Seiji Miura

Thermoelectric semiconductor iron disilicides produced by sintering elemental powders

Abstract Development of a sintering process to fabricate iron disilicides with a fine grain structure is pursued using elemental powders as starting materials with additions of Al. The Al additions are expected to involve liquid Al-rich phase during sintering to accelerate the reaction kinetics. At the same time, additions are made of Co and Cu, the former being an n-type dopant while the latter might promote metal-to-semiconductor transition upon annealing after sintering. The effects of such additions on the sintering kinetics, constituent phases in the products, and their thermoelectric properties are examined. It is shown that fabrication of sintered iron disilicides using elemental powders, heretofore believed to be difficult, becomes possible with Al additions. Then a mechanism of sintering in the Fe–Si–Al ternary system is proposed, and finally a series of demonstrations is given for the changes in thermoelectric properties depending upon the doping element used. It becomes evident that the thermoelectric figure of merit of the present materials is equivalent to that of the conventionally fabricated iron disilicides.

Phase stability and relations of multi-phase alloys based on B2 CoAl and E21 Co2AlC

The CoAlC ternary phase diagram has been experimentally examined for the Co-corner with a particular interest in phase relations among cobalt primary solid solutions, denoted as (Co), the E21 type intermetallic compound Co3AlC, and the B2 type CoAl. Liquidus surface composed with monovariant and contour lines, and corresponding reaction scheme were determined by optical microscopy, differential thermal analysis, X-ray diffraction, and energy dispersive X-ray spectroscopy. Several isothermal sections and isoplethals with respect to a fixed concentration for one of the elements were also evaluated. It is shown that such multiphase alloys as B2/E21, B2/(Co), and B2/E21/(Co) can be produced in the ternary system by proper choices of alloy chemistry and heat treatment.

Amount of liquid phase during reaction synthesis of nickel aluminides

Abstract Intermetallic phases and the liquid phase are estimated using thermodynamic calculations as a function of temperature, the amount of phases and diluents, and Ni concentration in liquid Al during the reaction synthesis for NiAl binary alloys having arbitrary compositions under an adiabatic condition. Calculations use available thermodynamic data concerning intermetallic compounds and the liquid phase. Reaction synthesis experiments are then performed on compactions of powder mixtures which have several binary compositions. The effect of some fundamental conditions which relate to the conditions for calculation are investigated and the results obtained are discussed in terms of the amount of liquid phase estimated. It is found that the calculated amount of liquid phase in NiAl elementary powder compacts during the synthesis reaction are in good agreement with the experimental results; however, other factors are needed to understand the densification behavior of specimens with diluents such as liquid migration and formation of a skeleton structure.

Improvement in room temperature ductility of intermetallic alloys through microstructural control

A summary is presented of the recent advances in the development of multiphase intermetallic alloys towards improved room temperature ductility investigated by the present authors. The alloy systems of interest are Ni-Al-Be, Co-Al-Ni-Ti, and Co-Al-C, in which two- or three-phase alloys can be designed involving the Ll2, B2 and the primary solid solution, hereby denoted as (Ni) or (Co,Ni) in the first two systems, while the B2, E21, and the primary solid solutions, denoted as (Co) in the latter system. It is shown that by choosing a proper combination of phases with particular morphologies, good room temperature ductility can be obtained not only by compression and bending tests but also by tensile tests. Such a strategy has been so far most successful in the Co-Al-Ni-Ti system, where room temperature ductility of near to 20% is achieved in a B2/Ll2/(Co,Ni) three-phase alloy. It is pointed out that keys to design ductile multi-phase intermetallic alloys would be: (1) refinement of the microstructure utilizing solid state phase transformations such as an invariant reaction: (2) adjustment of amounts and compositions of the constituent phases, which is not possible in the binary system, but is in a multi-component system, and (3) hot fabrication to reduce solidification defects.

Positive temperature dependence of strength observed in ordered Cu3Pt single crystals

Abstract The composition and temperature dependences of strength of ordered and disordered single crystalline Cu3Pt alloys involving ternary additions are investigated. The relation between deformation behaviour and relative phase stability of L12 ordered phase with respect to L12 −s long period ordered phase is discussed in relation to the antiphase domain (APD) morphology observed in each alloy. Direct observation of APDs by transmission electron microscopy shows that maze-like APDs are formed during ordering in Cu-23at.%Pt alloy, while in (Cu-23at.%Pt)-1 at.%In and Cu-19at.%Pt alloys swirl-like APDs are formed. This implies that the relative phase stability of L12 phase with respect to L12 −S phase in Cu-23 at.%Pt alloy is lower than that in (Cu-23at.Pt)-l at.%In and Cu-19at.%Pt alloys. Compression tests of an ordered single crystal of each alloy at various temperatures reveal a positive temperature dependence of critical resolved shear stress (CRSS) in Cu-23 at. %Pt alloy, but not in (Cu-23 at.%Pt)-1 ...

Evaluation of the data for the plastic behaviour of Ni3(Al,X) single crystals

All the availablesshear stress data for various kinds of L12 single crystals which have been reviewed in 1989 are revised with the recent progress obtained with the last 2 years. Most of the findings in that review are strengthened further by the introduction of new data. Besides this, it is pointed out that the shear stress of L12 compounds such as Ni3Ge and Ni3Si with a strong positive temperature dependence of strength have a tendency not to obey Schmids law at low temperatures. At elevated temperatures, it is shown that the relation between shear stress and shear strain for the steady state stress obtained in the constant-strain-rate test and the strain rate obtained in the constant-load test, i.e. the creep test, can be expressed by a single straight line. The composition dependence of the creep resistance in Ni3Al is the most interesting result found. The creep resistance increases with increasing nickel concentration on both sides of stoichiometry but the value extrapolated to stoichiometry from the nickel-rich side is much larger than that extrapolated from the aluminium-rich side. The problems which must be solved in this field are suggested.

Dislocation morphology, structure and density on (001) slip plane in Ni3(Al,Ti) single crystals deformed at elevated temperatures

Abstract Ni 3 Al single crystals with the L1 2 structure and with a [ 1 23] orientation with respect to the stress axis were deformed by compression at temperatures above T p , the yield stress peak temperature, so that (001) cube slip dislocations were introduced. The dislocation morphology, configuration and density on (001) slip planes were examined as functions of the deformation temperature, strain rate and strain by transmission electron microscopy. It was found that dislocations on the cube plane were predominantly of edge character and that within the range studied the dislocation density was largely independent of the deformation temperature, strain rate and strain.

Microstructural control for ductilization of multi-phase alloys based on B2 CoAl

Abstract Tensile tests were carried out on several alloys based on the B2 intermetallic compound CoAl. The alloys are multi-phase with Co primary solid solution, L1 2 (Co,Ni) 3 Al, or E2 1 Co 3 AlC; they were developed by the present authors and showed some room temperature ductility previously under compression testing. Also, attempts to refine the microstructure of the selected alloys were made using heat treatment and hot or cold rolling. Although no drastic improvement in room temperature ductility is observed, indications of a microstructure favorable for improved ductility are obtained. Also, it is found that some type of solidification defect causes premature failure during tensile tests, but not during compression tests, for which a cure should be sought after further improvement in the mechanical properties.

Phase Stability and Mechanical Properties of Multi-Phase Alloys Based on the B2 CoA1 and the E2 1 Co 3 AlC

The Co-Al-C ternary phase diagram has been experimentally examined for the Co-corner with a particular interest in phase relations among B2 type intermetallic compound, the E2{sub 1} type Co{sub 3}AlC, and cobalt primary solid solution, denoted as (Co). Reaction scheme, liquidus surface, isothermal sections and isoplethals at constant concentration were determined. Mechanical properties of the B2/E2{sub 1}/(Co) three-phase alloys were investigated by compression tests carried out at a temperature range from 77 to 1,273 K, and by tensile tests at room temperature. It has been revealed that both excellent ambient temperature ductility and sufficient high temperature strength can be achieved by proper choice of alloy compositions.

Orientation Dependence of High Temperature Creep Strength and Internal Stress in Ni 3 Al Alloy Single Crystals

High temperature creep behavior of a nickel-rich Ni{sub 3}(Al,Ta) with the L1{sub 2} structure is investigated in order to clarify the influence of crystallographic orientation with respect to the stress axis. The single crystals with four different orientations are deformed in compressive creep at temperatures ranging from 1,123 to 1,273 K under a constant load, initial shear stress being 35 to 120 MPa for (111)[{bar 1}01] slip system. The results show a distinct orientation dependence of creep strength, although shape of creep curves, stress exponent and the activation energy seem to be independent of the orientation. It is shown, however, the internal stress, being measured by strain transient dip tests, is found to be orientation dependent and the creep behavior is independent on orientation if it is interpreted using the effective stress instead of the applied shear stress.