Materials Science Forum Vols. 747-748

Abstract: The transient temperature field in the dual-property disk of alloy FGH96 was investigated during the solution heating process of the gradient heat treatment by numerical simulation. The temperature curves for the different locations of the disk were attained. Then, the gradient heat treatment experiment was carried out, and the heat profiles were obtained. The numerical and the experimental results were almost consistent. The method of heat transfer for the bore of the disk was changed evidently through effective fixture design during the gradient heat treatment. The gradient of temperature (ΔT) can reach 121 or more, which lead to a supersolvus heat treated rim and subsolvus heat treated bore for the disk.

Abstract: Intermetallic compound NiAl is considered as a potential candidate material for bond coat in thermal barrier coating (TBC) system due to its capability of forming a continuous and uniform alumina scale at temperatures even high than 1200 °C, but its cyclic oxidation is rather poor. Previous study has found that cyclic oxidation resistance of NiAl alloys and coatings can be drastically improved by minor Dy doping as reactive element. In this paper, NiAlDy alloys were produced by vacuum arc-melting and the effects of various Dy contents on the microstructure and hardness of NiAl alloys were investigated. The results suggest that Dy tends to segregate at grain boundaries and precipitate within grains as brittle DyNi₂Al₃ phase with little DyNiAl needles in it. The addition of minor Dy resulted in grain refinement. The grain size of NiAl alloy were reduced from ~1 mm to ~300 µm, with increasing the content of Dy to 0.5 at.%. The addition of ~0.1 at.% Dy caused a 10 % improvement in both the microhardness and macrohardness due to Dy solid-solution and grain-boundary segregation, but the alloy revealed decreased microhardness and macrohardness with further increasing the content of Dy to 0.5 at.% as the formation of Dy-rich phase.

Abstract: The grain competitive growth and elimination during the directional solidification of a Ni₃Al-base single crystal superalloy IC6SX prepared by spiral grain selection method was studied systematically. The experimental results revealed that there were 5 kinds of mechanism during the grain competitive growth and elimination. The grains with preferred growth direction and smaller deviation angle to growth direction have stronger competitiveness, and the mutual thwarting of dendrites played an important role in the processing of grains competitive growth. The results can explain the competitive growth mechanism during the directional solidification and can be used to optimize processing parameters to lay an important foundation for improving preparation processes of single crystal superalloys.

Abstract: The creep behaviors of a Ni₃Al-base single crystal superalloy IC6SX prepared by spiral grain selection method was studied systematically under the testing condition of 760/540MPa. The microstructure evolution, movement of dislocations, formation of the dislocation networks and dislocation configuration during the creep process were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that the creep performance of the single crystal alloy IC6SX is excellent under 760/540MPa. The experimental results showed that the creep curve of the Ni₃Al-base single crystal superalloy IC6SX was divided into three stages, including decelerating creep stage, steady-state creep stage and accelerated creep stage. The microstructure and the dislocation configuration were different at different stage during the creep and the raft microstructure has not been formed. The creep mechanism was main slipping characterized by dislocation glide mechanism.

Abstract: A phase field model has been developed to simulate the dendritic growth of Ti-Ni alloy subjected to a strong magnetic field. The influence of a strong magnetic field on the microstructure morphology and its evolution was successfully investigated by the model. The effect of the magnetic field intensity on the dendritic evolution has been further discussed. The simulating results revealed that with greater magnetic field intensity, the primary dendritic arms and the side branches were easier to coarsen. Besides, the dendritic tip growth rate increased with increasing magnetic field intensity, while the curvature radius had an opposite tendency. The microstructure evolution under a strong magnetic field was also studied combined with solidification thermodynamics theory. The results indicate that, the temperature of equilibrium solidification of Ti-Ni alloy changes with the presence of a strong magnetic field, and the morphology of dendritic grains will be affected eventually.

Abstract: Some research results are introduced briefly in this paper. A new kind of titanium alloy was designed based on nine elements system, which belongs to (a+b) type. Bars with diameter of 14 mm were produced via Vacuum Arc Remelting (VAR) melting and conventional forging and rolling facilities. Influence of alloy elements contents and heat treatments on microstructure and mechanical properties of the new alloy were studied. The results show that Ultimate Tensile Strength (UTS) is about 1200~1700MPa and Elongation (EL) is normally less than 5% at R.T., though Electron Beam (EB) welding can be proceeded, but the toughness of the alloy is poor at R.T.High temperature (HT) short-term strength of the alloy is also acceptable up to 600. Based on the above performances, the new material can be applied in the corresponding circumstances.

Abstract: The microstructure, tensile properties, fracture toughness, fatigue crack growth rate and fatigue limit of different macrostructure regions in TC21 open-die forging were investigated. The results show that the microstructure is also uneven corresponding to non-uniform macrostructure. The ultimate tensile strength (UTS) and yield strength (YS) of coarse-grained zone are that UTS1070MPa, YS960MPa, which can meet the requirement of technical standard, whereas the strength of fine-grained zone drops more than 100MPa. The fracture toughness of this forging is that K_IC90MPa·m^1/2, being equivalent to the normal one. But when K=11MPa·m^1/2, the fatigue crack growth rate is that da/dN=2.488758×10^-5mm/cycle, higher than the normal TC21 open-die forging for an order of magnitude. The fatigue limit of fine-grained zone drops about 40MPa compared with coarse-grained zone. Uneven macrostructure and microstructure lead to a decline of tensile and fatigue property in TC21 forging.

Abstract: Based on microstructure of as-cast Ti-6Al-4V titanium alloy, an image processing to simplify model of β grain size, α lamella space was developed and a quantitative statistical method of β grain size, α lamella space in two-phase titanium alloys was established, with which β grain size and α lamella space of as-cast Ti-6Al-4V titanium alloy with different plate thickness were analyzed. The relationship between β grain size, α lamellar space and cast plate thickness was discussed. The results show that β grain size and α lamellae thickness nearly linearly increase as plate thickness increasing. β grain size has normal distribution and with cast plate thickness increasing, the dispersion gets larger.

Abstract: Four typical microstructures of TC18 titanium alloy (Widmannstatten structure, Fine basketweave structure, Bi-model structure, Coarse basketweave structure) were investigated, including the changes of intragranular α phase width and the relationship between intragranular α phase width and microhardness. The results show that the intragranular α phase width of the four typical microstructures varies greatly in accordance with the order from smallest to largest: Widmannstatten structure < Fine basketweave structure < Bi-model structure < Coarse basketweave structure. The microhardness of the TC18 titanium alloy reduces with the increase of intragranular α phase width.

Abstract: Evolutions of microstructure, micro-hardness in the central deformation area and spring-back, were studied in different bending conditions of Ti-15V-3Cr-3Sn-3Al titanium alloy sheet. The results show that, with the increase of bending deformation, i.e. increase of bending angle or decrease of bending radius, the grain area, the grain feret diameter ratio and micro-hardness in the central deformation area all show nearly linear growth trends. The spring-back angle has no clearly correlation with the degree of bending deformation. While the spring-back rate correlates positively with the length of deformation-affected zone, which can reflect the characteristics of bending deformation more intuitively than that of spring-back angle.