Advanced Materials Research Vols. 26-28

Abstract: The superplastic properties of Ti3Al base alloy have been investigated in the range of 900~1020°C and strain rate range of 3.3×10-4~3.3×10-2s-1 .The largest elongation of 1570% was obtained under the optimal superplastic deformation condition of 980°C and 3.3×10-4 s-1. The m-value varied from 0.46 to 0.59, its maximum value was obtained at 980°C.The microstructure of the alloy after heat treatment was composed of two phases of small spherical α2 phase particles distributing in βtransus matrix. Microstructure change has an obvious effect on the superplastic properties of Ti3Al-based alloy. At 980°C,when theα2 phase grain size changed from large to small then large and the cavitations in the fracture surface were larger and deeper with decreasing strain rate, the alloy showed the larger elongation and the lower flow stress.

Abstract: In this study, Nb-Cr binary mixtures were prepared by mechanical alloying. And mechanical alloying process of mixed Nb-Cr(1:2) powders was studied, and the phases of milling powders analyzed by XRD and SEM. It has been found that the powders form solid solution first, then amorphous phase appeared gradually and crystallization of amorphous takes place with the different milling time. A thermodynamic model of MA of Nb-Cr system was developed based on Miedema Semi-experimental theory to explain the phase transformations of Nb-Cr system during milling. Using Miedema semi-empirical model the free energies of the ordered Laves-phase NbCr2 intermetallics, solid solution and amorphous alloy were calculated. From XRD and SEM analysis of Nb/2Cr powders prepared by mechanical alloying in a planetary ball milling, it was found that the calculated results were in accordance with the experiments.

Abstract: In this study, the distribution of colloidal nano-gold particles on the nanoporous surfaces of FeAl single crystals was investigated. Colloidal nano-gold particles were dropped onto the nanoporous surface, and their distribution was observed by TEM. Some of nano-gold particles located at the edges of nanopores. A few additional attempts to improve the wettability of the surfaces were made using surface treatments. As a result, the frequency of nano-gold particles put into nanopores was achieved up to 98.3%.

Abstract: Ti-45.5Al-2Cr-4Nb-0.4B alloy was cast by vacuum arc melting at high purity Ar atmosphere using high purity sponge Ti, granular Al (99.99%), flake Nb (99.9%), lump Cr (99.9%) and TiB2 (99.5%) and subsequently heat-treated to obtain a couple of microstructures, i.e. lamellar and near γ. The heat treatment consisted of annealing at a high temperature (1200 ~ 1330oC) of different phase fields for 24 hrs and stabilizing at 900oC for 4 hrs followed by air cooling. Fracture toughness was measured on the specimens with different microstructures at room temperature. The value of KQ of specimen with fully lamella structure was obtained as 18.68 MPa √m, much higher than that of specimen with near γ structure (11.84 MPa √m). It was also revealed that the KQ value was decreased as the annealing temperature decreased.

Abstract: The characteristics of Al-coating materials on low carbon steel sheets during high temperature oxidation process were investigated. The phases’ evolution and growth were systematically evaluated by SEM/EDS and XRD. The hardness distribution from the surface to the steel substrate was measured to check the brittleness of iron aluminides coating layers. The oxidation properties of coating materials were investigated using weight gain method. Intermetallic compound recognized as FeAl3 was initially formed on the interface between steel substrate and coating materials. After exposing the specimens at elevated temperature, FeAl3 layer was gradually transformed into Fe2Al5, FeAl2, and FeAl+Fe3Al. Oxidation rate increased fastly after α-Al in the coating completely transformed at 650 °C. The brittle FeAl3 and Fe2Al5 have the lower resistance to oxidation because the easier cracking during intermetallic reaction.

Abstract: Effect of the ternary element of Mo on the crystal structure and thermoelectric properties of ReSi1.75 has been investigated. The crystal structure of Mo-containing ReSi1.75 has shear structure up to 1.5 at%, the structure changes into adaptive structure at larger Mo contents. The concentration of Si vacancies estimated from the crystal structures determined decreases with increasing Mo content. Thermoelectric properties of Mo-containing ReSi1.75 indicate that the character changes from p- to n-type semiconductor upon alloying with Mo, which is explained by the decrease in Si vacancy concentration.

Abstract: Hot rolled Mg alloy has a preferred orientation, which affects the deformation behavior of Mg alloy. In this work, load relaxation and tensile tests after hot rolling process were performed to clarify the effect of the texture on deformation behavior of AZ31 Mg alloy and the results were analyzed based on an internal variable theory proposed by one of the authors. The analysis of the deformation behavior is then correlated to XRD pole figure results of the texture. The analysis result of Mg alloy was found to be comparable to the results obtained from the test of Ti alloys, which has the same HCP structure.

Abstract: The thermodynamic properties of high temperature metals and alloys are studied using the statistical moment method, going beyond the quasi-harmonic approximations. Including the power moments of the atomic displacements up to the fourth order, the Helmholtz free energies and the related thermodynamic quantities are derived explicitly in closed analytic forms. The configurational entropy term is taken into account by using the tetrahedron cluster approximation of the cluster variation method (CVM). The energetics of the binary (Ta-W and Mo-Ta) alloys are treated within the framework of the first-principles TB-LMTO (tight-binding linear muffin tin orbital) method coupled to CPA (coherent potential approximation) and GPM (generalized perturbation method). The equilibrium phase diagrams are calculated for the refractory Ta-W and Mo-Ta bcc alloys.

Abstract: This work was studied for the changes of thermal properties on GTD-111 DS (Directional Solidification) gas turbine blade. In this study, gas turbine blades with 24,000~34,000 firing hours was used to get more effective result, gradually applied hot isostatic pressing (HIP) and post-heat treatment for these samples. In the latter steps, we observed changes of γ´ phase affected in material properties, and microhardness test was carried out to evaluate mechanical properties according to changes of γ´ fraction and shape. Experimental result shows, changes of γ´ fraction and shape were affected by HIP and post-heat treatment. And also mechanical properties changes such as micro-hardness related to γ´ phase. In this study, we explained changing transition of microstructure according to γ´ fraction distribution.

Abstract: The crystallographic orientation distribution, and its change as a function of creep deformation in Ni-based single crystal superalloys have been investigated by X-ray diffractometry. The distribution of the crystallographic orientation has significantly broadened by creep deformations. Directional broadening of the distribution agrees with creep dislocations having the burgers vector of 1/2<101>. High temperature creep strain of superalloys can be estimated by a non-destractive test where the width of rocking curve of a diffraction peak is measured.