Materials Science Forum Vol. 850

Abstract: The effects of different aging processes on microstructure and properties of Fe-26Mn-7Al-1.3C austenitic steel were investigated by mechanical properties testing, optical microscopy (OM). And scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and transmission electron microscopy (TEM) were also used to study the deposition, morphology and composition of the precipitates. The experimental results showed that after homogenized at 1050°C for 1 h, the best aging process was aged at 550°C for 2 h, then air cooled to room temperature. Through the best heat treatment process, some fine κ-carbides with a modulated structure ((Fe, Mn)₃AlC_x) were found to precipitate within the austenitic matrix, which significantly enhanced the austenitic matrix. Its best comprehensive mechanical properties were tensile strength of 789MPa, yield strength of 612MPa, impact toughness values (V-notch) of 168J/cm², surface hardness of HB272. Nevertheless, as the aging time prolonged, the fine globular κ-carbides became coarse and grew along austenite grain boundaries and were harmful to mechanical properties of the experiment steel.

Abstract: Al-5.0Cu-0.6Mn-0.6Fe alloy was obtained for the first time using ultrasonic vibration and squeeze casting simultaneously. The effect of ultrasonic vibration and applied pressure on the microstructures and hardness of Al-5.0Cu-0.6Mn-0.6Fe alloy were studied. The results indicated that the ultrasonic vibration and applied pressure promoted the formation of smaller α-Al globular grains. In particularly, with the treatment of ultrasonic vibration or applied pressure during solidification, the brittle Fe-rich imtermetallic compounds in Al-5.0Cu-0.6Mn-0.6Fe alloy became more refined and changed from Chinese script shape to polyhedral shape, which improved the mechanical property. Furthermore, these effects on the grain refinement and Fe-rich intermetallic compounds became more significant by using ultrasonic vibration and applied pressure concurrently during solidification. This process technology is helpful for the development of high performance aluminum alloys with low cost as well as for green casting.

Abstract: For developing of a ultra-purified ferritic anti-bacterial stainless steel, 1.4 Cu % was added in 21% Cr ferritic stainless steel. The effect of annealing time on the formability was analyzed. The micro texture of steel was measured by using electron back scattering diffraction analysis. The Cu precipitates through different annealing periods were investigated by means of transmission electron microscopy. Average plastic strain ratio was calculated by the r values at 0, 45°and 90° directions measured by tensile testing. The results shows that with the extension of annealing time, the morphology of the Cu precipitates became into rod from spherical and the size increased. Average plastic strain ratio increased first and then decreased, while the change trend of anisotropy index was opposite. The higher formability is due to the {111} texture components increase significantly, while more {001}<110> component at 60 min was adverse for the formability. And anisotropy index was determined by γ components and {110}<110> components.

Abstract: Due to its high silicon content, Fe-6.5wt%Si alloy has low iron loss and its magnetostriction is almost zero. And therefore Fe-6.5wt%Si alloy has good development prospect. However it has poor ambient temperature ductility and its cold rolling is very difficult. It’s important to study the effect of cooling rate on the microstructure and mechanical properties of Fe-6.5wt%Si alloy. In the present study the master alloy was melted and cooled through normalization, gas atomization, laser surface remelting and melt spinning. The microstructure, micro-hardness and X-ray diffraction were analyzed. The evolution of the microstructure at different cooling rate was summarized. The results indicated that under high cooling rate, the grain was obviously refined, and the microhardness decreased, but the change of phase was not obvious.

Abstract: The microstructure evolution during homogenization of 2A66 Al-Cu-Li alloy was investigated by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX), transmission electron microscopy (TEM) and differential scanning calorimeter (DSC). The results showed that the dendritic segregation can be found in the ingot of experimental alloy. Numerous eutectic phases can be observed in the grain boundary, and the distribution of the main elements along the interdendritic region varied asymmetrically. It was found that the main secondary phase was Al₂Cu. Differential scanning calorimeter (DSC) results showed that the over-burnt temperature of 2A66 Al-Cu-Li alloy was 515°C. With the increase of homogenization temperature in the area of 480°C~510°C, the boundaries of experimental alloy became sharp and the most of secondary phase dissolved into the matrix. The experimental alloy heat treated at 520°C was over-burnt. The Al₃Zr precipitated and α (Al) super-lattice was observed by TEM in the experimental alloy after homogenization.

Abstract: Two-stage aging treatments were applied to Al-6Si-2Cu-0.5Mg casting alloy, and the influence of aging treatment parameters on the microstructure and mechanical properties was investigated. The experimental results indicated that the microstructure and mechanical properties of Al-6Si-2Cu-0.5Mg alloy were significantly influenced by the aging time and temperature, and the elongation remarkably increased with the aging time increasing before 7 h. In the case of the alloy aged at 200°Cfor 2h, higher hardness and tensile strength were obtained, which may be attributed to precipitation of a large amount of Cu-rich phases. However, the higher elongation was achieved in the alloy under 200°C aging treatment for 5h, while its hardness and tensile strength slightly decreased. It is mainly due to the amount of the Cu-rich phases decrease slightly, but the morphology of the phases evolved from plate-like to bulk-like structure.

Abstract: Morphology evolution of eutectic Si in Ce-rich mischmetal (RE) modified Al-10wt.%Si alloy at different cooling rates was investigated. The morphology of eutectic Si and modification mechanism of RE was investigated by scanning electron microscopy (SEM) and Transmission electron microscopy (TEM). The results showed that the RE modified eutectic Si exhibited a plate-like morphology under the low cooling rate (~100K/s). When cooling rate increased to ~600K/s, some branches were observed on the eutectic Si. In the RE modified alloy with a higher cooling rate (~1000K/s), the number of the branches on RE modified eutectic Si increased, and the morphology of eutectic S was modified to coral-like structure. The addition of Sr caused a flake-to-fiber modification of eutectic Si at low cooling rate, and the fiber size decreased with cooling rate increasing. The morphological observations indicated that the morphology of eutectic Si in RE modified alloy was significantly influenced by the cooling rate, while the modification efficiency of RE was lower than that of Sr.

Abstract: The solidification behavior, microstructural evolution and mechanical properties of Al-Si-Mg foundry alloy with different Ti additions were investigated in the present study. The solidification behavior of those A357 alloys was analyzed through thermal analysis. The microstructures were examined by optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The results showed that the addition of Ti could refine grains of A357 as-cast alloy due to a good restriction on the grain growth, but Ti could not refine secondary dendrite arm spacing (SDAS), thus mechanical properties of the A357 as-cast alloy did not improved significantly. After T6 heat treatment, the microstructure with α-Al dendrites with the Al-Si eutectics at interdendritic space was replaced by a homogeneous α-Al matrix with a nonuniform dispersion of discrete, spheroidizing and coarse silicon particles. Hence, compared with the as-cast alloys, both of the strength and ductility of the T6 treated alloys are significantly improved, and an optimal combination of strength and elongation of the A357 alloy can be achieved by the 0.8 wt.% Ti addition after T6 heat treatment.

Abstract: Spray forming has attracted considerable attention for the production of high speed steels due to its potential and priority in the microstructure refining and cost saving. In this study, high-quality large billets of 2060 high speed steel were successfully produced by spray forming process using a twin-atomizer facility. As-deposited billet was subsequently processed by hot forging, quenching in oil at 1180 °C and a triple tempering in the temperature range of 500-580 °C. The microstructures and hardness of the deposit and their subsequent development resulting from hot forging and heat treatment were investigated. This paper was designed to provide insight and have a better understanding of such a system for the steel. The results showed that the as-deposited microstructure was composed of the fine equiaxed grains with V-rich MC and W-Mo-rich M₂C carbides non-uniformly distributed along the grain boundaries and inside the grains. M₂C presented rod-like or unconnected net-shaped morphologies in the as-deposited microstructure. Following hot forging, metastable M₂C carbides were completely decomposed into refined MC and M₆C nearly spherical carbides uniformly distributed throughout the microstructure. A hardness value of 31HRC was attained for the spray deposited and hot forged samples. With increasing the tempering temperature, hardness was increased firstly and then decreased. Secondary hardening peak appeared at 540 °C for spray formed 2060 steel austenitized at 1180 °C, and the corresponding peak hardness reached 71HRC.

Abstract: Surface segregation exists in two-phase zone continuous casting (TZCC) alloy with wide solid–liquid two phase zone. The surface segregation formation cannot be explained by the traditional solidification theories. ProCAST software was used to simulate the TZCC process for preparing the Cu–4.7 wt%Sn alloy with wide solid–liquid two phase zone. The Sn solute distribution in TZCC Cu–4.7 wt%Sn alloy was investigated, and the surface segregation mechanism of TZCC Cu–4.7 wt%Sn alloy was analyzed. The results showed that numerical simulation results were agreed with that of experimental. TZCC Cu–4.7 wt%Sn alloy in the center firstly started to solidify and resulted in “Λ” shape inclined solid/liquid (S/L) interface near the mold. Therefore, a narrow gap between the wall of the two-phase zone mold and the S/L interface formed. On the one hand, while Cu–4.7 wt%Sn alloy solidified along the opposite continuous casting direction, the solute redistribution between the solid and the liquid occurred, which lead to Sn solute decreased in solid and enriched in front of S/L interface. Because the narrow gap lies in front of inclined S/L interface near the two-phase zone mold, Sn solute enriches in liquid of the narrow gap. On the other hand, during the TZCC process, solid grains nucleate on the wall of the two-phase zone mold, while the melt feeds into the two-phase zone mold which the temperature is in the two-phase zone of the Cu–4.7 wt%Sn alloy. The solute redistribution also occurs while the solid grains grow, thus lead to Sn content increases in front of S/L interface near the wall of the two-phase zone mold. The enriched Sn solute is too late to diffuse, and will quickly flows into the narrow gap, resulting in further increasing of Sn content in the narrow gap. The liquid with enriched Sn solute in the narrow gap will become the surface layer after solidification, which lead to surface segregation layer during the TZCC Cu–4.7 wt%Sn alloy.