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이경섭,홍경태,박희섭,고홍석 대한금속재료학회(대한금속학회) 1998 대한금속·재료학회지 Vol.36 No.5
A study has been made to investigate the fracture behaviors of heat-treated polycrystalline NiAl from the controlled indentation flaw method in conjunction with the ring-on-ring disk bend test. Fracture initiated at the indentation load exceeding 196.14N. The fracture toughness was determined from an analysis of the dependence of fracture stress on the indentation load. The true fracture stress was determined by measuring acoustic emission during indentation and bending test. The fracture toughness was measured as 4.61±1.78 (MPam^(1/2)) for unheat-treated NiAl and 6.73±1.34 (MPam^(-1/2)) for 5 times cyclic heat-treated NiAl. The fracture mode of polycrystalline NiAl was primarily intergranular at room temperature, but the portion of intergranular fracture to transgranular fracture decreased in the heat-treated NiAl.
NiAl 합금의 열처리에 따른 점결함과 규칙화 거동 (Ⅰ)
이경섭,홍경태,박희섭,고홍석 대한금속재료학회(대한금속학회) 1998 대한금속·재료학회지 Vol.36 No.5
The thermal vacancy concentration and the long range ordering have been studied using density measurement and X-ray diffractometry in stoichiometric NiAl NiAl is well known for large amounts of vacancies in thermal equilibrium even at room temperature. The thermal defects can be formed on heating and readily retained by quenching, and then can be annealed out by annealing treatment. Density measurement revealed that the amounts of annealed-out vacancies decreased and the difference in density between as-quenched and annealed NiAl also decreased with the repeated heating-quenching-annealing cycles. The density of NiAl single crystal increased by up to 1.8% just by repeated heating-quenching-annealing cycles. The intensity of a superlattice line relative to that of a fundamental line in heat treated NiAl increased. This means that it is possibile for the thermal vacancy concentration in NiAl not only to depend on the temperature but also to be related to long range ordering.
NiAlFe 금속간 화합물에서 Fe 석출현상에 미치는 점결함의 영향
이경섭,홍경태,박희섭,고홍석 대한금속재료학회(대한금속학회) 1998 대한금속·재료학회지 Vol.36 No.5
The influence of point defects on the precipitation was investigated in NiAlFe by means of magnetic susceptibility measurement, differential scanning calorimetry, microhardness and TEM analysis. The magnetic measurements revealed that the ferromagnetic phases consisting of Fe atoms precipitated rapidly at about 700K and dissolved completely at temperatures over 1000K in NiAl alloys containing Fe over 6 at.%. Formation energy of thermal vacancies in NiAlFe was determined to be about 0.85 eV through analysis of quenching data in the range of 1000K-1500K. TEM analysis indicated that very fine α-Fe particles were distributed uniformly in NiAl matrix. Superparamagnetic behaviors were attributed to Fe particles in NiAl12Fe. Only the stoichiometric NiAl containing Fe showed a significant increase in magnetic susceptibility during aging treatment. It is considered that she formation of Fe particles is not dependent on consitutional vacancy but thermal vacancy.