Rongde Li

Influence of RRA Treatment on the Microstructure and Stress Corrosion Cracking Behavior of the Spray-Formed 7075 Alloy

The effects of retrogression under pre-aging on the microstructure, mechanical properties, and stress corrosion cracking behavior of spray-formed 7075 aluminum alloys were investigated by the transmission electron microscopy, tensile tests, and slow strain rate tests. The results show that as a result of aging at 120°C for 16 h (as pre-aging), the strength of the alloy can be preserved on a high level and the grainboundary precipitates are discrete after retrogression and re-aging treatment. However, the retrogression treatment is uncontrollable by the shortened retrogression period. After retrogression at 200°C for 8 min and re-aging, the ultimate tensile strength, elongation, and the stress corrosion cracking index of the alloy are 791 MPa, 8.5% and 0.155, respectively.

Cementites decomposition of a pearlitic ductile cast iron during graphitization annealing heat treatment

Cementites decomposition of a pearlitic ductile cast iron during graphitization annealing heat treatment was investigated. Fractographies and microstructures of heat treated samples were observed using a scanning electron microscope and mechanical properties were measured by a universal tensile test machine. The results indicated that during isothermal annealing at 750 °C, the tensile strength of pearlitic ductile cast iron was increased to a peak value at 0.5 h, and decreased gradually thereafter but the elongation was enhanced with the increase of annealing time. Moreover, the diffusion coefficient of carbon atoms could be approximately calculated as 0.56 μm2/s that could be regarded as the short-range diffusion. As the holding time was short (0.5 h), diffusion of carbon atoms was incomplete and mainly occurred around the graphites where the morphology of cementites changed from fragmentized shape to granular shape. In addition, the ductile cast iron with tensile strength of 740 MPa and elongation of 7% could be achieved after graphitization annealing heat treatment for 0.5 h. Two principal factors should be taken into account. First, the decomposition of a small amount of cementites was beneficial for increasing the ductility up to elongation of 7%. Second, the diffusion of carbon atoms from cementites to graphites could improve the binding force between graphites and matrix, enhancing the tensile strength to 740 MPa.