Materials Science Forum Vols. 546-549

Abstract: Mechanical Property and elastic modulus of Ti-Nb based alloys with various Si content, prepared by water quenching from high temperature β phase field, cold rolling and recrystallization heat treatment followed by water quenching, were investigated in terms of tensile test and resonance vibration method. TEM observations revealed that in addition to orthorhombic structured α" phase and bcc structured β phase, an intermediate ω phase is characterized in the microstructure. The volume fraction ratio of constituent phases was dependent upon Nb and Si contents. In as-quenched samples yield strength increased with increasing Si content. This propensity was similar to the cold rolled alloys. In recrystallized samples however, yield strength decreased slightly with increasing Si content even though the alloy containing higher Si content showed smaller grain size. From these results, it is found that strength of the present alloy influences largely on solid solution hardening and phase stability but does weakly on grain size. Yield strength of cold rolled sample was higher than as-quenched and recrystallized samples. Elastic modulus values were measured to 55GPa, 63GPa and 44GPa for as-quenched, cold rolled and recrystallized samples, respectively. The variations of yield strength and elastic modulus values were interpreted in terms of changes in microstructure as well as grain size and phase stability.

Abstract: The effects of rapid solidification on the microstructure and mechanical properties of two kinds of NiAl based eutectics were investigated. Rapid solidification resulted in the following effects, which encompass the refinement of the cell size, lamellar spacing and precipitates, deviation from the equilibrium composition, solubility extension, and a transition from a Heusler phase to an Hf-rich solid solution phase. Except for the deviation from the equilibrium composition, these microstructural characteristics are all beneficial to the improvement of mechanical properties. As a consequence, the room temperature compression yield strength and compression ductility were improved significantly and high temperature strength improved slightly. The high temperature compressive flow behaviour can be described by the temperature-compensated power-law equations.

Abstract: Fracture behavior of fully lamellar (FL) and duplex phase (DP) TiAl alloys is reported in this paper. It was found that the inverse behavior of coarse FL TiAl alloy showing inferior tensile properties but superior fracture toughness resulted from the different fracture mechanisms of these two types’ tests. In tensile specimens, the final fracture happened at a section that was most heavily damaged by the accumulation of large interlamellar microcracks and arbitrarily located within the gauge-limited volume. In 3PB notch specimens, the propagation of the main crack was constrained within a narrow strip nearby the centerline where the normal stress was the highest. Large lamellar grains caused serious damage in tensile tested specimens. However multi-oriented large lamellar grains formed seriously bifurcated crack tips, which made the crack propagation more difficult in 3PB notched specimen. The main mechanisms of toughening in FL specimens were the deflection of main crack, bifurcation and blunting of crack tip and formation of a diffuse zone of microcracks. These phenomena reduced the driving force for crack extending and then increased the fracture toughness.

Abstract: The effect of heat treatment on microstructures and stress rupture property of a Ni3Al base single crystal alloy DDIC6 was studied in the present investigate. The single crystal specimens were produced by screw selection crystal method. The heat treatment for the alloy was 1300°C/10h+1120°C/4h+870°C/32h and 1300°C/10h+870°C/32h.The microstructures were examined by SEM, TEM and X-ray EDS techniques. The stress rupture tests were carried out in air by constant load creep machines under 1100°C/130MPa with the specimens size of φ5×25 mm. The experimental results showed that the as-cast large size γ′ phases entirely dissolved after 1300°C/10h, and secondary fine γ′ phases precipitated by following aging at 1120°C and 870°C for certain periods of time. The stress rupture life under 1100°C/130MPa increased from 20~30hrs for as-cast condition to 60~100hrs for heat treatment condition. The improvement of the creep resistance of the alloy may attribute to the decrement of the elements segregation at dendrite and interdendritic areas, and the proper size and distribution of γ′ phases.

Abstract: The microstructure evolution of Ti-Al peretectic system in transient stage and steady state in directional solidification was predicted via theoretical analysis. The solute distribution controlled by diffusion at and ahead the solid-liquid interface will determine whether the properitectic and peritectic phases can nucleate and grow ahead of the opposing solid phase. The formation of banding structure is possible in a certain composition range. At the steady state, a microstructure selection map was set up based on interface response function model. The microstructure of TiAl alloys with different aluminum content was studied with Bridgman directional solidification method. Some evidence in the experiment has been found to support the theoretical prediction.

Abstract: Site occupancies of ternary additions (Ti, V, and W) in the C15 ZrCr2 and NbCr2 Laves phases were predicted theoretically by first-principles calculations based on density functional theory. The results suggest that Ti preferentially occupies the Zr and Nb sites in ZrCr2 and NbCr2, respectively. V and W substitute the Cr sites in both ZrCr2 and NbCr2. The calculations of heats of formation also show that the occupancy of W on the Cr sites and of Ti on the Zr sites stabilize ZrCr2. For NbCr2, the occupancy of V on the Cr sites and of Ti on the Nb sites increases the phase stability.

Abstract: The oxygen atom adsorption at Al-Al bridge, Ni-Ni bridge, Al top and Ni top site on the NiAl(110) surface by first-principles method within density functional theory has been studied in this paper. It has been found that the preferred adsorption position of the oxygen was at the Al-Al bridge site then the Ni-Ni bridge site. The charge transfer took place obviously between the O atom and the nearest Al atoms, but no charge transferred from the nearest Ni atoms to O atom. For the Al-Al (Ni-Ni) bridge adsorption site, the bond lengths of Al-O and Ni-O were about 1.741 Å (1.700Å) and 2.369Å (2.012Å), respectively, which means that the Al atom is easier to be oxidized than the Ni atom. It is revealed that the Al atom oxidized selectively and the chemical bond formed between the O ion and the nearest Al ions during the initial oxidation stage.

Abstract: The hot deformation characteristics of an as-cast Ti-22Al-25Nb alloy has been studied in the temperature range of 1323-1523K and the strain rate range of 0.001-10s-1, using hot compression tests. The experimental results indicated that discontinuous yielding occurs during the hot deformation performed at the strain rate of 10s-1, while the flow curves are of a steady-state type at lower strain rate range. Activation energy was obtained by analyzing the steady-state flow stress with a standard constitutive equation. They are 260-282kJ/mol in the temperature range of 1473-1523K, and 145-155kJ/mol in 1323-1423K. The processing map developed using the principles of dynamic material modeling exhibits three domains for the present alloy: 1) a domain of dynamic recrystallization of B2 phase in the temperature range of 1373-1423K at the strain rate range of 0.01-0.001s-1, with the power dissipation efficiency of about 35-50%, 2) a domain of dynamic recovery of B2 phase in the temperature range of 1473-1523K at the strain rate less than 0.01s-1, with the power dissipation efficiency of about 20-30%, 3) a domain of flow instability in the form of adiabatic shear band in the temperature range of 1323-1373K at the strain rate larger than 1s-1.

Abstract: The effect of NiCr-CrAl coating on the microstructure, oxidation and corrosion resistance, as well as mechanical properties of Ni3Al base alloy IC6AE has been studied in the present investigation. NiCr-CrAl coating for alloy IC6AE was prepared by the powder pack cementation method. The oxidation resistance tests were carried out under the condition of 1050°C/100h, while the hot corrosion resistance tests were under 900°C/100h. The tensile tests at the room temperature and the stress rupture life tests under 1050°C/90Mpa have been also conducted. The experimental results showed that NiCr-CrAl as-coated specimens had excellent oxidation and hot corrosion resistance compared with the alloy IC6AE blank specimens, and NiCr-CrAl coating has no obvious effect on the mechanical properties of alloy IC6AE. It may be concluded that NiCr-CrAl coating is suitable for the alloy IC6AE.

Abstract: A first principle study was performed to discuss the high temperature oxidation mechanism of NiTi alloys with the special emphasis on the effect of Nb on the oxidation behavior. The calculation results suggest that the Nb atom prefers the Ti site in Ni(Ti,Nb). The addition of Nb will not only reduces the electron density of Ti-d and Ni-d states near the Fermi energy level but the their electron contributions to the p-orbital of Ti. In addition, the Nb atom increases the formation energy of the Ti defect, which will decrease the diffusion of Ti atoms. All these Nb-induced effects account for the improvement of high temperature oxidation resistance, which agrees well with the experimental results.