Phase transformations in some TiAl-based alloys

Samples of a range of TiAl-based alloys have been cooled directly to room temperature at rates between 0.1 and 500 °C s⁻¹ in order to define the transformation behaviour during continuous cooling (CCT). In addition other samples have been cooled rapidly to predetermined temperatures where they have been held for times up to 18,000 s before cooling rapidly to room temperature in order to determine their time-temperature-transformation (TTT) behaviour. It has been found that the massive transformation occurs at the highest cooling rates used (500 °C s⁻¹) in all the alloys studied apart from Ti–44Al–4Nb–4Zr–0.2Si–1B. In this alloy the high-temperature beta phase partially transformed during rapid cooling to lenticular alpha which, together with the remaining beta, was retained at room temperature. The effects of holding at selected temperatures were as anticipated from the CCT curves and the equilibrium diagrams. In all cases the room temperature tensile properties were improved for the finest microstructures—i.e. for the fastest cooling rates used, although with alloys with B addition (i.e. grain-refined alloys) the effect of cooling rate was less important. The changes in microstructure and changes in the tensile properties and hardness of samples which have been tempered after quenching have also been determined. Appropriate tempering of samples which had been cooled at a rate which caused them to transform massively gives rise to fine microstructures of intimately mixed equilibrium phases. In the case of Ti–48Al–2Nb–2Cr this leads to a mixture of convoluted alpha and gamma grains of about 50 μm (even although it contains no B and is therefore not grain-refined) and to a plastic elongation of 1.3% which is significantly better than the 0.5% found in coarse-grained air-cooled or furnace-cooled samples of this alloy.     

The effect of boron and alpha grain size on the massive transformation in Ti–46Al–8Nb–xB alloys

Samples of Ti–46Al–8Nb containing up to 1 at.% B have been examined using optical microscopy after cooling over a wide range of cooling rates from the α phase field in order to understand the influence of boron and grain size on the massive transformation. The grain size of the samples was controlled either by varying the boron level or by appropriate processing of B-free and B-containing alloys. The results show that the addition of boron suppresses the feathery and the Widmanstätten transformation. The massive transformation and the lamellar transformation are strongly influenced by prior α grain size independent of whether the grain size was achieved by heat treatment or by addition of boron. In fine-grained samples the range of cooling rates over which the massive transformation occurs is restricted by formation of the lamellar microstructure at high cooling rates. These observations are discussed in terms of the factors controlling the nucleation and the progression of these transformations.     

Shape-Memory Wires

In wire made from high-temperature shape-memory alloys, partial substitution of palladium for nickel in NiTi increases the martensite to austenite transformation temperature, while minor boron additions increase the tensile ductility and formability. By heating deformed wires, transformation forces have been measured as a function of temperature. Further, the influence of variations in titanium level about stoichiometry on shape-memory behavior and tensile properties has also been determined.     

Effect of boron addition on tensile ductility in lamellar TiAl alloys

Various cast or wrought fully lamellar TiAl-based alloys with and without boron addition have been assessed. It has been found that titanium boride precipitates are the predominant factor influencing the room temperature tensile ductility. Large sized titanium boride precipitates often observed in high-alloyed TiAl alloys (such as Ti–44Al–8Nb–1B) cause premature failure in as-cast samples through promoting crack propagation via debonding between boride-matrix interfaces or cracking through boride precipitates themselves, giving rise to a typical tensile ductility of 0.3%. Refinement in titanium boride precipitates, via hot working or fast cooling during casting, will significantly improve the tensile ductility. In low-alloyed alloys (such as Ti–48Al–2Cr–2Nb–1B) the effect of boride precipitates is not as significant as it is in the high-alloyed alloys mainly because of their small sizes.     

Design optimization of cast Cu-Al-Be-B alloys for high clamping capacity

This paper investigated high-damping Cu-Al-Be-B cast alloys using metallographic analysis, X-ray diffraction (XRD) and electrical resistance measurements for transformation temperatures. The results showed that beryllium can stabilize β phase, resulting in a thermo-elastic martensite microstructure leading to high-damping capacity in cast Cu-Al-BeB alloys. Trace additions of boron to Cu-Al-Be alloys can significantly refine the grains, providing high strength and ductility to the alloys. A factorial design of experiment method was used to optimize the composition and properties of cast Cu-Al-BeB alloys. The optimal microstructure for thermo-elastic martensite can be obtained by adjusting the amounts of aluminum and beryllium to eutectoid or pseudo-eutectoid compositions. An optimized cast Cu-Al-Be-B alloy was developed to provide excellent mechanical properties, tensile strength σb = 767 MPa, elongation δ = 7.62 %, and damping capacity S. D.C =18.70%.     

热处理对压铸Mg-8Gd-3Y-0.5Zr合金组织性能的影响

采用气体保护法制备Mg-8Gd-3Y-0.5Zr(GW83K)合金,并冷模压铸成拉伸试样。通过光学显微镜、扫描电镜观察及力学性能测试等分析合金压铸态和不同热处理状态下的显微组织及力学性能。结果表明:冷模压铸GW83K合金经热处理后,其力学性能较压铸态均有所提高,尤其是经低温短时固溶处理(T4)后的合金,其晶粒度变化不大,组织比较均匀,片层状的共晶体消失,第二相以不连续的棒状或粒状分布于晶界处。GW83K-T4合金的室温拉伸性能可达到σb=261.7MPa,σs=240.8MPa,δ5=6.0%,比压铸态合金分别提高了21%,28.4%和30.4%,且该合金具有较好高温力学性能。     

Microstructure and technological plasticity of cast intermetallic alloys on the basis of γ-TiAl

This work is devoted to the estimation of the technological plasticity of binary and alloyed γ titanium aluminides by conducting compression tests at T = 1000°C. The technological plasticity was shown to grow with decreasing size of grains and grain colonies and with increasing amount of β-stabilizing elements in the alloys. The best technological properties are characteristic of the alloys that solidify completely through the β phase, containing β-stabilizing additions of niobium and molybdenum and microadditions of boron. These alloys are characterized by a small size of crystallites in the cast state; the use of special heat treatments makes it possible to substantially decrease the fraction of the lamellar component and to increase the content of the β(B2) phase in them. For the most technological alloy, tensile tests in the cast state have been carried out. In the temperature range of T = 900–1100°C, superplastic elongations have been achieved.     

Microstructure and high temperature tensile properties of Al–Si–Cu–Mn–Fe alloys prepared by semi-solid thixoforming

The differences in the microstructure and elevated temperature tensile properties of gravity die cast, squeeze cast, and semi-solid thixoformed Al–Si–Cu–Mn–Fe alloys after thermal exposure at 300 °C were discussed. The results demonstrate that the elevated temperature tensile properties of semi-solid thixoformed alloys were significantly higher than those of gravity die cast and squeeze cast alloys, especially after thermal exposure for 100 h. The ultimate tensile strength (UTS) of semi-solid thixoformed alloys after thermal exposure at 300 °C for 0.5, 10 and 100 h were 181, 122 and 110 MPa, respectively. The UTS values of semi-solid thixoformed alloys were higher than those of heat resistant aluminum alloys used in commercial applications. The enhanced elevated temperature tensile properties of semi-solid thixoformed experimental alloys after thermal exposure can be attributed to the combined reinforcement of precipitation strengthening and grain boundary strengthening due to thermally stable intermetallic phases as well as suitable grain size.     

Notch toughness in hot-rolled low carbon steel wire rod

Charpy V-notch toughness has been investigated in four hot-rolled, low carbon steels with different grain sizes and carbon contents between 0.019 and 0.057%. The raw material was wire rod designed for drawing and possible subsequent cold heading operations and manufactured from continuous cast billets. In this study, the influence of microstructure, mechanical properties, and alloying elements on the ductile-brittle transition behavior has been assessed. A particular emphasis has been given to the influence of boron with contents up to 0.0097%. As a result, transition temperatures between −29 and +50°C explicated by the material properties have been obtained. The examination also shows that the transition temperature raises with circa 0.5°C for each added ppm boron most likely as a consequence of an enlargement of the ferrite grain size and the reduction of yield and tensile strength. The highest upper shelf energy and lowest transition temperature can be observed in a steel without boron additions and with maximum contents of carbon, silicon, and manganese.     

Role of boron in TiAl alloy development: a review

Boron was found to be a unique grain refiner in cast TiAl alloys in the beginning of 1990 s and has become an element in most of the TiAl alloys developed to date.Over the past 25 or so years,efforts to understand the role of boron in solidification,solid-phase transformation,thermal and thermomechanical processing and mechanical properties of TiAl alloys and the relevant mechanisms never ceased.As a result,abundant knowledge on boron in TiAl alloys has been accumulated but scattered in various research papers and conference proceedings.This review summarises the progress in understanding boron and its impacts on the TiAl alloy systems.     

The influence of thermal processing route on the microstructure of some TiAl-based alloys

The influence of a range of different continuous cooling rates from the alpha phase field has been investigated in large-grain-sized (∼1500 μm) Ti48Al2Cr2Nb and in grain-refined Ti48Al2Cr2Nb1B alloys. In addition the microstructure of direct laser-fabricated samples of Ti48Al2Nb2Mn has been examined in order to assess the influence of the complex heating cycle and high cooling rate on this alloy. It has been found that the critical cooling rates to obtain a particular structure are higher in grain-refined Ti48Al2Cr2Nb1B than in the alloy without boron (B). The increase in the amount of the lamellar structure during cooling takes place in different manners in the two alloys. The formation of the lamellar structure in laser-treated samples has been found to be very different with the formation of Al-rich gamma regions separating regions with a lamellar structure. The possible factors affecting phase transformations are discussed in terms of the role of grain boundaries and B in solution in the solid state transformations and, in the case of the laser-treated samples, of melting and remelting.     

Effect of composition on grain refinement in TiAl-based alloys

Grain refinement through boron addition has been investigated in a range of cast TiAl-based alloys. The alloys have 44–50 at.% Al, up to 8 at.% alloying elements, and 1 at.% boron. The observed grain size ranges from 40 to 300 μm. The grain size in as-cast TiAl-based alloys is strongly composition dependent with aluminium having the strongest effect. Decreasing Al concentration from 50 to 44% the grain size can be reduced to 50 from 300 μm with similar casting conditions. Alloying element species and concentration also have strong effect on grain size. The strong boride formers like W, Ta and Nb increase the grain size and change the prevalent titanium boride form from TiB₂ to TiB. It is elucidated that the compositional effect on the grain size could be through affecting the local boron concentration in the solidification front during casting.     

Microstructure and tensile properties of cast Ti-44Al-4Nb-4Hf-0.1Si-0.1B alloy with refined lamellar microstructures

The tensile properties of a cast Ti-44Al-4Nb-4Hf-0.1Si-0.1B alloy with fine lamellar structures have been measured at room temperature. The average lamellar colony size was in the range of 70–130 μm, depending on heat treatment condition. All the tensile ductilities measured at room temperature are below 0.45% with the majority between 0.3 and 0.4%. Microstructural examination and fractographic examination revealed that there are many lamellar colonies with their lamellar interfaces parallel to each other, in contrast to observations on an extruded sample, which showed 1% ductility. Cleavage along the lamellar interfaces in those parallel colonies in the cast samples forms clusters of small parallel cracks close to each other and their collective effect, together with that from the debonding between boride ribbons and the metal matrix, may contribute to the low ductility in the cast alloy. Analysis shows that those parallel lamellar colonies probably originate from the Burgers alpha grains (obeying Burgers OR with the beta phase) formed during the beta-to-alpha solid phase transformation.     

Grain refinement in small-size ingots of intermetallic alloys Ti-46Al-8Nb and Ti-46Al-8Ta with the use of a massive transformation

An estimation of the efficiency of the refinement of colonies/grains of the as-cast structure of ingots of size Ø13 × 150 mm in the Ti-46Al-8Nb and Ti-46Al-8Ta (at %) alloys with the aid of a heat treatment including massive transformation has been performed. It is shown that the initial coarse-grained as-cast structure of these alloys can be refined using massive transformation without the use of a labor-consuming procedure of hot working. The method proposed is efficient for the alloys containing alloying elements that retard diffusion in the alloys, which makes it possible to obtain massive γ _m phase at moderate cooling rates. It is shown by X-ray diffraction that the massive γ _m phase is characterized by a reduced parameter of tetragonality, which can be restored via subsequent high-temperature aging.     

Microstructures and tensile properties of massively transformed and aged Ti46Al8Nb and Ti46Al8Ta alloys

Fully massively transformed samples of Ti46Al8Nb and Ti46Al8Ta have been HIPped (hot isostatically pressed) in the (α + γ) phase field in order to generate a fine convoluted microstructure and their tensile properties compared with those of samples with coarse lamellar microstructure. It has been found that the yield strengths and ductilities of the microstructurally refined samples were significantly improved with respect to those with coarse microstructures. The proof stresses of the microstructurally refined samples of the Ta- and Nb-containing alloys were almost identical (550 MPa) but the ductility in the Ta-containing alloy is about double that of the Nb-containing alloy, reaching values of up to 1.1% plastic strain. However, there is significant scatter in the elongation in both alloys, which in the case of the Ta-containing alloy has been shown to be associated with segregation which hinders the massive transformation and leads to the formation of some large grains. These observations are discussed in terms of the practicality of using massively transformed and heat-treated cast alloys in engineering components and in terms of the factors controlling the tensile behaviour.     

Phase transformations in powder iron-nickel alloys produced by mechanical alloying

Phase transformations in Fe-Ni alloys produced by mechanical alloying have been investigated by X-ray diffraction, Mössbauer spectroscopy, and magnetometric methods. The critical temperatures of phase transformations during heating and cooling of the alloys in the concentration range of 12–26 at % Ni have been determined, and the intervals of existence of phases at room temperature have been established. In the alloys that contain less than 16 at % Ni, a “normal” (diffusion-controlled) γ-α transformation was observed. In some alloys, an isothermal γ-α₂ martensitic transformation has been revealed, which was not earlier observed in cast Fe-Ni alloys. A considerable reduction in the start temperatures of the martensitic transformation in comparison with the cast alloys of the same nominal compositions has been found. Factors are considered that are responsible for the observed values of the temperatures of diffusionless transformations in the mechanically alloyed materials.     

On the massive phase transformation regime in TiAl alloys: The alloying effect on massive/lamellar competition

The influence of alloy composition on the solid-state transformations which occur during continuous cooling of TiAl-based alloys has been studied with the aim of defining the alloy compositions which most strongly influence the massive transformation. It has been found that heavy elements such as Nb and Ta decrease the cooling rates required to form massive gamma by suppressing the formation of feathery and lamellar structures to even lower cooling rates. It is proposed that the low diffusivity of such heavy elements retards these diffusion-controlled phase transformations (feathery and lamellar) so that the massive transformation, which does not require diffusion, but which occurs at a lower temperature can take place. The significance of the selection of alloying elements, which allow the formation of massive gamma, is briefly discussed in terms of the refinement of the microstructures of castings by heat treating massively transformed samples.     

Deformation and fracture of single crystals of VKNA-type alloys at 1100–1250°C

A comparative study of the structure and mechanical properties of single-crystal samples 〈001〉 of VKNA-1V and VKNA-4U alloys (cast and after heat treatment) has been performed using tensile tests in a temperature range of 1100–1250°C. In the case of the VKNA-4U alloy, samples of different crystallographic orientations have also been tested. At 1100°C, recrystallization was observed near the zone of fracture. The basic mechanism of relaxation at 1200–1250°C was found to be dynamic recovery. At 1250°C, the strength properties of the VKNA-4U alloy are higher than those of the VKNA-1V alloy.     

Microstructure and hot deformation behavior of two-phase boron-modified titanium alloy VT8

The results of studying the microstructure, compressive mechanical characteristics, and evolution of the microstructure of the cast two-phase titanium alloys VT8-xB (with x = 0, 0.2, and 0.4 wt %) during hot deformation are presented. Alloying with boron leads to the formation of borides, which are predominantly located at the boundaries of β grains and give rise to a considerable refinement of the original cast structure. The results of high-temperature compressive tests have shown that the strength characteristics of the boron-modified alloys noticeably exceed those of the base VT8 alloy. The experiments on the multiple isothermal forging of VT8 and VT8-0.4B alloys carried out at temperatures of 700–650°C have revealed the acceleration of the kinetics of dynamic recrystallization due to the presence of borides, which were efficiently refined during deformation. The alloying of the VT8 alloy with boron has been discussed from the viewpoint of improving the efficiency of the deformation treatment and final service properties of two-phase titanium alloys.     

Effect of Grain-Refined on Primary α Phase in Semi-Solid A356 Alloy Prepared by low superheat Pouring and Slightil Electromagnetic Stirring

The semi-solid slurry of A356 alloy, which is grain-refined by Al-Ti-B master alloy, is prepared by low superheat pouring and slight electromagnetic stirring. The effects of grain refining on the morphology and the grain size of the primary α phase in the slurry manufactured are researched. The results indicate that the slurry with particle-like and rosette-like primary α phases can be prepared by low superheat pouring and slight electromagnetic stirring from liquid A356 alloy grain-refined, in which the pouring temperature can be suitably raised. Compared with the A356 samples without grain refining, the grain size and particle morphology of primary α phase as well as the distribution of the grain with particle-like or rosette-like along radial in the ingot in A356 are markedly improved by grain refining.