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The silicon addition in the titanium alloys yields the solid solution and titanium silicides. The morphology and distribution of the titanium silicides depend upon the alloy chemistry or the heat treatment condition, and play an important role in improving the mechanical properties of the alloys. In this study, the morphology, formation process, and crystallographic characteristics of the titanium silicides in the Ti-Fe-Si alloy system were studied. Three types of silicides were found in the alloys; (1) interconnected chain-like silicides at grain boundary, (2) coarse silicides over 1 ㎛, (3) fine silicides less than 0.2 ㎛. Ti_3Si was dominant in cast + HIP condition while Ti_5Si_3 was dominant in as-cast state. It is recognized that Ti_5Si_3 → Ti_3Si transition occurred by the peritectoid reaction and it may be promoted by pressure during HIP. However, in the case of the fine silicides, Ti_3Si and Ti_5Si_3 were found simultaneously even after HIP. Such a fine silicide was found to have crystallographic orientation relationship with β matrix.

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Deformation induced martensitic transformation (DIMT) behavior of 304 stainless steel with and without nitrogen was investigated to elucidate the effect of nitrogen on DIMT, through the use of the proposed kinetics relation between the volume fraction f of transformed martensite and inelastic strain ε for DIMT; f/f_8 = 1 - exp[-β(ε-ε_o)^n]. The deformation mode parameter n characterizes the formation rate of nucleation sites and thus depends on the deformation mode providing the value of n = 2.2 for these steels regardless of N contents. Adding N, on the other hand, is found to reduce the value of austenite stability parameter β, which in turn implies more stable austenite to provide significantly enhanced ductility in 304N stainless steel below room temperatures. Nitrogen is also found to reduce the saturation volume fraction f_8 of transformed martensite and also the M_d temperature, above which martensitic transformation cannot occur by plastic deformation.

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TRIP steels exhibits low formability under multi-axial stress condition. The purpose of the present study is to suggest a way of improving the formability of TRIP steels under plane strain mode. For this purpose, the formability under different stress states and their relationships with the microstructure of the secondary phases have been investigated in the cold rolled 0.2C-1.5Si-1.5Mn TRIP steels experienced with various bainitic isothermal transformation at 300, 350, 400 and 450℃. Two kinds of mechanical tests were conducted to evaluate the formability: the tensile test under uni-axial stress condition, and the Limiting Dome Height(LDH) test for the plane strain mode under multi-axial stress condition. The formability of the present TRIP steels strongly depends on the isothermal bainitic transformation temperatures. The discrepancy between EL and LDH at 350℃ could be explained by the stability of the retained austenite and the distribution of the secondary phases. Consequently, this study revealed that effects of the microstructure on the formability were different depending on the deformation modes in TRIP steels. In order to improve the EL property, the stability of RA should be controlled, and in order to improve the LDH property, the continuously networked RA and the fine bainite should be formed.

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In order to clarify the behavior of precipitation and dissolution in Al-Mg-(La, Mm) alloys with a high Mg content(x≥7 wt.%) in detail, the characteristic parameter of DVRC (Differentiated values of the ratio of electrical resistivity change) depending on temperature has been investigated at a slow heating rate (0.05 K/min). Since the value of DVRC is very sensitive to its microstructural change, it enables us to understand the precipitation and dissolution behavior of phases clearly and quantitatively. During the heating, the transformation process of precursor phase, such as G. P. zone, shown as a positive peak at low temperature region, was retarded and the precipitation of β phase was suppressed by La or Mm addition. And the homogeneous precipitation of β phase was also induced by the additive elements of La or Mm.

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Microstructural changes in the artificially creep damaged Ni base superalloy, PM1000, were nondestructively evaluated by ultrasonic velocity measurement. The ultrasonic velocity decreased as the volume fraction of cavity in the crept specimen increased, which was caused mainly by the decrease of Young`s modulus. The change in theoretical ultrasonic velocity with amount of cavity could be calculated by using the composite material model assuming that cavities were dispersed in the matrix as second phases with zero elastic stiffness. The increase of dislocation density also resulted in the decrease of ultrasonic velocity. The discrepancy between experimental ultrasonic velocity and theoretical one was rationalized in terms of dislocation substructure.

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Dynamic deformation of metallic materials mostly accompanies substantial amounts of deformation heating. Since the flow stress of deformation is sensitive to temperature, implication of heat due to plastic work is essential to the evaluation of constitutive relations. In this study, a series of compression tests were conducted for SAF 2507 super duplex stainless steel at various temperatures and strain rates. The accumulation of plastic work was integrated through conventional analytic method and numerical method proposed in this study. Subsequent logarithmic interpolation deduced isothermal flow surfaces, which were primary input data of finite element analysis. Two compensation methods showed definite difference in resulting flow stress, especially at high strain level, which mainly due to the underestimated strain hardening by conventional one. Simple closed die forging process was analyzed and optimized with commercial FEM code applying both raw and calibrated material database. The effect of accounting deformation heat was more noticeable in high-speed forming process.

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Effects of Ni content(8.3∼12 wt.%) and deformation-induced martensitic transformation on the bending and zero to tension fatigue behavior of Type 304 stainless steel used for the membrane of LNG storage tank at room and cryogenic temperature were investigated. By lowering the Ni content, the bending fatigue life at room temperature was decreased at the high level of total strain amplitude. It would be attributed to the crack initiation at γ/α` interface and the low ductility with the increase of α` contents. However, at the low level of total strain amplitude, lowering the Ni content increased the bending fatigue life. It was due to the suppression of crack propagation by the α`-martensite transformation around crack tip and the increase of tensile strength. At cryogenic temperature, lowering the Ni content showed beneficial effect on the bending fatigue life due to the increase of slip deformation resistance. In the zero to tension fatigue test, lowering the Ni content showed better fatigue characteristics regardless of the temperature variation. In this case, the high tensile strength with the α`-martensite transformation mostly affects material`s fatigue life.

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Continuous indentation test with ball indenter has been studied for years because of its convenience and simpleness. Especially, the test is expected to supersede tensile test. Among tensile properties work-hardening exponents of various steels were evaluated by the test. We newly defined true stress and true strain with contact depth and indentation load. Finite element analysis shows that the contact depth is affected by elastic deflection and pile-up/sink-in. Therefore, the contact depth was calculated with taken both elastic deflection and pile-up/sink-in into account. The work-hardening exponents derived in this study show good agreement to those from tensile test.

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The fatigue crack propagation induced by σ phase precipitation of super duplex stainless steels aged at 600℃ was evaluated and the time-frequency analysis according to acoustic emission was conducted. The fracture mechanism by σ phase precipitation can be found from the time-frequency analysis of acoustic emission signals. As a result of aging at 600℃, the dispersed structure showed faster precipitation rate of σ phase than fibrous structure. The time-frequency analysis of acoustic emission signals obtained during the fatigue test revealed that, the specimens with non-precipitated σ phase induced a dominant frequency of 200∼300 ㎑ without any correlation with crack length. As the fatigue crack propagates (stage Ⅱ_c mode), the frequency range was in a high frequency range above 400 ㎑ in addition to frequency range of 200∼300 ㎑. On the other hand, the frequency range in the specimens with σ phase precipitated was not only 200∼300 ㎑ but also 400 ㎑ because of the fracture mechanism emanating high energy such as dimple fracture and separation of inclusion.

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This study is concerned with the effects of microstructural parameters on the quasi-static tensile and dynamic torsional deformation behaviors in Ti-6Al-4V alloys with Widmansta¨tten structures. Dynamic torsional tests were conducted using a torsional Kolsky bar for five Widmansta¨tten structures, which effective grain sizes such as colony size and α lamellar spacing were varied by heat-treatments, and then the test data were analyzed in relation to microstructures, tensile properties, and fracture mode. Under dynamic torsional loading, maximum shear stress showed a large dependence on the colony size as the effective grain size, while shear strain at the maximum shear stress point was largely affected by both the α lamellar spacing and the colony size. Adiabatic shear bands were found in the deformed region of the fractured specimens, and their width was smallest in the specimen having the large α lamellar spacing and colony size. The possibility of the adiabatic shear band formation was quantitatively analyzed by the theoretical critical shear strain (γ_c) condition. It was found to be most likely in the coarse Widmanstatten structure, whereas least likely in the fine Widmanstatten structure.

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