Tomei Hatayama

Impact properties of spark sintered titanium aluminides at elevated temperatures

Abstract The two kinds of spark sintered titanium aluminides, series 1 (70% TiAl, 30% TiAl/Ti 3 Al lamella) and series 2 (10% TiAl, 90% TiAl/Ti 3 Al lamella) specimens, having different volume fraction of the TiAl and TiAl/Ti 3 Al lamellar grains at or near the notch tip, are prepared for Charpy impact testing. These tests are conducted in air at various temperatures between 293 and 1373 K. Charpy impact value increases monotonously as temperature rises and its value is 2.0 to 8.1 kJ/m 2 , regardless of microstructural differences in the vicinity of the notch tip. However, the value (0.25×10 −3 m) of the deflection to failure is very low, even in the result obtained from the highest temperature (1373 K). On the other hand, the fracture load or maximum fracture load tends to increase as the test temperature is raised to 793 and 873 K in the series 2 and 1 specimens, respectively, and thereafter it decreases significantly. The maximum fracture load of the series 1 specimen is higher than that of the series 2 specimen throughout the test temperatures. The predominant mode of failure changes from transgranular fracture to intergranular fracture as the temperature is increased, which corresponds with the behaviour of the maximum fracture load.

Effect of heterogeneous precipitation on age-hardening of Al2O3 particle dispersion Al-4mass%Cu composite produced by mechanical alloying

The acceleration of aging kinetics has been frequently observed in aluminum matrix composites produced by ingot or powder metallurgy. Recently, in the mechanically alloyed (MA) Al-4mass%Cu/Al{sub 2}O{sub 3} composites, the authors have found that the age-hardening response significantly decreases, and that considerable stable {theta} phases are formed at a very short aging time. The purposes of this study are to investigate the local precipitation behaviors, and attempt to clarify the dominant microstructural factors of the decrease in the age-harden ability and the acceleration of the age-hardening kinetics in the Al{sub 2}O{sub 3} particle dispersion Al-4mass%Cu composites produced by mechanical alloying. In order to build a basis for comparison, the age-hardening behaviors of the unreinforced matrix alloy (IM alloy), which is produced by ingot metallurgy technique, are also investigated.

Microstructure of spark sintered titanium-aluminide compacts

Abstract The microstructural properties have been investigated for titanium-aluminide (Ti-53mol%Al) compacts spark sintered at four temperatures: 1573, 1623, 1648, and 1673 K after pulsed electrical discharge. The microstructure changes with sintering temperature and is classified into two groups. (1) Group 1: for specimens spark sintered at 1573–1648 K, the structure consists of three different grains which represent the core grain (core consisting of α-Ti and Ti3Al, surrounding grain consisting of Ti3Al), lamellar grain ( Ti 3 Al TiAl ) and equiaxed grain (TiAl). The structure of the core grain is the same as that of the as-received pre-alloy powder produced by combustion synthesis. (2) Group 2: for specimens spark sintered at 1673 K, the structure consists of two different grains which represent the lamellar ( Ti 3 Al TiAl ) and equiaxed (TiAl) grains. The grain growth is prevented during spark sintering. Vickers microhardness values of each phase in the spark sintered specimens are almost the same as those in specimens produced by other manufacturing methods. A simple rule of mixtures can be applied for the hardness of Ti 3 Al TiAl lamellar grains in spark sintered specimens. Specimens with high densities and approaching the equilibrium state can be obtained in a shorter time by spark sintering than conventional sintering. Such shorter high temperature exposure is important to prevent grain growth.

Rapid Consolidation by Spark Sintering of Rapidly Solidified 7075 Aluminum Alloy Powder

Compacts of 7075 aluminum have been produced from rapidly solidified powders by optimizing spark sintering parameters, such as pulse discharge time, fixed maximum temperature, holding time at this temperature, and method of cooling to room temperature after the sintering. High-grade compacts can be obtained by a short process (40-50 s) consisting of heating to 773 or 873 K at a heating rate of 9.6 K/s and holding this temperature for 10 s. The rapidly cooled compacts show the same supersaturated state at room temperature as the received as-atomized powder. Compacts quenched in water just after spark sintering at 873 K for 1.2 ks show the same age-hardening behavior as solution-heat treated compacts. Compacts that are quenched in water and aged after sintering at 873 K for 1.2 ks show the same elongation and flow stress as compacts aged after solution heat treatment. Elongation data suggest that compacts produced with longer holding time at a higher temperature and rapid cooling show a large amount of main alloying elements in solid solution and sufficient promotion of sintering.