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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.

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Formation and thermal decomposition behavior of nanocrystalline structure in NiTi intermetallics have been investigated by a combination of X-ray diffraction analysis, scanning/transmission electron microscopy, differential scanning calorimetry. Mechanically alloyed Ni_(50)Ti_(50) consist of 5-15 ㎚ size grains with a disordered b.c.c. structure and decomposes by eutectoid reaction into Ni₃Ti and NiTi₂ forming an endothermic peak at the temperature range of 450℃-550℃ Such a low-temperature eutectoid reaction is attributable to the ㎚-scale structure with a large amount of defects accumulated during mechanical alloying process. The occurrence of these transformation is closely related to the unique features of the microstructure in the mechanically alloyed powders.

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The dissolution behavior of secondary solidification phases in squeeze cast Al-4wt%Cu-1.5wt%Mg and Al-4wt%Cu-1.5wt%Mg-0.7wt%Ag has been studied using a combination of optical microscope, image analyzer, scanning electron microscope(SEM), energy dispersive spectrometer(EDS). X-ray diffractometer(XRD) and differential scanning calorimeter(DSC). Special emphasis was placed on the investigation of the effects of the non-equilibrium heat treatment on the dissolution of the second solidification phases, As-cast microstructure consisted of primary solidification product of α-Al and various types of secondary solidification phases such as Al₂Cu. Al₂CuMg and AlMgAg. Detailed thermal analysis showed that various types of dissolution occurred depending on the alloy composition. Based on the results from DSC, the effects of equilibrium and non-equilibrium solution treatments have been discussed. The volume fractions of the secondary phases decreased approximately from 27.3 to 11.6% after equilibrium solution treatment at 495℃ for 5hours and from 27.3% to 5.4% after non-equilibrium solution treatment at 515℃ for 5hours in Al-4wt%Cu-1.5wt%Mg-0.7wt%Ag. Non-equilibrium solution treatment resulted in higher hardness value than equilibrium solution treatment.

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Formation and growth of Widmansta¨tten HCP α phase in Mg-10wt%Li-3wt%Al has been investigated. Formation of Widmanstatten α phase has been investigated in detail by changing the cooling rate, i.e. by adopting the methods of water-quenching, air-cooling, and furnace-cooling. Growth of Widmansta¨tten α phase has been investigated by aging at 270℃. Widmansta¨tten type α phase becomes much finer with increasing cooling rate, resulting in higher hardness. The orientation relationship between Widmansta¨tten α phase precipitates and β phase matrix is:[21￣1]_α//[110]_β, (1￣11)_β//(101￣0)_α, Growth rate along the long axis of Widmanstatten α phase is 18.2㎛/hr. On the other hand, thickening rate is near zero, suggesting that ledges are not supplied sufficiently to the interface.

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The objective of the present study is to investigate the effects of isochronal and isothermal aging on microstructure and damping capacity of Cu-47%Mn and Cu-55%Mn alloys. In case of the isochronal aging for 8 hours, the maximum damping capacity was shown at 425 for the Cu-47%Mn alloy and 400 for the Cu-55%Mn alloy, respectively. This is due to the lowest value of tetragonality(c/a ratio) which is closely associated with the fcc→fct martensitic transformation. In case of the isothermal aging at 400℃, the aging time far highest damping capacity was 18 hours in the Cu-47%Mn alloy and 8 hours in the Cu-55%Mn alloy, respectively. TEM observation revealed that tweed microstructure of fct martensite is responsible for the high damping capacity.

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The damping capacities of fcc→bcc martensitic Fe-13wt%Ru and fcc→hcp martensitic Fe-25wt.%Ru alloys were studied as a function of martensite volume fraction. `Ihe damping capacity of Fe-25wt.%Ru alloy, which was strongly dependent on strain amplitude. was higher than that of Fe-13wt.%Ru alloy showing little dependence on strain amplitude. The damping capacity of Fe-25wt.%Ru alloy increased linearly with increasing volume fraction of ε martensite at a law strain amplitude, ε_(max)=2×10^(-4) but increased with increasing volume fraction of f martensite up to approximately 39vol.% and decreased with further volume fraction ε martensite at a high strain amplitude, ε_(max)=5×10^(-4). This implies that the damping mechanism of Fe-25wt.%Ru alloy is different depending upon the magnitude of strain amplitude : The damping capacity is due to the movement of stacking fault boundary in ε martensite plate at low strain amplitudes, while the damping mechanisms are the movement of stacking fault boundary and γ/ε interface at higher strain amplitudes. (Received January 15. 1998)

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In the designing the dispersion-strengthened Al alloy with Al₃M (M=Ti,V,Zr), the behavior of trialuminide second phase was investigated. Al alloys with transition elements Ti, V and Zr added by 2. 4 and 6at% were fabricated by vacuum arc melting and microstructure of those was studied in terms of phase and composition. Transformation of metastable trialuminide structure in the formation of stable second phase was analyzed by adopting simple lattice matching concept that is related with lattice stability. For the confirmation of analysis some sample Al alloys were produced by rapid solidification method. Heat treatments of produced alloys at different times and at different temperatures showed the convincing proof.

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A study was made of the effects of alloying elements on microstructural factors and wear properties of four high speed steel (HSS) rolls manufactured by centrifugal casting method. A simulation test was carried out using a high-temperature wear tester capable of controlling speed, load, and temperature. The test results revealed that the HSS roll containing a larger amount of vanadium showed the best wear resistance because it contained a number of hard MC carbides. However, it showed a very rough roll surface because of the cracking along cell boundaries, the preferential removal of the matrix, and the sticking of the rolled material to the roll during the wear process, thereby leading to increase in friction coefficient and rolling force. In order to improve wear resistance with consideration of the roll surface roughness, it is suggested that reduction in the vanadium content, solid solution hardening by adding alloying elements, secondary hardening by precipitation of fine carbides in the matrix, and formation of fine prior austenite grain boundaries by pre-austenitization treatment are required to strengthen the matrix which can hold hard carbides in it.

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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.

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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.

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