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In order to investigate the effect of Hf-content on phase transformation behavior and the microstructures of Ti-Ni-Hf shape memory alloys, differential scanning calorimetry, X-ray diffraction and transmission electron microscopy were performed. The one stage B2(cubic)-B19`(monoclinic) transformation occurred in the alloys whose Hf-contents are less than 10 at%, while both the B19(orthorhombic) and the B19` martensite coexisted in the alloys whose Hf-content is more than 10 at%, indicating that the B2-B19 and the B19-B19` transformations were not separated. Transformation temperatures decreased with increasing the Hf-content for the alloys whose Hf-contents are less than 5 at%, and then increased for those whose Hf-content is more than 5 at% and less than 15 at%. Further increasing Hf-content, however, the decrease in transformation temperatures which may be attributed to an unknown phase was observed. The (001) twin as an internal defect existed in the B19` martensite, while did not exist in the B19 martensite.

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The main objective of the present paper is to study the effect of heat treatment and temperature on mechanical properties of Ti-(43-54)Al-xMn-yNb (x=0, 3, 4, y=0, 4) alloys. The compressive yield strength as well as fracture strain was correlated with composition, microstructure, tetragonality, and strain rate. The controlled microstructures obtained by heat treatments were introduced in order to correlate the microstructural parameters with mechanical properties. The variation behavior of compressive yield strength of Ti_(49)Al_(51) base alloys showed the positive temperature dependence and the highest compressive yield strength among the measured six alloys was obtained in (Ti_(49)Al_(51))_(92)Mn₄Nb₄. The c/a ratio of (Ti_(53)Al_(47))_(97)Mn₃ alloy was measured using hot stage in-situ XRD technique at different temperatures. The c/a ratio was decreased significantly at 1273K. The c/a ratio of Ti-(43-54)Al-xMn-yNb (x=0, 3, 4, y=0, 4) base alloys also varied with the compositions. The compressive fracture strain of Ti_(53)Al_(47) base alloys was increased with decrease of the c/a ratio, while Ti_(49)Al_(51) base alloys had no correlation with the c/a ratio.

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A study was carried out to investigate the effect of Mn on the deformation and micro-fracture behavior in Ti-(43-54)Al-xMn-yNb(x=0, 3, 4, y=0, 4) alloys. No deformation twin was observed, while faulted dipoles were promoted during room temperature defermation of Ti_(49)Al_(51) alloy. However, in (Ti_(49)Al_(51))_(97)Mn₃ alloy, a certain deformation twins were observed. In addition to deformation twins and ordinary dislocations, superlattice dislocations and the faulted dipoles were observed in both (Ti_(53)Al_(47))_(97)Mn₃ and (Ti_(55)Al_(45))_(97)Mn₃. However, there was no remarkable difference in the operative deformation mode between (Ti_(53)Al_(47))_(97)Mn₃ and (Ti_(55)Al_(45))_(97)Mn₃ alloy. For both Ti_(53)Al_(47) and (Ti_(53)Al_(47))_(97)Mn₃, some recrystallized grains were found after deformation by 35% compressive strain at both 700℃ and 900℃. The recrystallized grains of (Ti_(53)Al_(47))_(97)Mn₃ alloy were smaller than those of Ti_(53)Al_(47) alloy. In-situ TEM experiment was carried out to observe the effeet of Mn on the fracture behavior in lamellar microstructure. In (Ti_(53)Al_(47))_(97)Mn₃ alloy, the fracture planes were determined as (111) and (001) planes and the main crack was propagated continuously across the lamellae. Also, in-situ SEM experiment was carried out to observe the effect of the lamellar orientation against the loading axis.

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Sliding wear tests of ordered Fe-28 at.% Al alloys with B2 and DO₃ structures have been carried out under various environmental conditions to investigate the effect of water (H₂O) on the sliding wear behavior of the alloy. The wear rate of the ordered alloy under wet (water lubricated) condition was found to be much lower than that under dry condition. The sliding-wear-test result was quite different from the abrasive-wear-test result, which reported higher wear rate under the wet condition. It was also observed that the higher the humidity of the ambient air, the lower the wear rate of the Fe-Al alloy becomes. SEM observations revealed that micro ploughing with some micro cutting was a major wear mechanism of the alloy under the wet condition, while under the dry condition with varying humidity, delamination and oxidation wear were two major wear mechanisms. It was found that during the sliding wear of the Fe-Al alloy, unlike during the three-body abrasive wear, water functioned as a lubricant under the test conditions and accelerated formation of the oxide layer. The water is found to decrease the sliding wear rate of the Fe-Al alloy, instead of increasing the rate by embrittling the ordered alloy.

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It was known that the wear characteristics of hypereutectic Al-Si alloys are mainly affected by the size and the volume fraction of primary Si phases. However, the effects of the size and the volume fraction of primary Si phases on the wear characteristics was not yet clear. Thus these were investigated in this study, and the effect of Fe addition on the wear characteristics was also studied. The wear test was carried out with variations of load (30∼2OON) and sliding speed (0.2, 1, 2m/s) at a constant sliding distance(2000m) on a pin-on-disk type wear testing machine. As a result, the Al-20Si-5Fe alloy, which had the primary phases of medium size(20∼30㎛), had the highest wear-transition-load(200N at sliding speed of 1m/s). The reason was related to MML (Mechanically Mixed Layer). MML, which was formed on alloy`s worn surface, had the higher hardness than the matrix and played a role as solid-lubricants. Therefore it seemed that the MML protected the matrix from contacting with counterface and held back the thermal softening of matrix by inhibiting the temperature rising of the surface. The MML of Al-20Si-5Fe, which had primary phases of 20∼30㎛, had the highest hardness and inhibited most effectively the temperature rising of the surface.

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The effects of temperature, contact stress, and environment on the sliding wear behavior of Fe-base NOREM 02 hardfacing alloy was investigated. At room temperature in air, NOREM 02 did not show galling even up to the contact stress of 30ksi owing to its low stacking fault energy and γ → α` strain-induced phase transformation. At 300℃ in air, however, NOREM 02 showed galling even under the contact stress of 15ksi. It was considered to be due to the increase of stacking fault energy with temperature and undevelopment of strain-induced phase transformation. In the mean time, it was confirmed that water environment increases the wear resistance of NOREM 02 owing to the lubrication effect and enhancing effect on the formation of Fe-oxide layers which prevent direct metal-to-metal contact between asperities.

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The main emphasis of the present study has been placed on understanding the effects of heat treatment and Si on microstructures and properties of the low carbon cold-rolled TRIP aided steel sheets. Two kinds of 0.10%C-1.5%Mn-Si-0.5%Cu cold-rolled steel sheets were prepared, and were intercritically annealed following isothermal treatment at bainitic region. Microstructural observation and tensile tests were conducted, and also the volume fractions of retained austenite were measured. The volume fractions of retained austenite and the mechanical properties of the steel sheets were changed dramatically by heat-treatment conditions. The volume fractions of retained austenite increased with decreasing intercritical annealing temperature and isothermal treatment temperature, resulting in the improvement of tensile strength, elongation, and strengthductility balance(TS×Elong.). In case of the ECO-5 steel sheet (high Si content), the higher volume fractions of retained austenite and mechanical properties were obtained compared to the ECO-4 steel sheet (low Si content). From the results of the present study, it is reached a conclusion that the conditions (temperature and time) of intercritical annealing and isothermal treatment should be established in consideration of the stability of austenite with the solubility of alloying elements in the austenite formed during the intercritical annealing.

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A mixture model (or composite model) in which a partially crystallized amorphous Al alloy is regarded as a nanocomposite of Al nanoparticles and remaining amorphous matrix is presented. The model aims at desoribing the hardening behaviour of primary heat treated amorphous Al-Ni-Y alloys, in which the strength (and hardness) changes according to the composition and microstructure from approximately 1000MPa for the as melt spun amorphous ribbon to 1500MPa after heat treatment. The nano-sized Al particles are treated as perfect materials because there are hardly any imperfections within the particles. The matrix is treated as amorphous materials, in which the solute concentration increases as the volume fraction of Al particles Increases. The calculated results for various compositions and heat treated specimens are compared with the experimental data in literature. It is shown that the hardness of the nanocomposite is independent of the solute concentration profile only if the hardness of the amorphous matrix is proportional to the solute concentration.

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Effects of cathodic hydrogen charging on fracture behavior of Al 8090 alloy were examined as functions of charging time, displacement rate, aging condition, applied potential and specimen orientations. Fatigue precracked SEN(single edge notch) specimens with various orientations were cathodically hydrogen charged in 0.1N HCl solution for 12 to 48 hours at an applied potential of -1800 and -2000mV vs. SCE, respectively. It was found that the cathodic hydrogen charging substantially reduced the initial fracture toughness value, K_(th), of Al 8090, while the reduction in K_(th) value was linearly proportional to (charging time)½. The dominant fracture mode of Al 8090 was intergranular dimpled rupture, and the plasticity around the dimples on the intergranular facets appeared to be reduced with hydrogen charging. The sensitivity to hydrogen assisted fracture of Al 8090 was greatly affected by specimen orientation, applied potential and aging condition. Like other high-strength Al alloys, the resistance to hydrogen assisted fracture was increased with increasing aging time. The reason for the improved resistance with prolonged aging in Al 8090 is believed to be related to the precipitate free zone formed along the grain boundaries.

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The thermal stability of the lamellar microstructure and the orientation control of the lamellar microstructure in a cast TiAl alloys with Mo and C were investigated. The lamellar stability of the seed material up to the melting temperature is important in the orientation control of the lamellar microstructure by the directional solidification. First, the partial liquidus surface was constructed for TiAl-C ternary and TiAl-1.5Mo-C quaternary systems. The lamellar stability was determined by quickly heating specimens to a temperature just below the melting temperature, holding, and then cooling to room temperature. Ti-46Al-1.5Mo-0.2C alloy was found to be a promising composition as a seed materials and ingots with a lamellar microstructure aligned along the growth direction were successfully grown by directional solidification process using the seed alloy. From the result of tensile and compression tests, directionally solidified Ti-46Al-1.5Mo-0.2C (at.%) ingots were found to exhibit a high yield stress together with a reasonably good tensile elongation at room temperature and a high compression stress up to 1000℃.

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