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Shape memory characteristics of (50-X)Ti-Ni-XHf(X = 0∼20 at.%) alloys have been investigated by means of the thermal cycling tests under constant load. The transformation elongation increased with increasing applied stress. Stress dependence of the martensitic transformation start temperature in Ti-Ni-Hf alloys was higher than that in the Ti-Ni binary alloy. Maximum recoverable elongation decreased with increasing Hf-content and was less than 1% in 15, 20 at.% Hf alloys because preferential arrangements of martensite variants were not developed well. Transformation hysteresis of Ti-Ni-Hf alloys almost did not change with increasing Hf-content and were larger than that of the Ti-Ni binary alloy. Residual elongation of Ti-Ni-Hf alloys was much smaller than that of the Ti-Ni binary alloy and that of the 35Ti-50Ni-15Hf alloy did not occur under applied stress of about 120 ㎫.

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The amorphization behavior of a Zr_(65)A_(7.5)Ni_(10)Cu_(17.5) alloy powder fabricated by a vacuum gas atomization has been investigated in this study. The thermal stability of the present alloy powder was investigated using DTA, and was compared with that of the bulk amorphous alloy produced by casting. The ΔT_x(T_x-T_g) value of the amorphous powder was is smaller than that of the bulk amorphous alloy. This decrease in thermal stability was due to the two-step crystallization process of the amorphous phase. Each particle of the alloy powder was solidified to either fully amorphous phase or fully crystalline phase, indicating that the crystallization behavior of this powder was highly dependent on the isolation effect. Fully vitrified powder could be obtained when the powder size was below 45㎛. As the powder size increased, the fraction of the amorphous powder decreased. From these results, it is considered that the control of heterogeneous nucleation sites such as impurities and oxides is very important factor of amorphization of the present alloy powder.

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A transformation dilatometer was used to formulate the dilatation amount of plain carbon steels during phase transformation as a function of carbon content and temperature. The mean carbon concentration in austenite during phase transformation could be calculated from the formulated equation and the mass balance of carbon in each phase. Through this calculation, a dilatometric characterization method, which can determine the volume fraction of each phase taking into account the effect of carbon enrichment in austenite during phase transformation, is suggested. The volume fractions of ferrite and pearlite and the beginning temperature of austenite-to-pearlite transformation, which were obtained from metallographic examination, were compared with the ones obtained by the developed dilatometric characterization method.

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Smooth cylindrical specimens of 1Cr-Mo-V steel were tested under HTLCF( high-temperature low-cycle fatigue) conditions to investigate the growth behavior of microcracks. HTLCF crack growth rate is analyzed based on the concept of J-integral by monitoring the hysteresis loop. A new approach that the crack growth rate is measured by the striation counting from the observation of SEM(scanning electron microscope) fractographs was shown to be successful in determining the LCF crack growth rate as well as the LCF life. It is shown that ΔJ is reasonable in characterizing the growth rate of fatigue microcracks at elevated temperatures but results in unacceptable deviations for the hold time cycling owing to the relative contribution of time dependent processes at the crack tip. Approach of C(t) correlated with ΔJ gives recommendable results for interpreting the crack growth rate with hold time.

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The effects of Mo addition on both the precipitation kinetics of Nb carbide and the high temperature strength have been investigated in hot rolled HSLA steels containing Cr and Mo. These steels were hot-rolled and coiled at 550, 600, and 650℃ respectively. Microstructural analysis of the carbides has been made using Field-Emission-Gun TEM attached EDS. The steels containing Cr and Mo exhibited higher strengths both at room temperature and high temperature(600℃) in comparison to the conventional HSLA steel containing only Cr. Refined bainitic structure could be obtained in the Cr-Mo contained steels due to the increase of hardenability and the fraction of bainitic phase increased with the increase of Mo content. TEM observations revealed that the Cr-Mo contained steels exhibited fine($lt;10 nm) and uniformly distributed Nb carbides, while the Cr contained steel exhibited coarse and sparsely distributed. The EDS analysis also indicated that the fine carbides was composed of complex form of carbide (Nb, Mo)C and the Mo content was higher in the finer carbides. The results clearly indicated that the addition of Mo affected the initial stage of Nb carbide precipitation. That is, Mo caused both transformation hardening and the uniform distribution of fine Nb carbides.

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The high temperature deformation behaviors of Cu-6 wt.%Ni-2 wt.%Sn and Cu-6 wt.%Ni-2 wt.%Sn-0.6 wt.%Si-0.6 wt.%Zn alloys were investigated in the temperature range of 550-900℃ at strain rates between 0.05-1.5s^(-1) by hot compression test. Flow curves were analysed by hyperbolic sine relationship and the activation energy of deformation was determined at high and low temperature ranges. The activation energy of deformation for Cu-6 wt.%Ni-2 wt.%Sn alloy exhibits almost the same as that for Cu-6 wt.%Ni-2 wt.%Sn-0.6 wt.%Si-0.6 wt.%Zn alloy in the low temperature range below 700℃. While, in the high temperature range above 700℃ the activation energy of deformation for Cu-6 wt.%Ni-2 wt.%Sn alloy exhibits much higher value than that for Cu-6 wt.%Ni-2 wt.%Sn-0.6 wt.%Si-0.6 wt.%Zn alloy. The difference of the activation energy of deformation was attributed to the presence of Sn-rich inclusion in the matrix.

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The effects of deformation-induced martensite on three-point bending fatigue properties of austenitic 304 stainless steel sheets at room temperature and -162℃ were investigated. The fatigue properties at -162℃ were superior to those at room temperature due to the higher volume fractions of deformation-induced martensite. In room temperature testing, the mean load amplitude increased steadily with cycles, which means that cumulative plastic incubation strain is required for martensite transformation. On the contrary, in specimen tested at -162℃, the mean load amplitude increased rapidly within a few cycles due to the rapid transformation of austenite to martensite and slightly decreased after the maximum due to dynamic recovery. Two kinds of deformation-induced martensite, namely ε-martensite (hcp structure) and α`-martensite (bcc or bct structure), were found depending on the testing temperature and total strain. The ε-martensite nucleated from overlapped stacking faults and grew into $lt;112$gt;_γ direction. The α`-martensite nucleation was associated with the formation of dislocation pile-ups on active slip planes of austenite and α`-martensite was found to be the dominant phase in high strain or low temperature.

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Indentation tests using a spherical indenter for split-type two half specimens, so called bonded-interface technique, have been proposed as a means to index deformation behaviour or damage behaviour of the material during indenting the conventional one piece specimen. The purpose of this study is to assess the validity of the split-type specimen technique and to better understand the mechanics of indentation of the split-type specimens with respect to the effect of the interface condition between the two split-type half blocks. Accordingly, we performed indentation tests of Al alloy specimens with elasto-plastic stress-strain behaviour. The finite element method was also employed to investigate the deformation behaviour of the two (single piece and split-type) test systems loaded with a spherical indenter. The simulated geometry of the specimens and load-displacement curves during indentation are compared with the experimental results of the Al specimens. The similarity and difference in deformation behaviour and stress distribution between the single-piece specimen method and the split-type specimen method are investigated. The effect of the friction between the two half blocks in the split-type test is discussed.

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The main emphasis of the present study has been placed on understanding the effect of Cu on the formability of TRIP-aided cold-rolled steel sheets. Two kinds of 0.15C-1.5Si-1.5Mn (ECO-0) and 0.15C-1.5Si-1.5Mn-0.5Cu (ECO-Cu) cold-rolled steel sheets were prepared. These steel sheets were intercritically annealed at 790-800℃, and isothermally treated at 430℃. Tensile tests and limiting dome height (LDH) tests were conducted, and the changes of retained austenite volume fractions as a function of tensile strain were measured using X-ray diffraction. The ECO-Cu steel sheet had higher volume fraction of retained austenite about 15%, and smaller size of ferrite grains and retained austenite particles than the ECO-0 steel sheet. On the other hand, the stability of retained austenite of the two steel sheets were the same. The tensile properties and the formability of the ECO-Cu steel sheet was superior to the ECO-0 steel sheet, due to high strain hardening rate maintained in the high strain region by the strain induced transformation of retained austenite to martensite. Containing Cu as a tramp element in the TRIP-aided controlled steel sheets showed beneficial effects on the mechanical properties and formability.

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Dissolution and precipitation reactions of the carbides occurred during heat treatment of 12Cr-1MoVW steel were studied. Transmission electron microscopy and energy dispersive X-ray spectroscopy were used to determine types and chemical compositions of precipitates. The dissolution of the M_(23)C_6 carbide took place during normalizing treatment through the diffusion of chromium atoms. Upon tempering, types of M₃C and M_(23)C_6 precipitates were formed depending on the tempering conditions. The chemical composition of M₃C formed at low tempering temperature is similar to the base material, but the enrichment of chromium and depletion of iron in M₃C occur with increasing the tempering time and temperature. On the other hand chromium content decreases and iron content increases in M_(23)C_6 carbide with increasing the tempering temperature. M_(23)C_6 forms at the δ-ferrite/tempered martensite interface regardless of tempering temperatures. The chromium content of M_(23)C_6 formed at low temperature is low, but chromium content increases and iron content decrease in M_(23)C_6 with increasing the tempering time and temperature.

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