Yoshinao Mishima

Martensite transformation temperatures and mechanical properties of ternary NiTi alloys with offstoichiometric compositions

Abstract Effects of ternary additions, Co and Cr, on martensite transformation temperatures and mechanical properties of offstoichiometric NiTi alloys were investigated. Martensite transformation start temperature (M s ) and austenite transformation start temperature (A s ) were determined by means of differential thermal analysis (DTA) in the temperature range between 77 and 423 K. Mechanical properties of yield stress and work-hardening coefficient were investigated using B2-stabilized NiTi alloys, whose alloy compositions were based on Ni-49 mol% Ti. It has been shown that (1) M s and A s considerably decrease with decreasing Ti concentration on the Ti poor side of stoichiometry (the concentration of Ti is less than 50 mol%); (2) M s and A s change slightly when Ti concentration is more than 52 mol%, where the alloys are in the two phase region; and (3) M s and A s decrease with increasing Cr and Co content with a constant Ti concentration. In the present analysis for evaluating the effect of ternary additions on M s , we use M s change by adding 1 mol% of ternary elements. M s changes by Cr addition are −65 K mol% −1 on the Ti rich side and −46 K mol% −1 on the Ti poor side. M s changes by Co addition are −15 K mol% −1 on the Ti rich side and −30 K mol% −1 on the Ti poor side. Both Cr and Co atoms are suggested to locate Ni sites preferably if only M s changes are considered. Characteristic stress-strain curves have indicated that the stress induced martensitic transformation (SIMT) occurred at 77 K. Above room temperature, work hardening coefficient at a permanent strain of 1% was found to be between 2 and 11 GPa. It should be emphasized that yield stress and work hardening coefficient increase with increasing test temperature between room temperature and about 650 K in most alloys. This strength anomaly is not related to SIMT but to precipitation hardening and/or anomalous dislocation motion, probably in a similar manner to B2-type CoTi.

Compatibility between carbon fibre and binary aluminium alloys

Compatibility between carbon fibre and various binary aluminium alloys is investigated. Series of aluminium alloys are coated onto the fibre surface and the wetting behaviour of these alloys are observed by scanning electron microscopy after heating the coated fibres at 1073 K, well above the melting point of the alloys. It is found that the aluminium alloys containing such elements as indium, lead and thallium showed excellent wetting behaviour. These alloying elements have significantly smaller surface energies as compared to aluminium and they are practically insoluble with aluminium, even in the liquid states. Tensile test shows that the strength of carbon fibres is not degraded after heating at 1073 K when fibres are coated with an Al-1 at % Tl alloy. The reasons for these elements in aluminium to substantially improve the compatibility against carbon fibres are discussed from the thermodynamical point of view.

High temperature thermoelectric properties of p- and n-type β-FeSi2 with some dopants

Abstract Cr doped p-type and Co doped n-type β-FeSi2 alloys have been prepared using a powder metallurgy technique. Then the effects of Cu and Ge additions on their thermoelectric properties were investigated. Thermoelectric power α, electrical conductivity, σ, and thermal conductivity, κ were measured in a temperature range from 373 to 973 K. Their thermoelectric power is found to decrease with increasing an amount of both Cu and Ge in both type of β-FeSi2. In contrast, their electrical conductivity exhibit opposite tendency with respect to the amount of both dopants. Their thermal conductivity is more effectively reduced in the Ge addition to them than that in the Cu addition. The figure of merit, Z of binary β-FeSi2 is 0.19×10−4/K, and increases by the addition of Cr, Co, and Ge. The maximum figure of merit, Z of 1.3×10−4/K was obtained in Fe0.95Co0.05Si1.958Ge0.042 at 845 K.

Accurate Determination of γ′ Solvus in Ni-Al-X Ternary Systems

An attempt has been made to determine the γ/γ′ phase boundaries with particular emphasis on the γ′ solvus in ternary Ni-Al-X systems with X being one of the VA or VIA transition metal elements. It is shown that the γ′ solvus in each ternary system can be obtained as a surface with respect to composition and temperature by employing differential thermal analysis (DTA) as a key experimental technique with support from energy dispersive X-ray spectroscopy combined with scanning elecron microscope(SEM-EDX). Also shown is that the DTA results often provide useful information on the phase relation in the vicinity of γ and γ′ phase fields. In the Ni-Al-V ternary system, for example, the pseudo-binary constitution between Ni 3 Al and Ni 3 V is suggested. The results presented here are systematic and consistent and therefore are reliable as compared with the previous results reported fragmentarily.

Interfacial reaction and strength of SiC fibres coated with aluminium alloys

Silicon carbide fibres (Nicalon) were coated with pure aluminium and aluminium alloys containing silicon. The coated fibres were annealed to produce an interfacial reaction zone between the coated layer and the fibre. The effect of this reaction zone on the tensile strength of the fibre was investigated. During the early stages of growth the reaction zone of the fibre is thin, and the strength of the fibre is controlled by inherent defects so that the fibre retains its original strength. After the early stages, notches are formed in the reaction zone of the fibre on loading at a small strain and the fibre fractures when a notch extends into the fibre. In this stage the fibre strength is dependent on the thickness of the reaction zone. An alloying addition of 1 or 5 at % Si to the aluminium matrix was found to be effective in retarding the growth rate of the reaction zone.

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.

Design of Laves phase strengthened ferritic heat resisting steels in the Fe-Cr-Nb(-Ni) system

Abstract A new class of heat resisting ferritic steels is investigated in the Fe–Cr–Nb and Fe–Cr–Nb–Ni systems, in which the major strengthener is the Fe2Nb Laves phase. Ferritic steels strengthened by Laves phase are expected to show excellent high temperature strength, while the brittleness of Laves phase may lower the toughness of the alloy. The α-Fe/Fe2Nb two-phase microstructure is selected to improve mechanical properties through changing volume fraction and morphology of Laves phase. Effects of Nb and Ni contents on the microstructure and mechanical properties of the alloys, fixed at 10 at.% Cr, have been systematically investigated. Mechanical properties were evaluated by tensile tests conducted at room temperature, 873 and 973 K. The tensile test revealed that the room temperature ductility decreases with increasing Nb content. The alloys with 1.0–1.5 at.% Nb are found to exhibit a good balance between room temperature ductility and high temperature strength.

Effect of alloy composition on microstructure and high temperature properties of Nb–Zr–C ternary alloys

Abstract The effect of alloy composition on microstructure and mechanical properties of Nb–Zr–C ternary alloys were investigated. Alloys were prepared arc melting and homogenized. The microstructure of the alloys were characterized by optical microscopy, X-ray diffraction and electron probe microanalysis. Compression tests were conducted from room temperature to 1773 K to determine the stress–strain curves and 0.2% flow stress. Fracture toughness of the alloys was determined using a single-edge-notched specimen under a three-point bending test. The results indicate that the 0.2% flow stress of the Nb–ZrC alloys increases with increasing ZrC content from room to elevated temperature. However, the compressive ductility and fracture toughness decrease with increasing ZrC content. A proper alloy design to obtain balance of high temperature strength and room temperature ductility is also discussed.

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.

Heterogeneous ordering and antiphase domain morphology in Cu3Pt (19 at.%Pt)

Abstract The ordering process in a Cu[sbnd]19at.% Pt alloy has been investigated. Differential thermal analysis revealed that the ordering occurs in two stages, a low-temperature stage I and a higher-temperature stage II. Examination of the stage I ordering during isothermal annealing by X-ray analysis showed that the variation in lattice constant and domain size with annealing time exhibit a step for annealing temperatures above 540 K, and that line broadening of both fundamental and superlattice peaks is observed irrespective of the annealing temperature. Combining this information with observations of the morphology of antiphase domains by transmission electron microscopy, it is confirmed that stage I ordering is a process of heterogeneous nucleation and growth of the ordered domain from the grain boundary, assisted by the migration of excess vacancies introduced by quenching. It is concluded that the kinetics of stage I ordering is controlled by the stability of the defect formed by the clustering of ...