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A theoretical analysis was carried out on the torsional characteristics of directionally solidified Al-Al₃Ni eutectic composites. The torsional properties of the eutectic composites and the matrix were tested in order to estimate the torsional behavior of pure Al₃Ni fiber by means of the rule of mixtures. The Ludwik`s equation T=Hγ^n was used to represent the shear stress and strain relation. The stress coefficient, H and strain hardening coefficient, n were obtained by minimizing the errors between the Torque-twist angle curve and the experimental data. The calculated Torque-twist angle curves using Ludwik`s equation were shown to closely fit the experimental results, and the calculated stress and strain hardening coefficients of pure Al₃Ni fiber were 836,000∼881,600 KN/㎡ and 1, respectively. The rule of mixtures was more applicable for the torsional behavior than for the tensile behavior.

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Blisters and internal cracks below free surface of a prestrained ASE 4340 steel were observed after hydrogen was cathodically charged in the steel. It is hypothesized that blistering which is a result of the local plastic deformation is taking place due to a modified triaxial stress state around a void in which hydrogen molecules generate gas pressure. Triaxial stress state can be modified by the external or residual stress to generate the shear component which induces the plastic deformation. It was observed that the maximum depth of cracks was increased with the amount of cold working, such as tensile, rolling, and bending deformation. To see the possible effect of the microstructural change by the cold works on the crack depth, cold worked specimens were stress-relief annealed and then hydrogen was charged. In these cases, it was observed that the crack depths did not increase as much as those of cold worked specimens with residual stress developed due to cold work. These results give a confirmation of the proposed, hypothesis.

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The effect of two step ageing (TAA) on the microstructure, fatigue and stress corrosion resistance was investigated on the 7075 aluminium alloy. The width of precipitate free zone (PFZ) and the size of precipitates along grain boundries were decreased with the TAA treatment without substantial loss of mechanical strength. Specimens which were failed by fatigue, showed various fracture modes depending on applied stress and heat treatment. The variation of fatigue fracture modes was caused by the initiation conditions of the fatigue cracks. The propagation of the stress corrosion cracks was controlled by the microstructure near the grain boundaries and by the deformation characteristics of the matrix. The fatigue limit at 10^7 cycles was improved up to 10% due to the possible introduction of matrix precipitate incoherency. The narrow PFZ and the small size of grain boundary precipitates have reduced the depth of stress corrosion cracks from 170㎛ to 110㎛.

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The variations in tensile properties and volume fractions of retained austenite in three 0.27%C steels containing chromium and silicon during tempering following 900℃ oil quenching were investigated. Microscopic changes during tempering were also studied using transmission electron microscopy. Silicon raised the tensile strength of as quenched and tempered steels and delayed the decomposition of retained austenite up to 300℃. Chromium also delayed the decomposition of martensite, but the effect was far less significant than that of silicon. The yield stress of silicon containing steel was raised while the tensile stress was decreased with increased tempering temperature up to 400℃. This yield behavior was discussed in relation to the carbon clustering and the decomposition of retained austenite during tempering. Silicon not only delays the recovery and recrystallization of martensite but also refines the carbide particle size.

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