Materials Science Forum Vols. 706-709

Abstract: Pre-alloyed beta gamma titanium aluminide powder with a nominal composition of TiAl-4Nb-3Mn is consolidated by hot isostatic pressing. After consolidation, a step cooled heat treatmentis performed to homogenize the material and produce a fully lamellar microstructure. Various agingheat treatments are then performed with the goal of forming interfacial beta phase precipitates alonglamellar interfaces. The step cooled heat treatment produces a relatively fine microstructure with alamellar spacing of 0.04 μm and an average lamellar colony size of 60 μm. The aging heat treatmentsgenerate beta phase precipitates along lamellar colony boundaries but not along lamellar interfaces,and result in lamellar degredation and grain growth. Constant load tensile creep and room temperaturehardness tests are performed on step cooled heat treated and step cooled heat treated and aged specimens.Creep resistance, generally, improves with aging time, even with no interfacial precipitation,and the lamellar degredation that occurs with aging. The microstructures of the as-tested specimensare characterized and related to the creep properties. The hardness values are also compared as afunction of selected heat treatment condition and microstructural features.

Abstract: A first principles study of adsorption of oxygen atom on surfaces of γ-TiAl was performed to investigate the intrinsic mechanism dominated the adsorption behaviours. First surface stability was evaluated. It was shown that the (100) surface is the most stable surface followed by (111), (110), and (001) surfaces. Then adsorption of oxygen on the (100) and the (111) surfaces was studied. Oxygen atom prefers the Ti-rich environment and has high potential to generate TiO₂. Competition between the O-Al bonding and the O-Ti bonding was observed. However, the O-Ti interaction dominates the adsorption behaviours in all considered systems. A linear relationship between adsorption energy and integration of orbital overlaps of O and metals was observed, which indicates that the electronic structure controls adsorption behaviours of oxygen on surfaces of γ-TiAl and provides a guidance to improve the oxidation resistance of γ-TiAl based alloys.

Abstract: In this study, long period stacking ordered structure (LPSO) type magnesium alloys have been developed. They consist of LPSO phase and α-Mg phase and have excellent mechanical properties at both ambient and elevated temperatures. In the present study, the influence of electromagnetic stir (EMS) on the structure of LPSO type Mg₉₆Zn₂Y₂ (at. %) billets of φ200 mm in diameter was investigated in order to establish the large-scale casting technology of LPSO type Mg alloy. EMS refined the grain size of α-Mg up to approximately 1/3.5, and reduced the size difference between the surface and the center of the billets. By applying EMS, it was found possible to provide large-scale casting billets with fine and homogeneous microstructure.

Abstract: Zn and Al are major alloying elements of Mg alloys. Main slip system of Mg is a basal slip and the CRSS increases with Zn or Al content. According to von-Mises criterion, five kinds of independent slip systems are required for uniform deformation, so it is necessary to activate non-basal slip systems to show good ductility. However, it has not become clear the effect of Zn or Al for non-basal slip systems yet. To investigate deformation behavior of magnesium crystal by non-basal slip, Mg-Zn and Mg-Al single crystals were stretched in the [110] direction and Mg-Zn single crystals were compressed in the [0001] direction. {112}<23> second order pyramidal slip was activated in Mg-0.1at%Zn and Mg-0.5at%Al. On the other hand, {101} twin was mainly activated in Mg-1.0at%Al alloy. Yield stress due to the pyramidal slip of magnesium decreased by 0.1at%Zn addition, however they increased with addition of aluminum..

Abstract: A way to overcome the low deformability of magnesium alloys at room temperature is toincrease the temperature of forming operations. The stress exponent n, which is known to be a keyparameter in the control of plastic stability, generally decreases when temperature increases.Nevertheless, low n-values are not enough to ensure large capacity of deformation since fracturecan also result from strain induced cavitation. In the present investigation, both the mechanisms ofhigh temperature deformation and damage were studied in selected Mg alloys. Since damage datacan also give information on the deformation mechanisms, the strain induce cavitation behaviourwas mainly studied thanks to X-ray micro tomography which provides 3D information like thecavity shapes or the variation with strain of the number of cavities. Moreover, additionally toconventional post mortem analyses, it was attempted to perform the 3D damage characterisation inin situ conditions, namely directly during high temperature deformation tests.

Abstract: The phase relationship of the Mg-Ni-Y system along the Mg-NiY line has been investigated using electron probe micro analysis (EPMA) at 673 K. Three two phase regions (Mg₂Y+τ₆, Mg₂Y+ MgNiY(τ₃), MgY+Mg₃Ni₂Y₄(τ₁)) and two three phase regions (Mg₂Y+Mg₃Ni₂Y₄(τ₁)+ MgNi₂Y₂(τ₂) and NiY+Mg₃Ni₂Y₄(τ₁)+MgNi₂Y₂(τ₂)) have been identified. One new ternary compound Mg₅NiY(τ₅) was found and reported for the first time. Ternary solubility of the Mg_8-13NiY(τ₆) compound has been established which extends from 77.10 at.% Mg until 86.77 at.% Mg along the Mg-NiY section. Solubility of the third element in the binary compounds, NiY, Mg₂Y and MgY has also been determined. Keywords: Mg alloy, phase identification, phase equilibrium, isothermal section.

Abstract: It is well-known that age hardening occurs in Mg-Al system alloys, when the alloy containing aluminum exceeds 6mass%. This precipitation reaction depends on aluminum content and aging temperature. The aging behavior in AZ91 magnesium alloy was investigated and it is the subject of this paper. However, for the Mg-Al system alloys, the influence of aluminum content on aging hardening characteristics has not been researched in detail so far. In this study, continuous and discontinuous precipitations during aging in Mg-Al system alloys cast into sand and iron molds were investigated by means of hardness measurement and microstructure observation with optical microscopy and transmission electron microscopy. Variation of hardness with aging was found to be caused mainly by the discontinuous precipitation along the grain boundaries from the composite rule in hardness. In iron mold castings, It was found that the variation of hardness with aging was found to be caused mainly by the continuous precipitation inside the crystal grain.

Abstract: AS is a relatively new series of Magnesium alloys. The microstructure of this alloy can be improved for semisolid processing. The current research is concerned with the microstructure evolution of AS21 under the strain induced melt activated (SIMA) process. For this purpose, the AS21 alloy is cast and compressed 10-40% at 200 °C. The semisolid heat treatment is completed in a carbonate salt bath at different temperatures between 600-620 °C. The microstructure studies show that there is no favourable microstructure evolution between 600-610 °C. At 615 °C fine globular grains are obtained with the most desired mean grain size and sphericity of 67 µm and 81%, respectively. At 620 °C an undesirable coarsening phenomenon occurs that damages the microstructure globularity. SEM micrographs show that in a successful SIMA processing, the Mg₂Si phases are broken into fine particles distributed within the grains and grain boundaries.

Abstract: Roll caster to cast a three layers clad strip was invented and effect of casting conditions on the clad strip was investigated. Two vertical type twin roll casters were set at vertically tandem position to cast three layers clad strip. Roll speed, roll load (separating force), melt temperature and elements of the aluminum alloy were chosen as the experimental conditions to be investigated. The roll load (separating force) and elements of aluminum alloy were important factors to cast sound clad strip. The clad strip could be cast at the speeds up to 40m/min. Interface between the strips were clear and elements of each strip did not diffuse to other strip. The clad strip was not peeled off at the interfaces by continuous bending. Strips were strictly connected at the interfaces.

Abstract: Microstructural factors that govern the plastic deformation of the long-period stacking ordered (LPSO) phase were clarified. The decrease in length of the long-axis for the plate-like shape of LPSO-phase grains increases the yield stress of the alloy in which basal slip is predominant in deformation. On the other hand, the yield stress tended to increase as the thickness of the plate-like shapes of the grains decreased for the alloy in which the formation of deformation kinks carried the strain.