Materials Science Forum Vols. 747-748

Abstract: Based on the advantages of Mg alloys, AZ31 alloy profiles were designed for mobile phones cell, and cars, which expanded Mg alloy application and achieved the purpose of environmental protection and energy saving. Tensile strength of the profile reached to 256 MPa, and elongation was 17.9%. The effects of different heat treatment parameters on microstructure and plastic property of AZ31 alloys profiles were studied. After aging heat treatment (T5), the comprehensive mechanical properties at room temperature of the profiles increased. The tensile strength reached to the maximum of 261 MPa, and the elongation reached to 14.3%, after ageing at 225 for 20h. The elongation reached the maximum value of 18.3%, however, the tensile strength decrease to 241 MPa, after aging at 175 for 15h. Moreover, the influence of cryogenic treatment on microstructure and plastic property of the AZ31alloy profiles were studied. The results showed that elongation increased with the increase of treatment time, which would reach to 18.5% after cryogenic treatment. In addition, by cryogenic treatment and aging treatment, the tensile strength improved to 271 MPa. The cryogenic treatment improved obviously the plastic ability of AZ Mg alloys, while normal heat treatment improved it unobviously.

Abstract: Orthogonal experiments were used to investigate the porous magnesium CPU heat dissipation under the forced convection condition. Three factors were involved in the experiment, i.e. different porosity level (50%, 60%, and 70%), different pore size (0.8-1.6mm, 1.6-2.2mm, 2.2-3.2mm) and different thickness (20mm, 35mm, 50mm) of open-cell porous magnesium. The materials used in the present study were prepared by upper-pressure infiltration method. Experimental results indicated that cooling effect factors of porous magnesium are relevant with the pore structure while the most important one is pore size, followed by porosity. Under the experimental conditions with the pore size of 2.2~3.2mm, porosity of close to 70%, and thickness of 35mm, the maximum heat dissipation benefit was obtained. Compared with the commercial radiator, the fluid flow and heat transfer on the surface of radiators integral fins are better than the others when the CPU is fully busy. Average reduced temperatures of porous magnesium CPU by using upper-pressure infiltration method are 4.4 - 8 lower compared with the commercial ones.

Abstract: The effects of minor Al and Ce on the microstructures, room-temperature and high-temperature mechanical properties of as-cast Mg-6Zn magnesium alloys were investigated. With the Al addition into Mg-6Zn alloy, the coarse eutectic Mg₅₁Zn₂₀ phases were refined and distributed discontinuously. After adding 0.5wt.% Ce into Mg-6Zn-1Al alloy, a new needle-like Al₂CeZn₂ phase was observed. Meanwhile, the volume fraction of Mg₅₁Zn₂₀ phase decreased and the semi-continuous Mg₅₁Zn₂₀ phase became discontinuous globular morphology. It has been observed that the addition of Ce element coarsens the grains, and 1wt.% Al addition enhanced the yield strength and ultimate strength from 86.35MPa, 229MPa to 90.7MPa, 238MPa, respectively. Moreover, the Ce addition can significantly increase the high-temperature mechanical properties of cast Mg-6Zn-1Al alloy.

Abstract: The tensile properties of a low-cost first generation single crystal superalloy DD16 have been investigated. The results show that values of the tensile strength and yield strength of DD16 alloy were similar at typical temperatures; from room temperature to 760, the yield strength of DD16 alloy increases; However, above 760, the yield strength of DD16 alloy decreases remarkably, and the maximum of the yield strength was 1145.5MPa at 760. From room temperature to 760, the fracture mode was cleavage fracture; But above 760, the fracture characteristics changed from cleavage to dimple.

Abstract: The fully heat-treated DD6 single crystal turbine blades were treated in three ways: (1) surface grinding, without coating; (2) firstly surface grinding, then surface coated with HY₃(NiCrAlY) anti-oxidation coating; (3) after surface grinding, coated surface with the bond layer of NiCrAlY and top layer of YSZ(Y₂O₃ stabilize ZrO₂) for the double-layer thermal barrier coatings. All these three experimental blades were treated with vacuum heat-treatment at 1120°C for 4h. The effects of coating on surface recrystallization of single-crystal blades have been investigated. The results indicate that: just with surface polishing blade generated a 6~9μm thick cellular recrystallization zone on the surface. The γ' presented coarse morphology and distributed discretely within cellular, and the γ' growth direction was perpendicular to the cellular interface; blade surface coated with anti-oxidation coating after polishing formed a 3~6μm thick cellular recrystallization zone in the matrix interface. But the grain boundary was fuzzy and recrystallization morphology was incomplete; the matrix interface morphology of blade coated with thermal barrier coatings after polishing changed insignificantly, but some local cellular recrystallization was found. The results indicate that the coating changes the cellular recrystallization morphology of the original matrix of the blade, which can effectively reduce the occurrence degree of recrystallization.

Abstract: With cold rolling deformation between 12.5% and 60% for GH 536 superalloy strips, effect of deformation on the mechanical properties, texture formation and microstructure evolution have been investigated. The results show that an increase of deformation will lead to an increase of mechanical properties. As the deformation was 60%, the tensile strength and yield strength of the strip were 1430MPa and 1370MPa, respectively, and the elongation of strip was 4.5%. Also, an increase of deformation will lead to an increase of hardness, yield ratio and deformation resistance as well as an increase of rolling textures such as Goss {110}<001>, S{213}<4>, Copper {112}<1> and especially Brass {110}<12>. The optimized processing parameters of cold rolling deformation between 25% and 37.5% have been proposed to obtain an excellent formability and cold workability for GH 536 superalloy strips.

Abstract: Powder metallurgical TiAl alloy was fabricated by gas atomization powders, and the effect of heat treatment temperature on the microstructure evolution and room tensile properties of PM TiAl alloy was investigated. The uniform fine duplex microstructure was formed in PM TiAl based alloy after being heat treated at 1250/2h followed by furnace cooling (FC)+ 900/6h (FC). When the first step heat treatment temperature was improved to 1360/1h, the near lamellar microstructure was achieved. The ductility of the alloy after heat treatment improved markedly to 1.2% and 0.6%, but the tensile strength decreased to 570MPa and 600MPa compared to 655MPa of as-HIP TiAl alloy. Post heat treatment at the higher temperature in the alpha plus gamma field would regenerate thermally induced porosity (TIP).

Abstract: The hot components of gas turbine are loaded by high stresses in presence of high-temperature oxidation and hot corrosion environment during service process. It is important to simulate the interaction of stress and hot corrosion on the components in real service situation. Effect of the ratios of salt mixtures of Na₂SO₄/NaCl and the coating amounts of salt mixtures on the microstructure and the high temperature stress-rupture properties of a corrosion resistant directionally solidified superalloy at 850/500MPa were investigated. The results demonstrated that the stress-rupture life decreased with the coated salt mixture ratio of Na₂SO₄/NaCl changing from 3:1 to 9:1. Stress-rupture lives of the alloy decreased from 63 h to 42 h, while the coated salt mixtures amount increased from 1 mg/cm² to 4 mg/cm², hereby the salt mixture contained 75% Na₂SO₄ and 25% NaCl. When the coating ratio of Na₂SO₄/NaCl was 9:1, the stratified corrosion products layers were composed of the (Ni, Co)O, Ni (Al, Cr)₂O₄ layer, bright grey phase that be identified as CrWO₄, and the underlying dispersed particle shape phase CrSx, the underlying layer forms the sulfides eutectic and leads to catastrophic destruction for the alloy, which decreased the stress-rupture lives dramatically.

Abstract: The oxide-dispersion-strengthened (ODS) ferritic alloy powders (Fe-14Cr-3Al-2W-0.1Ti-0.35Y₂O₃) were prepared by mechanical alloying (MA) at a rotation speed of 720 rpm for 24 h. All the elements were mixed homogenously in the powder, and Cr and Al dissolved in α-Fe after MA. The bulk samples were produced by spark plasma sintering (SPS) at 950 and 50 MPa and by hot isostatic pressing (HIP) at 1150 and 130 MPa, respectively. The SPS sample showed a tensile strength of 730 MPa and a poor ductility due to the existence of pores in microsize. The HIP sample had a high tensile strength of 980 MPa, yield strength of 710 MPa and elongation of 10.3 %. The excellent mechanical properties of the HIP sample was due to the small grain size of the matrix of about 400 nm and the fine oxide particles of 5-40 nm.

Abstract: The non-metallic inclusions in master alloy, P/M superalloy and HIP powder billet were studied in this paper. The results show that the amount of inclusions in master alloy is higher than that of the superalloy powers. The EB-button analysis shows that the main non-metallic inclusions in both the master alloy and the HIP powder billet is Al₂O₃.The amount of the inclusion in master alloy is about 0.166cm²/kg and the size of most inclusions is in the range of 100μm to 200μm, while the maximum inclusion size reaches 400μm.In the P/M superalloy billet, the content of inclusion is only 0.01cm²/kg and the size of most inclusions is less than 50 μm.