Microstructures and phase transformation in directionally solidified TiAl-Nb alloys

Intermetallic Ti-45Al-8Nb-(W,B,Y)(at.%)and Ti-46Al-5Nb alloys are directionally solidified at a constant growth rate of 30μm·s-1 using a Bridgman type apparatus.The quenched microstructures and lengths of different phase regions were observed and measured after various growing times of 0-30 min.Results show that the phase transformations in different phase regions are mainly depending on the high temperature microstructure and the supercooling degree during quenching process.After isothermal holding,the primary phase grows into the liquid phase,the dendrites change from equiaxed to columnar grains,and the length of the L+βphase region,L+β+αphase region and mushy zone varies,indicating that the entire directional solidification process can be described by a static equilibrium-nonequilibrium-dynamic equilibrium evolution process.In addition,the gap between the original growth interface and front interface shows that the actual crystal growth rate is not equal to the drawing velocity during directional solidification.

Tensile and compressive behavior of Ti-based bulk metallic glass composites

This article focuses on the tensile and compressive characteristics of a Ti-based bulk metallic glass composite （BMGC）. It is found that the yield stress, maximum strength, and fracture strain are 1380 MPa, 1516 MPa, and 4.3% for uniaxial tension, but 1580 MPa, 4010 MPa, and 29% for uniaxial compression, respectively. The composite displays a linear ＂work hardening＂ capacity under compression; however, the ＂work softening＂ behavior is observed in the true engineering stress-strain curve upon tensile loading. The fracture surfaces of specimens also exhibit dissimilar properties under the different loadings.

Effect of Nb and alloying elements on interface reaction between high Nb-containing TiAl alloys and ZrO_2-based ceramic moulds

In the present study, Ti-45Al-(6, 7, 8)Nb(at%) and Ti-45Al-8Nb-0.5(Mn, Si, Y, B) alloys were prepared by arc melting and casting into Zr O2(Y2O3 stabilized) ceramic moulds to study the effect of alloying elements Nb and Mn, Si, Y, B on the interfacial reaction between casting Ti Al alloys and ceramic moulds by SEM, and the elements' distribution in the interface reaction layer by line scanning. The results showed that with an increase in Nb content, the interfacial reaction weakened and the thickness of the reaction layer decreased gradually. The interface reaction thickness of the alloys with Nb content of 6, 7, 8at% were 60, 34 and 26 μm, respectively. Clearly, the addition of 8at% Nb to Ti-45 Al is the best for the thickness of the reaction layer. The addition of Nb would form a Nb-rich film in the reaction layer, which could reduce the solubility of oxygen in the interface, and suppress further diffusion of oxygen to the matrix. If the same content of Mn, Si, Y, or B alloying elements were added respectively to Ti-45Al-8Nb, the thickness of the interface reaction layer from large to small was as follows: Mn>Si>Y>B. The interface reaction thickness increased after 0.5at% Mn added, had no obvious change after 0.5at% Si addition, and decreased after adding 0.5at% Y or B. The introduced elements, which formed a protective film or/and promoted the formation of a dense aluminum oxide layer, would be of benefit to the resistance of interfacial reaction.

Reaction behaviors occurring in Ti/Al foil metallurgy

Reaction behaviors occurring in Ti/Al foil metallurgy were systematically investigated.Particular emphasis was focused on the reaction between solid Al and Ti as well as subsequent reaction between TiAland Ti layer.In the solid reaction between Al and Ti,the presence of residual Al is mainly caused by inhomogeneous growth of TiAllayer and micro-voids existing at the interface.However,through reaction between molten Al and Ti,TiAl/Ti multilayer can be achieved with complete consumption of Al.During subsequent high-temperature heat treatment,TiAl/Ti multilayer will eventually turn into TiAl/TiAl multilayer accompanying with simultaneous formation and successive disappearance of intermediate phases,such as TiAland TiAl.Moreover,it is found that the growth direction of TiAl layer changes as a function of annealing time between different couples in multi-intermetallics system.

Advances in phase relationship for high Nb-containing TiAl alloys

γ-TiAl alloys,including two categories（the conventional TiAl and the high Nb-containing TiAl（high Nb-TiAl））,are technologically intriguing because of their applications at high temperatures.Specifically,the service temperature of the high Nb-TiAl alloys is 60-100℃higher than that of conventional TiAl alloys.Recently developed TiAl alloys,for example TNB,TNM,β-γ alloys,belong to the high Nb-TiAl alloys,displaying similar behavior in phase transformation,strengthening,oxidation at high temperatures,and relationships between composition,microstructure,and mechanical properties.This work presents an in-depth review of the high Nb-TiAl alloys regarding the advances in phase diagram,formation mechanism of the new γ_1 phase,microsegregation induced by adding a high content of alloying element Nb,and the mechanism of the B2/ω phase formation.Some challenges in developing the high Nb-TiAl alloys are also discussed.

Turning machining induced microstructural stability of a high Nb-containing TiAl alloy during high temperature exposure

Turning machining induced microstructural instability was investigated in a fully lamellar Ti-45Al-8.5Nb-（W,B,Y） alloy during high temperature exposure.After turning machining followed by thermal exposure at900 or 1000℃ for 100,300 and 500 h,a depth-dependent gradient microstructure with random orientations was produced in the region close to the machining surface.Two typical layers,a fine-grained（FG） layer with equiaxed grains and a coarse-grained（CG） layer with elongated grains,are formed in this region in transversal direction.The thickness of the two layers is up to 120 urn after thermal exposure at 1000℃ for 500 h,which is less than the depth of the hardened region（200 μm） after turning machining.Most of the new grains in FG and CG layers are constituted of γ single phase,while short α_2 segments and few B2 particles are precipitated at the γ/γ interface or inside the γ grains.Recrystallization and phase boundary bulging are found to be the major mechanisms responsible for lamellar degradation in FG layer and CG layer,respectively.The residual deformation energy stored is considered to be the main driving force of this process.

Gelcasting of titanium hydride to fabricate low-cost titanium

In this work, low-cost titanium was fabricated by gelcasting of titanium hydride powder. The effects of morphology and grain composition of powder raw material and solid loading on the rheological behavior of gelcasting slurry were studied. The degreasing, dehydriding and sintering behaviors of gelcasted green body were investigated by differential thermal analysis （DTA） and dilatometer. The results show that the solid loading of titanium hydride slurry reaches 50 vol%. Combination of dehydriding and sintering in one process accelerates the densification, and the relative sintered density of the final part achieves 96.5 %. In order to test the ability of gelcasting process for fabricating structural materials, a resin handle produced by 3D printing technology was used as a model and a titanium handle was successfully fabricated. Higher solid loading and better sinterability of titanium hydride powder promote manufacture of bulk titanium with high relative density, complex shape and well-defined microstructure.

Growth rate and composition of directionally solidified intermetallic TiAl–Nb alloys with different solidification conditions

Intermetallic Ti-xAl-8Nb（x = 41,43,45,47,49;at%） alloys were solidified unidirectionally upwards with a constant temperature gradient of G=3.8 K·mm^（-1）at wide range of growth rates of v=10-400 μm·s^（-1）using a Bridgman directional solidification（DS） furnace.Microstructural parameters including the primary dendrite arm spacing（λ_1）,secondary dendrite arm spacing（λ_2）,dendrite tip radius（R） and mushy zone depth（d） were measured statistically.The values of λ_1,λ_2,R and d decrease as the growth rate increases for a given composition（x）.The values of λ_1,λ_2,R and v increase with the increase in x value,while the value of d firstly increases and then decreases with the increase in x value for a given v.The relationships between λ_1,λ_2 and R were analyzed by the linear regression.The average growth rate exponent of λ_1 is 0.29,which is in accordance with the previous experimental observations,and that of λ_2 is close to the previous experimental results,while those of R and d are lower than the results in other alloy systems.In addition,theoretical models for λ_1,λ_2 and R were compared with the experimental observations,and a comparison of the present experimental results with the theoretical models and previous experimental results was also made.

Microstructure and microsegregation of directionally solidified Ti–45Al–8Nb alloy with different solidification rates

Microstructure and microsegregation of directionally solidified Ti-45Al-8Nb alloy were investigated by scanning electron microscopy（SEM）,transmission electron microscopy（TEM） and electron probe microanalyzer（EPMA）.For the alloy solidified at the solidification rates（v） ranging from 10 to 400 μm·s^（-1）,the microstructure of the mushy zone exhibits a cellular-dendritic structure at lower growth rate（v=10-50 μm·s^（-1）） and a typical dendritic morphology at higher growth rate（v = 100-400 μm·s^（-1））.The relationship between primary dendrite arm spacing（λ_1）and v is λ_1=1.08×10~3v^（-0.35）.Al and Nb elements segregate at interdendritic zone and in the dendritic core,respectively.In solid of mushy zone,a relatively flat concentration profile is observed for the typical dendrite structure,and Nb enriches in B2 phase induced by β→α＋βtransformation.The content of B2 phase is hardly affected by v.The extent of microsegregation in steady-state zone decreases at a lower growth rate because holding the samples at higher temperature after solidification for a long time can homogenize the solid effectively.