Morihiko Nakamura

Elastic constants of MoSi2 and WSi2 single crystals

Single crystals of MoSi2 and WSi2 with a body-centred-tetragonal C 1 1b structure were fabricated using a floating-zone method. The elastic wave velocity was measured for samples with various orientations using a simple pulse echo method at room temperature, and six elastic stiffness constantscij were calculated. The stiffness constants were a little higher for WSi2 than for MoSi2.c11 andc33 of these compounds were approximately equal toc11 of tungsten and molybdenum, respectively, althoughcij (i ≠j) was a little higher for these compounds than for molybdenum and tungsten. Youngs modulus 1/s11 was the highest in the <0 0 1> direction, and the lowest in the <1 0 0> direction. The shear modulus 1/s66 was high on the {0 0 1} plane and independent of shear direction. It was generally low on the close-packed {1 1 0} plane and largely dependent on shear direction. The elastic constants for the polycrystalline materials were estimated fromcij andsij. Poissons ratiov was 0.15 for MoSi2 and for WSi2, and these values were much lower than for ordinary metals and alloys. The Debye temperature θD was estimated using the elastic-wave velocity of the polycrystalline materials via the elastic constants such as Youngs modulus and shear modulus: it was 759 K for MoSi2 and 625 K for WSi2.

Elastic constants of TiAl3 and ZrAl3 single crystals

The elastic stiffness constants, cij, were measured from the velocity of ultrasonic waves for TiAl3 and ZrAl3 single crystals with tetragonal D022 and D023 structures, respectively. The value of c11 for the 〈1 0 0〉 direction was approximately equal to that of c33 for the 〈0 0 1〉 direction in both TiAl3 and ZrAl3. Youngs modulus for a single crystal was the highest in the 〈1 1 0〉 direction in which titanium or zirconium atoms and aluminium atoms were arranged in the closest packed manner, although it was not so high in 〈0 1 2〉 and 〈0 1 4〉 directions which showed the other closest packed array of the constituent atoms for TiAl3 and ZrAl3, respectively. The elastic constants, such as Youngs modulus, shear modulus and Poissons ratio, were approximately estimated for ideal polycrystalline TiAl3 and ZrAl3 from the stiffness constants and the compliance constants for single crystals. The Poissons ratio of these materials was about 0.16 and 0.19 for TiAl3 and ZrAl3, respectively, and these values are much lower than those of ordinary metals and alloys. Debye temperatures were estimated at room temperature from the average velocity of ultrasonic waves and were 681 and 577 K for TiAl3 and ZrAl3, respectively.

High temperature deformation behaviour of MoSi2 and WSi2 single crystals

High temperature deformation behaviour of MoSi2 and WSi2 single crystals, which both oriented near 〈001〉 and near 〈100〉, have been studied by compression tests over the temperature range of 1100 to 1500° C in a high vacuum of less than 6×10−4 Pa. At elevated temperatures, several per cent compression deformation is possible in both MoSi2 and WSi2. Slips on {110∼ and {013∼ planes, the dislocation with the direction of Burgers vector 〈331〉 and the stacking fault on {110∼ plane are observed in both deformed MoSi2 and WSi2. In MoSi2, the 0.2% offset stress of the sample oriented <001> is higher than that of the sample oriented <100>. The higher strength of the sample oriented <001> is related to the higher CRSS for the main slip plane of it. The reverse orientation dependence of the strength in WSi2 is also correlated with the difference in CRSS on {110∼ and {013∼ planes, which shows the opposite result to MoSi2. The higher CRSS on {110∼ plane in WSi2 compared to that on {013∼ may be caused by the formation of a large number of stacking faults on {110∼ plane.

Compressive deformation of CoZr and (Co,Ni)Zr intermetallic compounds with B2 structure

B2 type (Co,Ni)Zr compounds which were prepared by arc-melting were deformed in compression at temperatures from liquid nitrogen temperature to 973 K. Their flow stress was anomalously dependent on the testing temperature, decreasing with increasing temperature up to room temperature and then increasing with temperature up to about 673 K, followed by a decrease. The peak of the flow stress was higher for PE specimens than for PA ones which were machined perpendicular and parallel to the direction of grain growth of the ingot, respectively. It is considered that this behaviour of the flow stress is caused, not by the phase transition but by the motion of superlattice dislocations. The ductility of CoZr was lowered by cracking at grain boundaries at which secondary phases were observed. The substitution of nickel for cobalt suppressed the grain boundary cracking and (Co,Ni)Zr had a higher ductility than CoZr.

Rapid quenching and properties of hard magnetic materials in MnAl-X (X=Ti, Cu, Ni, C, B) systems

MnAl-X (X=Ti, Cu, Ni, C, B) ribbons were fabricated by a rapid quenching from the melt using a single roller method. As-rapid quenched ribbons were quenched completely into high temperature h c p phase. The rapid quenched ribbons were brittle especially carbon doped ribbons, however, the titanium doped ribbons were relatively ductile. Hard magnetic properties were measured using a vibrating sample magnetometer for the ten kinds of ribbons annealed at 410–700° C for 10 min–100 h. Phase transformation characteristics and magnetic properties were discussed by comparison with the samples produced by the conventional method. Titanium, boron and carbon doped ribbons showed good hard magnetic properties.

The mechanical properties of Fe-Co heterogeneous alloys fabricated by powder metallurgy techniques

Fe-Co heterogeneous alloys fabricated by powder metallurgy techniques were hot rolled, cold rolled and then heat treated. These processes produced a type of fibre-reinforced composite which consisted of fibrous Fe and Fe-Co phases. The tensile strength of the alloys depended on the composition and degree of order of the Fe-Co phase. The rule of mixtures was applicable, provided that the dependence of strength in the Fe-Co phase on Co content was considered.

Mechanical and magnetic properties of the rapidly quenched Cu2MnAl

Rapidly solidified Cu2MnAl ribbons were fabricated by the chill-block melt-spinning technique as a function of rotation speed of an iron roller. The rapidly quenched ribbons were relatively ductile, and the total strain at failure for the bend test increased with increasing rotation speed of the roller. The effect of rapid quenching on long-range ordering in Cu2MnAl alloy was studied by X-ray diffraction. The decomposition characteristics during isothermal ageing at temperatures between 350 and 600° C were also examined by X-ray diffraction, magnetization and Vickers hardness measurements. The decomposition reaction at temperatures below 400° C was Cu2MnAl→γ-Cu9Al4+T-Cu3Mn2Al+β-Mn. However, at temperatures between 500 and 600° C, Cu2MnAl decomposed into a new L21 type Cu2MnAl andβ-Mn, and further annealing caused the appearance ofγ-Cu9Al4. The decomposition rate of the rapidly quenched ribbons was faster than that of the water-quenched alloy.

Permanent deformation of (Co, Ni)Zr intermetallic compounds through phase transformation

Abstract(Co, Ni)Zr intermetallic compounds, which have Bf structure at room temperature and B2 structure at elevated temperatures, were heated and cooled repeatedly in the temperature range between room temperature and 1223 K. This process produced the large permanent deformation in Co30Ni20Zr50. Permanent elongation by as much as 0.2 to 0.3% per cycle was observed for the specimen parallel to the columnar structure, which grew from the bottom to the top of the button ingot, and permanent shrinkage by as much as 0.2% per cycle was observed for the specimen perpendicular to the columnar structure. For example, permanent elongation of about 30% was obtained after 93 cycles of the heating and cooling process in the former specimen, but no permanent elongation nor shrinkage was observed in the specimen parallel and perpendicular to the columnar structure for Co36Ni14Zr50. The latter alloy had a different preferred orientation of the columnar structure from the former. These facts show that the crystal orientation influenced the permanent deformation caused by this transformation.