To further investigate the microstructure characteristic and solidification mechanism, so as to provide knowledge for the microstructure control of a NiTi-AI based high-temperature structural material, the microstructure of Ni-43Ti-4AI-2Nb-2Hf （at.%） alloy ingots prepared by conventional casting （arc-melting） and directional solidification （DS） at various drawing velocities （2 mm.min-＇, 18 mm.min-1, 30 mm-min-＇ and 60 mmmin~, respectively） was investigated by means of electron probe microanalyses. Experimental results reveal that the microstructures are composed of NiTi matrix phase,/3-Nb phase and Ti2Ni phase for samples obtained by both conventional casting and DS. Conventional casting has an equiaxial structure, while DS has a slender and acicular cellular structure which grows along the [001] orientation preferentially. Small amounts of white/3-Nb phase and black Ti2Ni phase co-exist at the grain boundaries or intercellular regions. With an increase in drawing velocity, the NiTi matrix phase is inclined to grow along （100） and （200） crystallographic planes, and the cellular arm spacing reduce gradually, but the directionality of the solidified structure weakens significantly. The homogeneous dispersion of,8-Nb phase and the decrease of Ti2Ni phase in DS samples are beneficial to improving the mechanical properties. Solidification mechanism analysis indicates that the dark grey NiTi matrix phase initially precipitates from the liquid phase, and then the divorced eutectic reaction takes place, which produces the light gray matrix phase and/^-Nb phase. Finally, the peritectic reaction happens, which generates the black Ti2Ni phase.
Pan LiwenZheng LijingZhou LeiZhang HuaruiTang XiaoxiaMa LiminZhang Hu
The refinement of large boride ribbons in the as-cast TiAl alloy is the essential issue for aerospace industry application,which is difficult to avoid by classical casting techniques.The present paper seeks to explore the possibility of the semi-solid process in manufacturing Ti-47Al-2Cr-2Nb-0.8 B(at%)alloy.An important result is that,except forming a nondendritic globular structure,the semi-solid process also plays a crucial role in refining large borides for the TiAl alloys with boron.In the current alloy,the long ribbon borides can be successfully converted to fine,equiaxed particles with an average size of 2-5μm.Furthermore,subsequent proper heat treatment is necessary to control and achieve a fine,nearly fully lamellar microstructure.The key microstructure-refinement mechanism is due to the fine borides obtained during semisolid process,which act as the effective nucleation centers for the formation of interdendritic fine y grains,providing the pinning effect on priorα-grain boundaries.These results prove that semi-solid process can be an ideal candidate for the microstructure refinement in TiAl alloys.
In this study, the effect of temperatures and cooling rates of heat treatment on the microstructure of a powder metallurgy (PM) Ti-46Al-2Cr-2Nb-(B,W) (at.%) alloy was studied. Depending on the cooling rate and temperature, the different structures were obtained from the initial near-γ (NG) microstructures by heat treatment in the α+γ field. The results show that the microstructures of samples after furnace cooling (FC) consist primarily of equiaxed γ and α 2 grains, with a few grains containing lamellae. Duplex microstructures consist mainly of γ grains and lamellar colonies were obtained in the quenching into another furnace at 900°C (QFC) samples. However, further increasing of the cooling rate to air cooling (AC) induces the transformation of α→α_2 and results in a microstructure with equiaxed γ and α_2 grains, and no lamellar colonies are found.
By liquid metal cooling(LMC) process,the Ni-43Ti-4Al-2Nb-2Hf(%,atomic fraction) alloy was directionally solidified(DS).The microstructure and tensile properties at room and elevated temperature were investigated.It was found that the DS process significantly improves the room temperature tensile strength,increasing by 70% compared with the as-cast alloy.After appropriate heat treatment(HT),the average tensile strength reaches above 1900 MPa,nearly twice of the as-cast one.At 800 and 900 ℃,the tensile strengths are about 308 and 169 MPa,respectively.
Jiang, Dongwen Zheng, Lijing Zhou, Lei Pan, Liwen Tang, Xiaoxia Zhang, Hu
This article presents the microstructure and hardness variation of an Al 8.5Fe-1.3V 1.7Si （wt%, FVS0812） alloy after selective laser melting （SLM） modification. Three zones were distinguished across the melting pool of the SLM-processed FVS0812 alloy： the laser melted zone （LMZ）, the melting pool border, and the heat affected zone （HAZ） in the previously deposited area around the melting pool. Inside the LMZ, either an extremely fine cellular-dendritic structure or a mixture zone of the α-Al matrix and nanoscale Al12（Fe,V）3Si particles appeared. With a decreased laser beam scanning speed, the cellular-dendritic structure zone within the LMZ shrank significantly while the mixture zone expanded. The α-Al and Al12（Fe,V）3Si mixture zone was also observed in the HAZ, but another phase, submicron θ-Al13Fe4 particles with rectangular or hexagonal shapes, formed along the melting pool border. Microhardness tests indicated that the hardness of the SLM-processed FVS0812 samples far exceeded that of the as-cast FVS0812 alloy.
