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Precipitation characteristics and crystallographic features of 9R precipitate in Fe-0.15C-1.5Mn-1.5Si-1.0Cu (all in weight percent) alloy were investigated utilizing conventional and high-resolution transmission electron microscopy (TEM). The modulated or tweed structure which contained bumpy morphology and distinguished striations perpendicular to [100]_α direction was observed at the early stage of aging at 500℃. The precipitates with banded structure were found to be lying parallel to the {110)_α planes within ferrite matrix after aging at 500℃ for 10 hours and it was confirmed from the HRTEM investigations that this banded contrast from the precipitate corresponded to the intervariant boundary of twinned 9R precipitate. After electron irradiation, it was founded that the close-packed (009)_9R planes rotated and aligned more closely with matrix {110}_α planes and the regular stacking faults on every third (009)_9R planes were eliminated. This 9R-to-3R transformation was due to the relaxation of elastic strain produced by the bcc-to-9R martensitic transformation.

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A mathematical approach using the phenomenological theory, originally developed by Kajiwara for Cubased shape memory alloys, has been applied to the bcc-to-9R martensitic transformation in Fe-0.15C-1.5Mn-1.5Si-1.0Cu alloy. Analytical equations for the lattice invariant shear and habit plane were derived, and the calculated values of the lattice invariant shear and habit plane were g`=0.09669 and (0.34179, ±0.80472, 0.48539)_bcc, respectively. The orientation relationship between the ferrite matrix and 9R precipitate was calculated using the transformation matrix as follows: [110]_9R//[1.00131, -1.00379, 1.00186]_bcc and (114)_9R//(0.07769, 1.07566, 1,00008)_bcc The deviation angles of orientation relationship between the calculated and the measured by HRTEM were found to be 0.06071 in direction and 3.67590 in plane, respectively. The present model successfully predicted the crystallographic features of bcc-to-9R martensitic transformation in ferritic steels.

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Transformation behavior and shape memory characteristics in Ti-(45-X)Ni-5Cu-X(Mo, Cr, Al)(at.%) alloys were investigated by means of differential scanning calorimetry, X-ray diffraction and thermal cycling tests under a constant load. The substitution of Mo, Cr and Al for Ni in Ti-45Ni-5Cu alloy induced the B2→B19 martensitic transformation prior to the B2→B19` transformation, and consequently Ti-(45-X)Ni-5Cu-X(Mo, Cr, Al)(at.%) alloys transformed in two stages; the B2→B19→B19`. The B2→B19 and the B19→B19` transformations were completely separated in Ti-(45-X)Ni-5Cu-X(Mo, Cr) alloys, whereas they were not done in a Ti-44.3Ni-5Cu-0.7A1 alloy without applied stress. The substitution of Mo, Cr and Al in a Ti-45Ni-5Cu alloy decreased transformation hysteresis and the residual elongation. The maximum recoverable elongation of Ti-(45-X)Ni-5Cu-X(Mo, Cr, Al) alloys was about 5%.

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The Ni-rich β-NiAl alloys are considered as potential materials for high temperature shape memory alloys because of their higher thermoelastic martensite transformation temperature. But the transformation to Ni_5Al_3 phase occurring at 450∼550℃ during heating of NiAl martensite interrupts the reverse martensitic transformation, and causes the shape memory effect in NiAl martensite to disappear. In the present study, the phase transformation process in binary Ni-(33∼37 at.%)Al martensite was investigated by DTA, and it was found that the blocker of reversible martensitic transformation was not the β→Ni_5Al_3 transformation but the M→Ni_5Al_3 transformation occurring at 250∼300℃. Therefore, the transformation temperature of M→Ni_5Al_3 determined the critical temperature for operation of shape memory effects. In addition, for verifying the critical temperature, the phase transformation process was investigated for various ternary Ni-33Al-X alloys (X = Cu, Co, Fe, Mn, Cr, Ti, Si, Nb). As a result, Ti, Si and Nb were very effective to lower the A_s, temperature of Ni-33Al-X alloys, facilitating the shape memory effect in Ni-33Al-X alloys. Especially, Si and Nb additions were beneficial for increment of the transformation temperature of M→Ni_5Al_3, resulting in higher operating temperatures of NiAl-based shape memory alloys.

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The effect of prior austenite grain size on mechanical properties in a medium carbon ferrite-pearlite steel was investigated to find the optimum condition to minimize the deviation of mechanical properties. It is shown that the variation of hardness of a medium carbon steel containing ferrite and pearlite with cooling rate becomes smaller as prior austenite grain size becomes finer. The hardness of specimen with fine prior austenite grain increases less sharply than that of specimen with coarse one for the change of same amount of pearlite volume fraction in as-rolled state. It is concluded that the deviation of mechanical properties of medium ferrite-pearlite carbon steels can be reduced effectively by making prior austenite grain size finer.

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The main emphasis of the present study has been placed on understanding the effects of heat-treatment (austenitizing and isothermal treatment) on the microstructures and mechanical properties of a 0.20C-0.25Si-2,0Mn high strength steel for cold forming. The steel was austenitized, and isothermally treated at bainitic region followed by air cooling. Microstructural observation, tensile tests, and Charpy impact tests were conducted. The mechanical properties and microstructures of the steel were changed by heat-treatment conditions. The size of prior austenite increased with increasing austenitizing temperature, resulting in decrease of impact toughness. Microstructure was changed from bainite + pearlite to martensite + bainite with decreasing isothermal treatment temperature, and yield or tensile strength, yield ratio, and reduction of area were increased. From the results of the present study, it is confirmed that it is possible to develop a new C-Si-Mn high strength steel for cold forming through optimization of heat treatment.

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Tensile, fatigue, creep-fatigue, creep tests were conducted at R.T.∼600℃ to evaluate the effect of nitrogen on mechanical properties and the optimum nitrogen content for type 316L(N) stainless steel containing the different nitrogen content from 0.04% to 0.15%. Tensile strength decreased with temperature and increased with the addition of nitrogen but elongation was not decreased with the addition of nitrogen. Fatigue life was almost same up to 300℃ and decreased drastically at 600℃. Fatigue life increased with the addition of nitrogen but decreased at above 0.10% nitrogen content. Fatigue crack propagation increased with temperature, decreased with the addition of nitrogen, and increased at above 0.10% nitrogen content. The time to rupture for creep increased with the addition of nitrogen and saturated at above 0.10% nitrogen content. Minimum creep rate of creep decreased with the addition of nitrogen. Creep-fatigue life increased with the addition of nitrogen but decreased at above 0.10% nitrogen content. Mechanical properties were improved up to 0.10% nitrogen content but saturated or decreased at above 0.10% nitrogen content. The optimum nitrogen content for the high temperature mechanical properties of type 316L(N) stainless steel was evaluated as 0.10% in the 0.02% carbon content.

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The thermal stability of lamellar microstructure in TiAl-Mo PST crystals containing C or Si was investigated. In addition, the variation of α-phase volume fraction in Ti-Al-Mo-(C, Si) systems was also investigated at several temperatures. It was found that Ti-46Al-1.5Mo-O.2C and Ti-46Al-1.5Mo-1.0Si alloys were very stable during heat treatments at various heating rates and temperatures. Moreover, the, α-phase volume fraction of Ti-46A1-1.5Mo-0.2C and Ti-46A1-1.5Mo-1.OSi alloys (stable composition) was less changed than those of Ti-47Al and Ti-46Al-1.5Mo alloys (unstable composition). From these results, the instability of the latter alloys was thought to be resulted from relatively higher variation of α-phase volume fraction during heating. Therefore, it could be suggested that the variation of α-phase volume fraction is an important factor that control the thermal stability of lamellar microstructure.

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Fracture resistance is one of the most important properties of TiN coatings for the wear application. Therefore, this study was focused on improving the fracture resistance of TiN films and a new structure TiN film with Ti/TiN/Ti buffer layer/TiN were made. These new type TiN films were synthesized by cathodic arc ion plating with various thicknesses of the Ti buffer layer. The structure and phase compositions of the films were examined by scanning electron microscopy(SEM), X-ray diffraction(XRD) and Auger electron spectroscopy(AES) and the mechanical properties were studied by a micro knoop hardness tester and an impact wear tester. The results from XRD analysis showed that the Ti interlayer between the substrate and the TiN layer has a preferred growth orientation of (111) but the internal Ti buffer layer within TiN film showed a preferred orientation of (002) plane, which matches with TiN (111) plane coherently, resulting in reduced possibility of failure between the Ti buffer layer and TiN layer. Microhardness of TiN film without the Ti butter layer was approximately Hk=2850 but in case of TiN films with the Ti butter layer, microhardness decreased slightly to approximately Hk=2300∼2800, depending upon the thickness of the Ti buffer layer. Results from the impact wear test showed that impact wear volume of the TiN film with Ti buffer layer decreased approximately three times than that of the TiN film without Ti buffer layer. This tendency could be attributed to the stress accumulation and relaxation at the soft Ti buffer layer.

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Investigations of fatigue failure mechanism have been carried out to study the contrary fatigue properties of nitrided steel and titanium. In the case of nitrided steel, the starting point of fatigue failure is the surface of specimen. The fatigue limit received nitriding treatment is improved by the increase of surface slip resistance. On the other hand, in the case of nitrided titanium, the hardened compound layer formed on surface by nitriding treatment is broken in brittle manner by the concentrated stress located at the boundary between compound layer and matrix grain. Also, the intensity of this concentrated stress is depended on the slip length of matrix grain and the slip length is decided by the grain size. Consequently, in nitrided titanium Hall-Petch relationship between matrix grain size and fatigue limit remarkably appears. Furthermore, as the hardness of this compound layer is very high(Hv>2000), i.e. low toughness, the brittle fracture of it happens in low stress level and directly results in the overall fracture of specimen. By this reason, the fatigue limit is decreased.

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