| Budget Amount *help |
¥3,500,000 (Direct Cost: ¥3,500,000)
Fiscal Year 1999: ¥1,100,000 (Direct Cost: ¥1,100,000)
Fiscal Year 1998: ¥2,400,000 (Direct Cost: ¥2,400,000)
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| Research Abstract |
Point defects which exist as a stable state in metals and alloys in any temperatures and compositions are well established to strongly affect their material properties. Aim of the present research is to determine the point defect structure in the intermetallic compounds with B2-type ordered structure by means of powder X-ray and neutron diffraction measurements and also to discuss the defect formation mechanism in these B2-type intermetallics by thermodynamic treatment.
Point defects determined for B2 FeAl alloys showed a certain hybrid state in which four kinds of point defect generally appears, similarly as has already been reported for B2 NiAl and CoAl alloys by our research group. Further, two curious features were found in point defect behavior, that is with increasing in temperature, first, thermal vacancies are created on both Fe and Al sites (random vacancy distribution, RVD) and second, antisite atoms tend to recover into their own sites (antisite atom recovering, ASAR). These
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situations can never be interpreted within a framework of the defect model previously proposed.
Thermodynamic treatment on the point defect formation in B2-type alloys was developed based on the Bragg-Williams method. In constructing the defect model, it was assumed : (1) constitutional defect in the ground state is the antisite atom and (2) defect behavior due to the temperature increase can be described as a hybrid state where the RVD reaction and the ASAR and other reactions succeeding the RVD process simultaneously occur. Numerical results for NiAl and FeAl alloys reproduced well their observed trends, e.g., the preferable formation of Ni (Fe) vacancies and a certain creation of the antisite Al atoms in both systems, and in the FeAl, the RVD and ASAR like behaviors.
Vacancy concentration in B2 AuCd alloys was obtained as a function of composition. It showed a rapid increase above 49.5%Cd and reached to about 1% at 51.5%Cd, which is comparable with the results in B2 FeAl alloys. So, the point defect behavior can be expected to be a similar hybrid one as in the FeAl. B2 AuCd alloys transform to the martensite phase when decreasing in temperature. The 'martensite ageing effect' appearing in ageing in the martensitic state is believed to related to the point defect behavior. Changes in the lattice constant and the integrated intensities of fundamental and superlattice reflections due to martensite ageing were measured for slow-cooled and water-quenched Au-49.8Cd alloy. The results showed no detectable change, indicating that vacancy migration and also long-range ordering during ageing do not occur. This suggests an independence of the long-range order on the martensite ageing effect in this alloy. Less
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