Materials Science Forum Vols. 475-479

Abstract: Most of materials have long been considered to be mechanical and/or physical anisotropy. Permitting materials to grow along specific orientation by means of directional solidification technique can optimize their structural or functional properties. The present paper 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 directional solidification of following advanced materials: column and single crystal superalloys under high thermal gradient, Ni-Cu alloys under deep supercooling of the melt, intermetallic compounds with selected preferential crystal orientation, superalloys with container less electromagnetic confinement, high Tc superconducting oxides, high temperature structural ceramics, continuous cast single crystal copper and copper-based composites. The relevant solidification phenomena, such as morphological evolution, phase selection, peritectic reaction and aligned orientation relationship of crystal growth for multi-phases in the processing of directional solidification, are discussed briefly. The trends of developments of directional solidification technique are also prospected.

Abstract: A new nickel-base superalloy Ni-25Cr-20Co-0.5Mo-2Nb-1.7Ti-0.8Al has just developed by Special Metals Corporation in the application to ultra-supercritical boilers with steam temperatures up to 700°C. The structure stability of the alloy was studied in detail in combination of SEM, TEM, XRD and micro-chemical phase analyses. Experimental results show that the most important structure instability is as follows: 1) g¢ coarsening; 2) g¢ to h transformation; 3) G phase formation. The phase computation by means of Thermo-Calc has been adopted in chemical composition modification for structure stability improvement. Two suggested new modified alloys in adjustment of the Al and Ti contents and in control of Si level were designed and melted for experimental study. These 2 modified new alloys exhibit more stable structure at 760oC long time aging. A bright perspective has been shown for the development of this new alloy in the near future.

Abstract: Internal elastic strain, and its change accompanied with the raft formation during creep deformation in the Ni-based single crystal superalloy (TMS-26) have been investigated by X-ray diffractometry. The elastic strain caused by the lattice misfit between g and g' phases has markedly been changed by creep deformation especially in the directions perpendicular to the [001] tensile axis. The change in the elastic strain can be explained by the effect of creep dislocations stacked at g/g' interfaces. The evolution of the elastic stress field estimated from the elastic strain has explained well the transition from primary creep stage to the second one.

Abstract: The creep behavior and microstructure of several nickel-base single-crystal superalloys after high-temperature low-stress creep have been investigated. These alloys were designed with varying content of the alloying elements Mo and Ru. At 1100°C and 137 MPa, the large g/g¢ lattice misfit in negative with the addition of Mo leads to the formation of dense interfacial dislocation networks. These dislocation networks are effective to strengthen the alloys during creep by preventing the penetration of the g dislocations into the g¢ phase.

Abstract: The mechanical properties of chromium (Cr) and Cr-base alloys are reviewed, with particular emphasis on the ductility at ambient temperature and the strengths at high temperature. Analysis of rather scattered data and our recent results suggest that Cr-base alloy can be ductilized greatly at ambient temperature and is quite capable of being strengthened to high levels at high temperature. New designs on composition would give high possibility to Cr-base alloys as structural materials used in high-temperature applications.

Abstract: A recrystallized Co-Ni-Cr-Mo based superalloy was produced by cold working of 72% and subsequent recrystallization heat treatment. Microstructural observation revealed that a full recrystallization of the cold-worked alloy occured when heat treatment was performed at and above 1273K for 1h. So that, recrystallization heat treatment was carried out in a temperature range from 1273K to 1473K for 1h~24h, by which the average grain size was controlled to 28µm～238µm. Tensile tests were carried out from room temperature (RT) to 1073K in order to understand the effect of grain size on the mechanical properties of the Co-Ni-Cr-Mo based superalloy. At RT and 943K, yield strength, tensile strength and elongation of the recrystallized alloy were improved with decreasing grain size. The alloy having a grain size less than 42µm exhibited a steady-state flow behavior in the true stress-true strain curve at 943K. However, the alloy having a grain size of 28µm showed lower yield strength than that of 42µm at 1073K. It was found that the steady state flow is closely related to the occurrence of {111}<112> deformation twinning in the Co-Ni-Cr-Mo based superalloy.

Abstract: Hot isostatic pressing(HIP)/diffusion bonding(DB) was employed for cast superalloy MM247 and P/M superalloy Udimet720. The effects of P/M Udimet720 particle size, holding time for HIP/DB and post heat-treatment on the microstructure and tensile properties were investigated. Tensile properties of HIP/DB’ed MM247/Udimet720 were improved at room temperature and 650°C with decreasing powder size from –140mesh to –270mesh, and with increasing HIP/DB time from 2h to 3h. Post-HIP heat-treatments, which strengthen the soft MM247 with cuboidal ’ precipitation, are proposed to enhance the mechanical properties of the HIP/DB’ed MM247/Udimet720.

Abstract: This study was carried out to investigate the effect of heating rate on the phase transition between additive base metal powder and liquid insert metal during transient liquid phase (TLP) diffusion bonding of Ni-based superalloy GTD-111. Heating rates studied were 10, 1, 0.5 and 0.1°Cs-1 in high vacuum conditions (3×10-5 torr) by means of a high frequency induction furnace. When heated at lower than 0.5°Cs-1, the transition of dissolution to solidification occurred during heating. In the case of very slow heating, the dissolution quickly finished at a lower temperature, and solidification soon started. The separated grains of additive base metal powders supplied the lager interface area for the diffusion of boron. Solidification transition temperatures of liquid phase were affected by the increase of diffusion interface and of duration time during slow heating. A minimum heating rate required to heat insert materials to a normal TLP isothermal bonding temperature of GTD-111 (≈1150°C) without a dissolution-solidification transition during heating should be higher than 1°Cs-1.

Abstract: Iron-base superalloys are well known materials having excellent high temperature properties .They are used in turbo superchargers and turbine engines required for aerospace and power plants. In this investigation precipitation hardenable X5NiCrTi26-15 was used to study the influence of microstructural changes on the creep behavior at different conditions . Different creep cycles were applied for both base alloy and laser beam welded alloy (6kW CO2 ) namely at 600 , 625 and 650C at applied controlled creep stresses of 400 and 450 MPa . The base material sheet was used in as solution annealed state ( 30 min, 960 C, WQ ).The specimens were hardened in two steps (24h,760 C, FC and 16 h, 705 C,AC ) before being investigated . The microstructural changes due to grain boundary sliding, intergranual fracture perpendicular to the metal flow axis , and the type ,morphology of different secondary carbides were measured and discussed . To examine the changes in microstructure Philips EM 400 TEM with an acceleration voltage of 120KV, and SEM as well as light microscopy were used . It was found that , laser beam welded structure investigated after creep deformation at temperatures lower than 650C and at controlled stress of 400 and 450 MPa , showed a textured weld metal zone with dendrite having lower hardness combined with a higher creep resistance than that for base material .It was found also that creeping at 650C at the same stress values offsets any gain in creep resistance of welded joints as compared with that for the base material at the same conditions.

Abstract: An attempt was made to assess the surface damage in the isothermally degraded FSX414 Co-based superalloy using ultrasonic Rayleigh surface wave. Microstructural analysis (amount of solute Cr near the surface) and measurement of ultrasonic velocity and attenuation were performed after isothermal degradation heat treatment at 1100°C for up to 4000 hours. Rayleigh wave velocity was observed to increase with aging time, which was attributed to the increased depletion of Cr near the surface. Strong frequency dependence of Rayleigh wave velocity was found in the specimens with surface depletion layer. Attenuation coefficient of Rayleigh wave increased with increasing aging time. Therefore, both attenuation and velocity of ultrasonic Rayleigh wave are suggested as potential nondestructive evaluation parameters for the characterization of the thermally aged FSX414 superalloy.