In this study, the crystallization behavior of melt-spun Al87Ni10La3 amorphous phase was investigated by using X-ray diffraction and non-isothermal differential thermal analysis techniques. The results demonstrated that the amorphous phase exhibited two-stage crystallization on heating, i.e., at first step the amorphous phase transforms into α-Al phase and at second step Al11La3 and Al3Ni intermetallic phases precipitate, simultaneously. The activation energies of the crystallizations of the amorphous phase were evaluated by the Kissinger equation using the peak temperature of the exothermic reactions. The values of two-step crystallization activation energies were approximately 173.7±6 and 278.4±5 kJ/mol, respectively. The Avrami index was calculated for the first and second step of crystallizations and obtained 0.9 and 3.8 respectively. This kinetics investigation indicated that in the first step of crystallization of Al87Ni10La3 alloy, the nucleation rate decreases with time, and the crystallization is governed by a three-dimensional diffusion-controlled growth, while the second stage was in interface control regime. پرونده مقاله

In this research, microstructure and mechanical performance of dissimilar resistance spot welded DP780/DP980 dual-phase steels were studied utilizing optical microscope, microhardness, and tensile shear tests. Resistance spot welding (RSW) was performed in the current range of 7 to 12 kA, with 0.5 kA steps. At welding currents lower than 7 kA low amount of melting led to the very low strength of the joints due to small weld nugget diameter. The results showed that an increase in welding current from 7 kA up to 11 kA, result in an increase in weld nugget diameter. Further increase of welding current (higher than 11 kA), however decreased the weld nugget diameter due to severe melt expulsion. Microstructural studies showed that weld nugget was primarily comprised of martensite, and the heat-affected zone (HAZ) of both sides of the joint was comprised of three different microstructural zones; upper-critical HAZ (UCHAZ), inter-critical HAZ (ICHAZ), and sub-critical HAZ(SCHAZ). Microhardness test showed that at both sides, softening occurred at SCHAZ. The results of the tensile shear test showed that both peak load and fracture energy of the joints followed approximately the same trend as weld diameter with welding current. Two different fracture modes of interfacial failure (IF) and pullout failure (PF) were observed in the tensile-shear test. At welding currents lower than 10 kA, the failure occurred in IF mode, while at higher welding currents, PF was dominant. Weld nugget diameter at welding current of 10 kA; i.e., critical weld nugget diameter, was ~8.5 mm. پرونده مقاله