Heat treatment response and influence of overaging on mechanical properties of C355 cast aluminum alloy

The research activity was focused on the optimization of heat treatment parameters for C355 (Al-Si-Cu-Mg)cast aluminum alloy and on its microstructural and mechanical characterization in T6 condition, also evaluatingthe effect of subsequent high temperature exposure. Differential thermal analyses were carried out to identifythe solution heat treatment optimal temperature. After solution heat treatment and quenching, samples weresubjected to artificial aging, at different times and temperatures, as to obtain the corresponding hardnesscurves. Samples for successive hardness and tensile tests were subjected to hot isostatic pressing (HIP) and T6heat treatment, according to the parameters optimized in the foregoing research phase. Some of the T6 heattreated samples were also characterized after overaging, induced by holding at 210 °C for 41 h. Aiming to carryout a comparative study, tensile properties of C355 alloy, both in T6 and overaged conditions, were comparedto those of A356 alloy (results from a previous study), which is currently more widely employed than C355.Experimental results showed how C355-T6 alloy is characterized by superior mechanical properties as comparedto A356-T6, especially in the overaged condition, due to the higher thermal stability induced by Cu-basedstrengthening precipitates

Monte-Carlo simulations of the recombination dynamics in porous silicon

A simple lattice model describing the recombination dynamics in visible light emitting porous Silicon is presented. In the model, each occupied lattice site represents a Si crystal of nanometer size. The disordered structure of porous Silicon is modeled by modified random percolation networks in two and three dimensions. Both correlated (excitons) and uncorrelated electron-hole pairs have been studied. Radiative and non-radiative processes as well as hopping between nearest neighbor occupied sites are taken into account. By means of extensive Monte-Carlo simulations, we show that the recombination dynamics in porous Silicon is due to a dispersive diffusion of excitons in a disordered arrangement of interconnected Si quantum dots. The simulated luminescence decay for the excitons shows a stretched exponential lineshape while for uncorrelated electron-hole pairs a power law decay is suggested. Our results successfully account for the recombination dynamics recently observed in the experiments. The present model is a prototype for a larger class of models describing diffusion of particles in a complex disordered system.Comment: 33 pages, RevTeX, 19 figures available on request to [email protected]

Effect of Different Heat Treatments on Tensile Properties and Unnotched and Notched Fatigue Strength of Cold Work Tool Steel Produced by Powder Metallurgy

The present study investigates the effect of two heat treatments on the microstructure, the tensile and the fatigue properties of a powder metallurgy tool steel that has undergone two heat treatments: quenching and multiple tempering (conventional for powder metallurgy tool steel), and quenching and multiple tempering with an intermediate cryogenic step at −80◦ C (new solution). The findings of the research indicated that the new heat treatment promotes the development of a homogeneous distribution of carbides in the martensitic matrix, with an increase of about 10% in tensile strength and about 7% in elongation to failure. This combination of exceptional strength with a high degree of toughness leads to an improvement in the fatigue behaviour of the steel, which exhibits a higher unnotched and notched fatigue strength (about 15% and 25% respectively) and a lower fatigue notch factor (about 15%) compared to conventionally heat-treated steel. These results highlight that the powder metallurgy tool steel, with the new heat treatment, could be a viable option for the production not only of tools and dies, but also for high-performance automotive components, including even those with complex geometries, such as camshafts or crankshafts