Materials Science Forum Vols. 539-543

Abstract: Directional solidification technique permits materials to grow along specific orientation, in order to obtain mechanical and/or physical anisotropy. The present research attempts to introduce the research work in the field of processing of some advanced materials by innovative directional solidification techniques performed at State Key Laboratory of Solidification Processing and with author’s intended research work. The paper deals with the specific topics on state of the art of directional solidification: single crystal superalloy and Nd-Fe-B alloys under high thermal gradient, Cu-Ni alloys under deep supercooling of the melt. The relevant solidification phenomena, such as morphological evolution, crystal growth for multi-phases in the processing of directional solidification, are discussed briefly. The trends of developments of directional solidification techniques are also prospected.

Abstract: Degradation mechanism and life prediction method of high chromium ferritic creep resistant steels have been investigated. In the high stress condition, easy and rapid extension of recovered soft region results in significant decrease in creep strength, however, ductility is high. In the low stress condition, extension of recovered soft region is mainly controlled by diffusion and it is slow, therefore, deformation is concentrated in the recovered soft region along grain boundaries and ductility is extremely low. Delta-ferrite produces concentration gap due to difference in equilibrium composition of austenite and ferrite phases at the normalizing temperature. It increases driving force of diffusion and promotes recovery of tempered martensite adjacent to delta-ferrite. Concentration gap may be produced also in heat affected zone (HAZ), especially in fine grain HAZ similar to dual phase steel, and it has possibilities to promote recovery and, therefore, to decrease creep strength. It has been confirmed the advantage of region splitting analysis of creep rupture strength for high chromium ferritic creep resistant steels, through a residual error analysis. It is important to avoid a generation of concentration gap in order to improve stability of microstructure and to maintain high creep strength.

Abstract: Alloy 718 ingot with a diameter of 400mm was made by the vacuum melting process ; VIM followed by VAR. Compression tests were conducted on samples collected from columnar zone of the VIM/VAR-processed Alloy 718 ingot in wide temperature and strain rate ranges, i.e. 750~1,200OC and 10~0.001s-1 in order to understand the deformation behavior and evolution of microstructure. Tensile tests at high temperature were also conducted on samples in temperature ranges, i.e. 750~1,100OC. Effects of process parameters on the flow behavior as well as on the microstructure evolution during compression tests at high temperatures are considered. As a result of the deformation simulation, The VIM/VAR ingot was heat-treated for homogenization, and casting structure of the ingot was broken down for uniform microstructures and mechanical properties by controlled cogging process using a hydraulic press. The observation of the microstructure and grain size distribution was carried out to evaluate the effects of optimum process parameters during cogging and mechanical property tests were performed in this study.

Abstract: The deformation features are analyzed for titanium alloy and superalloy during isothermal/hot die forging process, and proper finite element models with appropriate parameter values are determined. On the platform of DEFORM software, the formation processes of vane-integrated disk and compressor disk, made of titanium alloy and superalloy respectively, are simulated and analyzed. Based on the simulation results, some important suggestions to the process design and parameter determination are brought forward, which have been taken into consideration or adoption in practice. As a result, the production yield is promoted, and a large amount of expenses of testing and die trial-manufacture are saved.

Abstract: Boron-fiber-reinforced Al-matrix composite was fabricated by a pulsed current hot pressing (PCHP) process at a pressure of 32MPa for 600s. It was found that the boron fiber and the Al-matrix were well bonded when the PCHP process was performed at a holding temperature of 773K. No interfacial reaction layer was observed along the interface between the boron fiber and the matrix when PCHP was done at 773K for 600s. Tensile deformation carried out at room temperature for the composite showed that the tensile yield stress increased with increasing volume fraction of the boron fiber in the composite. The composite with 17.2 vol.% of boron fiber presented a tensile yield stress of 600MPa. This value was about 90% the yield stress estimated by a force equilibrium equation of a composite taking into account the direction of fiber axis.

Abstract: The growth of TiNi thin films by ion beam sputter deposition using a Kaufmann type ion source is described. Argon ions are used to sputter separate Ti and Ni targets to deposit nearequiatomic TiNi thin films. Typically, ion energies and current densities of 1500 eV and 1 mA cm-2 respectively are used, with an argon overpressure of around 0.05 mtorr, to achieve deposition rates of order 1 μm hr-1. The thermophysical properties of the deposited films were investigated by thermal imaging. Patterning of TiNi films and foils with micrometre resolution using KrF excimer laser ablation at 248 nm wavelength, with beam fluence up to 2.5 J cm-2, 15 ns pulse duration and pulse rates up to 100 Hz has also been investigated.

Abstract: Effect of chemical composition was investigated on martensitic transformation temperatures, Curie temperature, magnetization and microstructures for Ni-(Mn, Fe, Co)-Ga and Cu-Mn-Ga systems. In the Ni-(Mn, Fe, Co)-Ga alloys, which is a modification of Ni-Mn-Ga systems, the Af and TC over 400 K were achieved. Cu-Mn-Ga alloy exhibited shape memory effect at temperatures above 373 K and had TC over 400 K. Furthermore, Cu-Mn-Ga exhibits good ductility even in polycrystalline condition unlike the case of Ni-Mn-Ga. Effect of addition of the fourth element to improve the magnetic property is under investigation.

Abstract: Effect of isothermal aging on martensitic transformation temperatures, mechanical properties and microstructure was investigated for a Ni-rich Ti-Zr-Ni shape memory alloy at temperatures ranging from 673 K to 773 K. The aging behaviour was two stage process: the first stage associated with an increase in the Vickers hardness and a decrease in martensitic transformation temperatures and the second stage with a decrease in the hardness and increase in the transformation temperatures. Second stage was also characterized by the appearance of nano-scale precipitates, which has never been reported.