Kazuhiro Matsugi

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.

Observation of particle behavior in copper powder compact during pulsed electric discharge

Abstract The behavior of spherical copper powder particles of uniform size (average diameter: 550 μm) in a powder compact was observed under an optical microscope during a single-pulse electric discharge of 500 ms duration. The morphologies of necks formed between powder particles were observed under a scanning electron microscope, and their diameters were measured. The results obtained are summarized as follows: pressure and pulsed current density determine whether or not a spark occurs. Spark is more likely to occur at interparticle contacts under low pressure and high current density. Where a spark occurs, particles are joined together by melting. Regardless of whether or not a spark is observed, necks are formed at points of contact between particles and neck diameter increases with pulsed current density. These results suggest that microscopic sparking, melting, and vaporization occur by means of extremely high temperature attained by local heat generation at the interparticle contacts in the initial stage of compaction.

A case study for production of perfectly sintered complex compacts in rapid consolidation by spark sintering

Abstract The distribution of voltage, temperature and titanium-compact density in the spark sintering was investigated for clarification of phenomena in the sintering stage governed by plastic deformation mechanism. The largest heat source in a punch–die-two step cylindrical compact system was in the punch with a smaller sectional area. The heat flow was mainly from the smaller punch to the compact. The sintering was mainly promoted by the mechanisms of plastic deformation and power law creep during the continuous current discharge. The effect of the plastic deformation mechanism on the final relative density of compacts was large in the press method and flanged compact used in this study. A procedure, particularly the press method, was proposed on the basis of this result for the production of homogeneously perfect sintered products. This proposed procedure could produce perfectly sintered complex compacts with their designed final dimension.

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.

An electronic approach to alloy design and its application to Ni-based single-crystal superalloys

Abstract The d-electron concept was applied to the design of nickel-based single-crystal superalloys. This concept was devised on the basis of molecular orbital calculations for transition-metal-based alloys. Two calculated parameters were mainly utilized in this alloy design. One is the d-orbital energy (evel (Md) of alloying transition elements and the other is the bond order (Bo), which is a measure of the covalent bond strength between atoms. Using these parameters, accurate prediction is possible for the appearance of the γ + γ′ eutectic phase, the topologically close-packed phases and the α-W phase in the olloys. Also, the strengthening of both the γ and γ′ phases in the alloys can be treated with these parameters. On the basis of this concept, single-crystal superalloys were designed successfully. Six kinds of designed alloys lay in the compositional range Ni–10mol.%Cr–12mol.%Al–1.5mol.%Ti–(1.8–2.7)mol.%Ta–(1.8–2.7)mol.%W–(0.6–0.9)mol.%Mo–0.25mol.%Re It was confirmed experimentally that these designed alloys had excellent creep rupture properties, high hot corrosion resistance, low density and loow material cost. In this respect the designed alloys are much superior to any single-crystal superalloys published in the literature to date.

Melting and solidification of TiNi alloys by cold crucible levitation method and evaluation of their characteristics

Abstract The addition of Re, Fe and Cr into Ti–50 mol.-%Ni has been carried out to improve the oxidation and mechanical properties. The mono phase consisting of TiNi with the B2 type structure was identified in micro-alloyed materials proposed on the basis of the d-electrons concept. Experimentally, TiNi alloys were melted and solidified by the cold crucible levitation melting (CCLM) method. The TiNi–(Cr, Fe, Re) alloys with high purity and without contamination from a crucible were prepared, and the homogeneous microstructure was achieved by the diffusion mixing effect of CCLM even in the as-cast alloys which contained Re and Cr with higher melting temperatures and different specific gravities. The transformation from austenite to martensite phases occurred in all alloys below or above room temperature. Some alloys had the ability of shape memory even at room temperature. Ternary alloys showed a higher flow stress level compared with the binary TiNi alloy. On the other hand, the oxidation at 1273 K was promoted by the formation of titanium oxides (TiO2) on the alloy surfaces. The oxidation resistance was improved by the formation of the continuous Cr2O3 film in TiNi–Cr alloys. The alloying effects by ternary elements (Re, Fe, Cr) in the intermetallic TiNi as well as metallic materials were explained well using two parameters used in the d-electrons concept.

Design and manufacture of an impact–electric current discharge joining machine

Abstract An impact–electric current discharge joining machine, which can simultaneously apply impact load and electric current, has been designed and manufactured to produce a high-strength joint between materials with little change in their appearance. The objective functions of the designed and manufactured impact–electric discharge machine are almost satisfied. Partial joining is achieved between FCD450/FCD450 samples and between FCD450/Al samples and the joints exhibit low fracture stress. Process parameters can be controlled to improve joint strength, by adjusting the setting values of the functions provided in the machine.

Fabrication and characterization of unidirectional CF/Al composites

Abstract The unidirectional carbon fiber (CF) reinforced aluminum (Al) composites have been fabricated by the low pressure infiltration (LPI) of molten Al into porous CF preform. Prior to the fabrication of the unidirectional CF/Al composites, the unidirectional CF preform was prepared by sintering of CFs and copper (Cu) particles under the spark plasma sintering (SPS). The compression strength of CF preform was examined to determine the infiltration pressure of molten Al into CF preform. The effects of the different infiltration pressures and sizes of Cu particles on the densification of unidirectional CF/Al composites have also been investigated. The compression strength of CF preform increased with increasing of the contact area between CFs and Cu particles. The density of CF/Al composites improved with the increase of the infiltration pressure. The CF/Al composites, into which the bimodal Cu particles are added, have average particle sizes of 2.55 μm and 11.79 μm, with a high relative density of about 95% with the nearly homogeneous fiber distribution.

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.