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The effects of addition of Mn, Al, Ti and Si on the aging behavior in Cu-6Ni-6Sn-xMn (wt%) alloys were investigated by means of X-ray side band observation around (200)_α peak, hardness measurements and microstructural observations. The supersaturated solid solution α of Cu-6Ni-6Sn alloy decomposed via the spinodal mechanism to form the modulated structure on aging at temperatures below 350℃, then the matastable phase of γ` with DO_(22) structure formed later. However, when the alloy were aged at at temperatures above 450℃, the cellular structure consisting of αand γ phases formed. The decomposition process of Cu-6Ni-1.5Mn-6Sn alloy was similar to that of the Cu-6Ni-1.5Mn-6Sn alloy. In the alloys consisting more than 3 wt% Mn, however, neither the spinodal decomposition nor the formation of γ` were not occurred. The decomposition mode of the alloys were significantly altered by the small additions of Al, Ti or Si. Al, Ti and Si added alloys were hardened by the metastable CuAl₂, CuTi₂ and MnSi precipitates, respectively.

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The deformation-induced transformation behavior of an austenitic Fe-25Mn-0.2C steel has been investigated in this study. The recently proposed transformation-kinetics equation between the volume fraction (f) of transformed phase and the inelastic strain (ε), which is in the form of (f/f_s) = 1 - exp(-βε^n), has been employed to analyze the transformation behavior during deformation of the Fe-25Mn-0.2C steel at various temperatures ranging from 7.5℃ to 177℃. In the equation above, f_s, denotes saturation value of f and parameters β and n characterize the stability of austenite and formation rate of nucleation sites to inelastic strain, respectively. The deformation-induced phases in this steel below and above 100℃ were ε-martensite and deformation twin, respectively. The formation rates of nucleation site to inelastic strain, n, for the ε-martensite and deformation twin were obtained as 1.4 and 1.0, respectively. The values of f_s and β appeared to decrease with increasing temperature for both (-martensite and deformation twin. A new method to determine the M_d temperature, above which the deformation-induced phase cannot be formed regardless of the amount of inelastic strain, was suggested and obtained as 67℃ and 290℃ for the formation of the ε-martensite and deformation twin, respectively. These values appear to be very close to previously reported ones.

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Metastable liquid phase separation(MLPS) in Cu-Co binary system was investigated using an electromagnetic levitation technique. In Co-rich alloys, Phase separation generally appeared as dispersed L2(Cu-rich) droplets in a L1(Co-rich) matrix, whereas for alloys containing Co less than 55 wt%, L1 droplets formed in a L2 matrix. Secondary MLPS was occurred in the primary separated L1 droplets by increasing the cooling rate during phase separation. The effect of undercooling and cooling rate on the microstructure of L1 phase was investigated. The microstructure of L1 phase was found to fall into three categories of dendrite, spherical and mixed (dendritic plus spherical) morphology. As the undercooling and/or cooling rate was increased the spherical morphology became a dominent structure.

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High temperature deformation behavior of Ti-6Zr-6Nb-6Sn alloy was studied using compression test at strain rates from 0.001s^(-1) to 10s^(-1) and at temperature from 650℃ to 900℃. High temperature deformation efficiency has been evaluated following the Prasad et al.`s model and flow instability condition has been also estimated according to the Kumar et al.`s model. The results showed that a dynamic recrystallization of α and β-grain is primarily responsible for high deformation efficiency and that an unstable flow condition is closely related with the formation of deformation shear band. The optimum working condition has been estimated to be the strain rate 10s^(-1) at the temperature 750℃, which results in a fine and equiaxed α + β duplex microstructure.

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A study has been made to investigate the effect of tungsten(W) addition on the microstructures, tensile properties and low cycle fatigue(LCF) properties of 9Cr-1Mo steels at 298K and 873K. Four steels containing different amount of W (0W, 1.2W, 1.8W and 2.7W) were normalized at 1050℃ for 1 hour and tempered at 750℃ for 1 hour. Microstructural analysis revealed that no significant difference was observed in their tempered martensite microstructure of 0W, 1.2W and 1.8W, but δ-ferrite was observed to form at the prior austenite grain boundaries of 2.7W alloy. With the increase in W content, the yield and tensile strength were increased at both temperatures. Low cycle fatigue life was also increased with the W content up to 1.8%, but decreased in the 2.7W alloy, which was primarily due to the presence of soft δ- ferrite acting as a crack initiation site. The fatigue life of 873K was reduced as compared to that of 298K, due not only to the decrease in the strength at high temperature but also to the formation of oxide layers along slip bands, which increases slip irreversibility during cyclic deformation.

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The objective of this study is to obtain better understanding of Delayed Hydride Cracking(DHC) of Zr-2.5Nb alloys. Three kinds of Zr-2.5Nb tubes with different yield strength and texture were subjected to DHC testing at temperatures ranging from 230-300℃ to investigate a DHC growth pattern, DHC velocity and an activation energy for DHC of Zr-2.5Nb alloys. Cantilever beam specimens of 38㎜ long with the notch of 0.5㎜ deep were subjected to charging of 50 to 200 ppm H at 400℃ and to homogenization treatment at 400℃ for 24 h while the DHC velocity was determined by an Acoustic Emission method. The DHC growth in the radial direction showed a zigzag pattern through cracking of hydrides reprecipitated preferentially on {1017} planes. The DHC velocity of these Zr-2.5Nb tubes was compared and discussed based on texture and strength. The DHC velocity showed no dependence on hydrogen content ranging from 50 to 150 ppm, whose activation energy was determined to be 70.7 kJ/㏖ for Tube A.

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A new method for fatigue life extension has been proposed in this study. To confirm the effectiveness of the proposed method, an austenitic stainless(SUS304) steel(pre-strain : about 2%) was prepared. It was shown from transmission electron microscopy(TEM) observation that micro-sized α martensite particles (0.02∼0.1㎛ in diameter) were dispersed in the SUS304 steel by cooling at -78℃. No decrease in ductility was observed in tensile tests for the material with dispersed micro-sized martensite particles as compared to the as-received material. After tension-compression loading, dislocation structure and surface microstructure observations were carried out using a TEM and a laser microscope, respectively. It was found that the re-arrangement of dislocations and the formation of large α martensite, which may cause fatigue crack initiation, were suppressed in the material with dispersed micro-size α martensite particles as compared to the as-received material. Fatigue life of the material with dispersed micro-sized α martensite particles did not increase in a low cycle regime($lt; 10⁴cycles) as compared to the as-received material. However, in a high cycle regime( $gt; 10⁴cycles), the fatigue life of the material with dispersed micro-sized a martensite particles was much longer than that of the as-received material.

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The Ti-aluminides have the potential for high temperature structural applications because of their interesting characteristics. As compared to conventional Ti alloys, Ti-aluminides offer higher stiffness as well as superior creep and oxidation resistance with a low density. In this work, Ti-aluminide intermetallic compound was formed from high purity elemental Ti and Al foils by self-propagating high-temperature synthesis(SHS) in hot, press. Formation of titanium aluminides at the interface between Ti and Al foil was observed to be controlled by pressure and heating rate. Especially, the heating rate was the most important factor to farm intermetallic compound by SHS reaction. In the effect of Cr and Mn additions, SHS reaction temperatures were decreasing and the resultant composite. microstructures were consisted of a fully dense, well-bonded intermetallic composite. After the SHS reaction of alternatively layered 10 Ti and 9 Al foils with evaporated Cr or Mn coting layer, intermetallic sheets of 700 ㎛ thick Ti-46%Al-2 wt%M (M = Cr, Mn) were formed by heat treatment at 1000℃ for 10 hours.

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Precipitation behavior of cast alloy 718 during thermal exposure and creep-rupture test was investigated. Coarsening of γ` and γ$quot; was accelerated with raising exposure temperature, but transformation from γ$quot; to δ was dominant at and above 816℃. The applied stress assisted in transformation from γ$quot; to δeven below γ$quot; solvus. The transformation nucleated at the stacking fault in γ$quot; phase. δ phase usually existed in the interdendritic region or at grainboundary in the as-cast condition. Generally δ had plate or needle-like morphology in the as-cast or heat treated conditions, however, the stress induced δ dispalyed curved morphology across dendrite.

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This paper describes the experimental results obtained from mixed mode (I and II) creep crack growth tests in Ti-25Al-10Nb-3V-1Mo alloy at 1033K. Mixed mode crack tip conditions were achieved using the compact mixed mode(CMM) specimen, in which initial crack tip conditions corresponded to K_I / K_(II) = 1.29 (mixed mode) and K_I / K_(II) = 0.06 (or almost pure mode II). Two tests have also been performed under pure mode I conditions using the standard compact tension(CT) specimen. The results of these tests have been compared with mode I data obtained from compact tension(CT) tests, using parameter of C^* and an effective stress intensity factor K_(eff). Mode I data show that K approach correlates the mode I data successfully, compared with C^*. This might be due to the environment brittlement effects. The correlation between the mode I, mode II and mixed mode data is reasonably good with the K_(eff). However, the scatter band is greater than that obtained from the mode I results only. Therefore, the mixed mode creep crack growth rates in Ti₃Al can be correlated with the pure mode I and II data, using an effective stress intensity factor, K_(eff) obtained from K_I and K_(II) , even though it predicted the growth rates conservately.

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