensite and residual austenite increase; lastly the randomly oriented and uniform-sized nanocrystallir^e layers are formed under continuous deformation.
LI Da-zhao1,2, WEI Ying-hui1, HOU Li-feng1, LIN Wan-ming1 (1. College of Materials and Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China
The effects of the solution and aging treatment on microstructures and mechanical properties of the Mg-10Zn-5A1-0.1Sb-XCu cast magnesium alloys were investigated by brinell hardness measurement, scanning electron microscopy （SEM）, energy spectrum analyzing apparatus and X-ray diffraction （XRD）. The experimental results show that the strip-like t-Mg32 （A1, Zn）49 phase is shown at the grain boundaries and Mg2Cu phase become smaller, even granular after solution treatment at 350 ~C for 24 h. By ageing treatment at 180 ~C, the ternary strengthening phase （r phase） precipitates gradually at or around grain boundary. With increasing aging time, the micro-hardness improves obviously and up to the maximum （105.9 HV） at aging time of 36 h. In addition, the tensile-strengths at room temperature and at an elevated temperature respectively reach 228 MPa and 176 MPa, which is increased by 20% and 10%, respectively.
To improve the comprehensive mechanical properties of Mg-10Zn-5Al-0.1Sb magnesium alloy, different amount of Ce-rich rare earth （RE） was added to the alloy, and the effect of RE addition on the microstructure and mechanical properties of Mg-10Zn-5Al-0.1Sb alloy was investigated by means of Brinell hardness measurement, scanning electron microscopy （SEM）, energy dispersive spectroscope （EDS） and X-ray diffraction （XRD）. The results show that an appropriate amount of Ce-rich rare earth addition can make the AI4Ce phase particles and CeSb phase disperse more evenly in the alloy. These phases refine the alloy＇s matrix and make the secondary phases [τ-Mg32（AI,Zn）49 phase and φ-Al2MgsZn2 phase] finer and more dispersive, therefore significantly improve the mechanical properties of the Mg-10Zn-5AI-0,1Sb alloy. When the RE addition is 1.0 wt.%, the tensile strengths of the alloy both at room temperature and 150℃ reach the maximum values while the impact toughness is slightly lower than that of the matrix alloy. The hardness increases with the increase of RE addition.
You ZhiyongZhang ZhaoguangZhang JinshanWei Yinghui
Based on its excellent tensile strength-ductility property combination,twinning-induced plasticity (TWIP) steel shows great potential in applications for structural components in automobile industry.The aim of this research is to investigate the corrosion resistance properties and corrosion mechanism under room temperature in TWIP steel.The influence of the deformation twin density on corrosion property was primarily considered by salt spray test.The specimens used in the investigation are as-annealed and as-deformed respectively.The microstructure and corrosion resistance property were characterized by scanning electron microscope (SEM),optical microscope (OM) and so on.There are some annealing twins distributed randomly in austenitic grains in the as-annealed specimen.After the specimen was subjected to tensile experiment,the density of the deformation twins increased sharply,which are different from the annealing twins in size and morphology.It was found that the corrosion potential of the as-annealed is lower than that of the as-deformed and the corrosion current density behaves contrarily.After immersed in 5% NaCl solution salt spray for 48h,the as-deformed showed a bit better than the as-annealed in corrosion resistance.With the time prolonged,the gap between the two specimens in corrosion resistance increased rapidly.The corrosion morphologies varied in color and shape.Further investigation,carried out by SEM and EDS,indicated that as-annealed and the as-deformed followed pitting corrosion and uniform corrosion mechanism respectively.The reason for the difference in corrosion mechanism is possibly the presence of the deformation twins.The deformation twins formed during the tensile test refine grains by way of segmentation.The twin boundaries largely belong to the coincidence site lattice (CSL),which is on lower energy state.It suggests that the twins not only play a role in strengthening,but also improve effectively the corrosion resistance in TWIP steel.
CHU Guo-dong,WEI Ying-hui,LI Da-zhao,TIAN Yun (College of Materials Science and Engineering,Taiyuan University of Technology,Taiyuan 030024,Shanxi,China)
To improve the strength, hardness and heat resistance of Mg-Zn based alloys, the effects of Cu addition on the as-cast microstructure and mechanical properties of Mg-10Zn-5Al-0.1Sb high zinc magnesium alloy were investigated by means of Brinell hardness measurement, scanning electron microscopy （SEM）, energy dispersive spectroscopy （EDS）, XRD and tensile tests at room and elevated temperatures. The results show that the microstructure of as-cast Mg-10Zn-5AI-0.1Sb alloy is composed of α-Mg, τ-Mg32（Al, Zn）49, Ф-Al2MgsZn2 and Mg3Sb2 phases. The morphologies of these phases in the Cu-containing alloys change from semi-continuous long strip to black herringbone as well as particle-like shapes with increasing Cu content. When the addition of Cu is over 1.0wt.%, the formation of a new thermally-stable Mg2Cu phase can be observed. The Brinell hardness, room temperature and elevated temperature strengths firstly increase and then decrease as the Cu content increases. Among the Cucontaining alloys, the alloy with the addition of 2.0wt.% Cu exhibits the optimum mechanical properties. Its hardness and strengths at room and elevated temperatures are 79.35 HB, 190 MPa and 160 MPa, which are increased by 9.65%, 21.1% and 14.3%, respectively compared with those of the Cu-free one. After T6 heat treatment, the strengths at room and elevated temperatures are improved by 20% and 10%, respectively compared with those of the ascast alloy. This research results provide a new way for strengthening of magnesium alloys at room and elevated temperatures, and a method of producing thermally-stable Mg-10Zn-5Al based high zinc magnesium alloys.
You ZhiyongZhang YuhuaCheng WeiliZhang JinshanWei Yinghui
