Designing, Processing and Properties of Advanced Engineering Materials

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Authors: Dong Bok Lee, Y.D. Jang
Abstract: Alloys of Ti39.4Al10V (at.%) that consisted mainly of ordered β-Ti, γ-TiAl and α2-Ti3Al phases were oxidized at 700, 800, 900, and 1000oC in air. The oxide scales formed consisted largely of an outermost, thin TiO2 layer, an outer, thin Al2O3 layer, and an inner, very thick (TiO2+Al2O3) mixed layer. Vanadium, which was uniformly distributed throughout the oxide scale, harmfully decreased oxidation resistance, and made thick, nonadherent scales owing to the formation of low melting compounds of V-oxides. The oxidation progressed via the outward diffusion of Ti, Al and V ions, and the concurrent inward transport of oxygen.
Authors: Dong Bok Lee, Seung Wan Woo
Abstract: The oxidation of Ti-(43~52%)Al-2%W-(0~0.5%)Si alloys between 900 and 1050°C in air progressed via the outward diffusion of Ti ions to form the outer TiO2 layer, and the inward transport of oxygen to form the inner (TiO2+Al2O3) mixed layer, between which the intermediate Al2O3 barrier layer existed. Tungsten tended to diffuse inward to be incorporated below the intermediate Al2O3 layer, while Si outward to exist over the entire oxide layer. Both W and Si tended to be dissolved in the oxide layer, rather than forming independent oxides.
Authors: Dong Bok Lee, Y.D. Jang, Sung Jin Kim
Abstract: The (Al+12.5 at.%Cu)3Zr alloy was prepared by mechanical alloying followed by cold isostatic pressing and sintering at high temperatures. The oxidation between 673 and 973 K in air progressed by oxygen dissolution arisen by the zirconium.s high affinity and solubility of oxygen. The porous (Al+12.5 at.%Cu)3Zr alloy oxidized even faster than Ti3Al alloys, because no Al2O<3/sub> barrier layers formed.
Authors: Y.D. Jang, Sung Jin Kim, Dong Bok Lee
Abstract: The oxidation behavior of (Al+8 at.%Mn)3Ti alloys prepared by mechanical alloying followed by cold isostatic pressing and sintering at high temperatures was investigated at 1000 and 1100°C up to 300 hr in air. Since the Al2O3 barrier layer having some dissolved ions of Ti, and Mn formed, the oxidation resistance of the prepared alloys was excellent.
Authors: Tae Kwon Ha, Jae Young Jung
Abstract: High temperature deformation behavior of a Ti-Al intermetallic compound has been investigated in this study. Specimens with a near gamma and a lamella structure were obtained by performing heat treatment at temperatures from 1200 to 1330°C for 24 hr, respectively, and stabilized at 900°C for 4 hr followed by air cooling. A series of load relaxation tests has been conducted on these samples at temperatures ranging from 850 to 950°C to construct flow curves in the strain rate range from 10-6/s to 10-3/s. Strain hardening was observed even at the temperature of 950°C in both the near gamma and the lamella structures. Further aging treatment for 12 hr at the test temperatures has been found to cause negligible softening in both microstructures, providing the strong applicability of this alloy system in the temperature range.
Authors: Jian Hui Zhang, Kenong Xia, Erik Strom, Zeng Yong Zhong, Chang Hai Li
Abstract: This paper presents the true stress - strain curves and data analyses of a Ni-containing TiAl and its reference alloy based on the isothermal compression tests at 1000°C and 0.01 - 1.0s-1 strain rates. The results show that the minor Ni addition makes the flow softening coming sooner and therefore significantly lowers the peak stress. Those effects, in addition with a better balance between the work hardening and flow softening during hot deformation, improve the steady state flow behavior of TiAl. The Ni-influence mechanisms are also suggested based on the TEM observation of dislocation configurations and lamellar breakdown during the deformation.
Authors: Keizo Hashimoto, Rieko Matsumoto
Abstract: In this study, Nb doped γ-TiAl is designed and examined their tensile properties at elevated temperatures. Small compositional changes cause drastic changes of the mechanical properties at 1273K. In order to clarify the deformation mechanism at elevated temperatures, dislocation structures have been observed using transmission electron microscope (TEM). All of the specimens observed by TEM show that at least two slip systems are operating in each grain. The specimen having relatively lower strength and higher tensile elongations shows more than four operating slip systems in the grains. The specimen having a medium strength shows many super-dislocations and their dissociations that block the other dislocation motions. The specimen having the highest strength shows many deformation twins. These observations suggest that deformation twins become the strong obstacles against moving dislocations at elevated temperatures. The relationships between observed dislocation structures and mechanical properties of γ-TiAl are discussed.
Authors: K.A. Lee, Young Won Chang, Chong Soo Lee
Abstract: A series of tensile and load relaxation behaviors in B2 type β-CuZn alloy have been examined to clarify the strain rate & the temperature dependence of yield stress anomaly. As strain rate increased, the anomalous peak moved to a higher stress and temperature region. The anomalous yielding region below peak temperature (Tp) revealed slightly positive strain rate dependence, while the region above Tp showed considerably positive strain rate dependence. Based on the different tendency of temperature and strain rate dependence of anomalous yielding, two types of deformation modes were suggested and incorporated into the yield stress vs. temperature curve analysis, which was in accord well with the experimental results. The less sensitive strain rate dependence of yield stress anomaly indicated that the yield stress anomalous behavior in β-CuZn alloy mainly seemed to be attributed to the vacancy hardening.
Authors: Ai Dang Shan, Jian Sheng Wu, Hitoshi Hashimoto, Yong Ho Park
Abstract: The tensile properties of two MoSi2 alloys with different grain sizes (1 micrometer and 10 micrometer) were evaluated in vacuum at temperatures ranging from 1400 to 1600K and initial strain rates ranging from 1×10-5/s to 1×10-3/s. For the alloy with 10micron grain size an m vale of 0.35 and an activation energy value of 350 kJ/mol were observed in the lower strain rate range while an m value of 0.12 and an activation energy value of 760 kJ/mol were observed in the higher strain rate range. For the alloy with 1micron grain size, a uniform m value of 0.55 and an activation energy value of 160 kJ/mol were observed. Moreover these two alloys showed remarkable ductility (maximum 33%) in the test temperatures. The deformation mechanism and the remarkable ductility are discussed in the light of the microstructural observations through SEM and TEM.
Authors: Zhong Guang Wang, H. Zhang, Q.S. Zang, Zhe Feng Zhang, Z.M. Sun
Abstract: Ti3SiC2 has been widely investigated for their intriguing physical and mechanical properties [1]. In the past decades, dense and pure polycrystalline Ti3SiC2 has been fabricated by some techniques [1,2]. Studies on deformation and fracture behavior at elevated temperature showed that Ti3SiC2 exhibited substantial plasticity with high strength [3]. It is pointed out that Ti3SiC2 can be possibly used for some contact applications, such as bearings, for its relative high compression strength, self-lubricating and machinability [1]. In the present paper, fatigue crack-growth rates and Hertzian contact damage of Ti3SiC2 were studied. Shielding ability of Ti3SiC2 was investigated specially in the small crack region. Damage evolution and strength degradation under Hertzian contact were studied to reveal damage behavior and mechanisms in Ti3SiC2.

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