Powder Metallurgy TiAl Research Articles

The Effect of Heat Treatment on the Microstructure and Mechanical Properties of Powder Metallurgy Ti-48Al Alloy

Heat treatment is the critical step in achieving a refined microstructure and enhanced mechanical properties of TiAl-based alloys. This study investigated the influence of heat treatment temperature, cooling method, and heat treatment time on the microstructure and mechanical properties of an extruded powder metallurgy Ti-48Al alloy, and achieved the control of fully lamellar fine microstructures and the enhancement of performance through a simple heat treatment, rather than the traditional approach of homogenization followed by heat treatment. The results indicate that the heat treatment temperature determines the type of microstructure, while the cooling rate dictates the lamellar width. As the heat treatment temperature was increased from the two-phase region to the α single-phase region, the microstructure transitioned from duplex to near lamellar, and the alloy strength initially increased and then decreased, influenced by both the lamellar colony ratio and grain size. A rapid cooling rate (water quenching) induces a non-diffusive massive phase transformation, whereas a slow cooling rate (air cooling) gradually forms α2/γ lamellar colonies. Therefore, a suitable heat treatment regime for the powder metallurgy Ti-48Al alloy was determined to be 1340 °C/5 min/air cooling. The microstructure of the alloy was near lamellar, consisting of lamellar colonies approximately 50 μm and a small number of γ equiaxed grains of about 10 μm. Subsequently, the alloy exhibited a room temperature tensile strength of 784 MPa and a yield strength of 763 MPa, representing improvements of 17.0% and 38.7% over the extruded alloy, respectively. This research provides a reference for establishing a heat treatment process for powder metallurgy TiAl alloys.

Cyclic-oxidation behaviours of the powder-metallurgy TiAl–4Nb–3Mn and TiAl–2Nb–2Mo beta-gamma alloys

ABSTRACTFor this study, cyclic-oxidation tests for the powder-metallurgy TiAl–4Nb–3Mn and TiAl–2Nb–2Mo alloys were carried out in air between room temperature and 900°C, and the oxidation behaviours under cyclic oxidation were compared with those under isothermal oxidation. The morphologies and structures of the oxides are identical to those from isothermal oxidation, except for the Mn-oxide formation on the top surface in the case of the TiAl–4Nb–3Mn alloy. The growth rate of the oxides in the TiAl–2Nb–2Mo alloy is slower and more stable than that of the oxides in the TiAl–4Nb–3Mn alloy, and severe internal oxidation is also evident in the latter alloy; however, the weight gain showed a sudden drop in the TiAl–2Nb–2Mo alloy that is mainly due to the enhanced formation of titanium nitrides at the interface.

Preparation and Properties of Ti-47Al-2Cr-2Nb-0.15B Alloy by Powder Metallurgy Route

Ti-47Al-2Cr-2Nb-0.15B alloy is a typical γ-TiAl alloy, and powder metallurgy (PM) as a near-net shape method was used to prepare it in this article. Clean pre-alloyed powders were prepared by argon gas atomization, and TiAl alloy was prepared by hot isostatic pressing (HIP) at 1150 °C and 1230 °C. However, surface contamination is inevitable due to chemical reactions with the residual O2 in the vacuum chamber during gas atomization, or due to physical adsorption of O2 and H2O during storage of the powder at room temperature. Infrared spectrometry was used to study this process. We found that the adsorption of gases is mainly H2O. The adsorbed gas in powders would deteriorate the performance of PM alloy, so a gas protection environment is suggested. Tensile properties of PM TiAl alloy were compared with as-cast alloy. Results showed that PM TiAl alloy had better strength which also had more fine and uniform microstructure.

Effect of Container on the Microstructure and Properties of Powder Metallurgy TiAl Alloys

Pre-alloyed powder of Ti-47Al-2Cr-2Nb-0.15B was prepared by a gas atomization process and powder metallurgy (PM) γ-TiAl alloys were made through a hot isostatic pressed (HIPed) route. The atomized powders were canned in containers, degassed, sealed, and HIPed. Effect of two different canning materials (mild steel and commercial pure titanium (CP-Ti)) on the microstructure and properties of as-HIPed γ-TiAl alloy were discussed. Due to the reaction between mild steel containers and γ-TiAl at relative high temperature (over 1230 °C), the γ-TiAl matrix is contaminated. CP-Ti canned γ-TiAl showed bigger yield and fracture strength than mild steel canned TiAl. PM γ-TiAl alloy parts having complex shape could be manufactured by the near net-shape process.

Microstructural evolution of a PM TiAl alloy during heat treatment in α+γ phase field

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 α2 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→α2 and results in a microstructure with equiaxed γ and α2 grains, and no lamellar colonies are found.

Cyclic oxidation behavior of a beta gamma powder metallurgy TiAl–4Nb–3Mn alloy coated with a NiCrAlY coating

A new beta gamma TiAl–4Nb–3Mn alloy was synthesized by powder metallurgy (PM) and coated with NiCrAlY powder by Air Plasma Spray (APS). The cyclic oxidation behavior of the coated beta gamma alloy was investigated between room temperature and 1000°C for up to 800cycles. During cyclic oxidation testing, a TiN layer formed at the interface between NiCrAlY and TiAl–4Nb–3Mn and the thickness of the TiN layer increased with the number of cycles. The outward Ti diffusion also resulted in the formation of titanium oxides on the coating top surface after 630cycles. Inward diffusion of Ni led to the formation of an inner diffusion zone containing NiAlTi and a mixture of NiAlTi+TiAl. Due to the spallation of coating scales on the circumferential surface, weight loss was observed after 120cycles although both the top and bottom surfaces of the specimens remain in contact with the substrate until 800cycles. Due to inward and outward diffusion, half of the NiCrAlY coating was consumed after 800cycles suggesting the need for diffusion barrier coating at the interface between the NiCrAlY coating and the TiAl–4Nb–3Mn alloy.

Comparative assessment of microstructure and compressive behaviours of PM TiAl alloy prepared by HIP and pseudo‐HIP technology

The present work discusses conditions of comparable microstructure and compressive behaviours of powder metallurgy (PM) Ti–46Al–(Cr,Nb,W,B) (at.‐%) alloy prepared by hot isostatic pressing (HIP) and pseudo‐HIP technology respectively. A PM route based on pseudo‐HIP of alloy powder prepared by plasma rotating electrode process was used for the rapid fabrication of high quality TiAl alloys. The pseudo‐hipped alloy shows a fine grained microstructure which is significantly finer than that of the hipped alloy consolidated under similar conditions. The pseudo‐hipped alloy exhibits more excellent compression properties than that of the hipped alloy at elevated temperatures. By using the kinetic rate equation, the stress exponent n and the apparent activation energy Q are determined and discussed respectively. The influence of microstructure on the flow behaviour during the total hot compressive deformation was studied. The role of submicrocrystalline forming and dynamic recrystallisation due to mechanical twinning and grain matrix deformation will be discussed.

Microstructure and Creep of γ-TiAl Containing β-Stabilizer

Fully lamellar structures of powder metallurgy (PM), investment cast, and directionally solidified (DS) TiAl alloys containing β stabilizer were produced after stepped cool heat treatment, and interface β precipitates were formed after aging at 950°C. In addition, a columnar grain structure combined with a fully lamellar structure aligned with the load direction and interface β precipitates were formed by directional solidification and subsequent heat treatments. Creep test results of PM TiAl indicate that controlling the initial microstructures is also critical for balancing the primary and steady-state creep resistance during short and long-term tests. DS TiAl alloy exhibits a significant reduction of the primary strain and creep rate compared to polycrystalline TiAl due to the unique DS microstructure. Therefore, a DS microstructure with proper lamellar orientation and controlled interface β precipitation is the ideal if maximum time to a relatively small (<0.5%) strain is the design criterion of merit.

Microstructures and properties of powder metallurgy TiAl alloys

The microstructures and properties of powder metallurgy (PM) TiAl alloys fabricated by hot-extrusion of gas-atomized powder at different temperatures (1150, 1250 and 1400 °C) have been investigated. Microstructure of the alloy fabricated at 1150 °C consists of fine grains of γ and α 2 phases in association with coarse grains of B2 (ordered bcc) phase. Metastable ω particles of ordered hexagonal phase were observed within some B2 grains. This alloy (containing metastable B2 phase) displayed a low-temperature superplastic behavior, i.e. a tensile elongation of 310% was obtained when the alloy was tested at 800 °C under a strain rate of 2×10 −5 s −1. Microstructure of the alloy fabricated at 1250 °C consists of coarse grains of α 2 phase and fine grains of γ and α 2 phases. This alloy (containing no B2 phase) on the other hand displayed no low-temperature superplasticity, as recorded from the alloy fabricated at 1150 °C. A refined fully lamellar (FL) microstructure was successfully developed within the alloys fabricated at 1400 °C. The widths of γ lamellae are in a range between 100 and 350 nm and the widths of α 2 lamellae are in a range between 10 and 50 nm. Solute effects on the creep resistance of refined FL-TiAl alloy were critically investigated.

Effects of alloying elements on creep of TiAl alloys with a fine lamellar structure

Creep experiments were conducted on five powder-metallurgy TiAl alloys with fine grains (65–80 μm), fine lamellar spacings (0.1–0.16 μm), and different compositions [Ti–47Al (+Cr, Nb, Ta, W, Si)] at temperatures of 760°C and 815°C and stresses from 35 to 723 MPa. Results show that at a given lamellar spacing, replacing 1% Nb (atomic percent) with 1% Ta and replacing 0.2% Ta with 0.2% W induced little effect, but addition of 0.3% Si decreased the creep resistance by a factor of 3–4 under otherwise identical conditions. Field emission TEM was used to characterize the changes of microstructure and alloy element distribution before and after creep. It was found that thinning and dissolution of α 2 lamellae and continuous coarsening of γ lamellae were the main creep processes and the microalloying elements tended to segregate at lamellar interfaces, especially at ledges during creep. The effects of different alloying elements are interpreted in terms of the interaction of alloy segregants with misfit and/or misorientation dislocations at the lamellar interface. That is, the interaction retards the climb of interfacial dislocations and thus the creep process in the case of large segregants (Nb, Ta, W), but facilitates the climb and creep in the case of small segregants (Si).

Superplastic behavior of a powder metallurgy TiAl alloy with a metastable microstructure

Superplasticity in a powder metallurgy (P/M) TiAl alloy (Ti–47Al–2Cr–1Nb–1Ta) with a metastable B2 phase coexisted with a fine-grained γ+ α 2 duplex structure has been studied. Alloy samples were tested at temperatures ranging from 650 to 1100°C, and at strain rates ranging from 10 −6 to 10 −4 s −1. An elongation of over 300% was obtained at a strain rate of 2×10 −5 s −1 and at a temperature of 800°C, which is close to the ductile-to-brittle transition temperature of the alloy. This is in contrast to the prior observations of superplastic behavior of TiAl alloys in which a typical temperature of 1000°C is usually required for achieving superplasticity. It is suggested that the occurrence of low-temperature (800°C) superplasticity in the present alloy is primarily due to the presence of a metastable B2 phase in addition to a fine-grained ( α 2+ γ) duplex microstructure. The metastable B2 phase continues to decompose into fine-grained α 2 and γ phases, which promotes grain boundary sliding during superplastic deformation. The retained fine B2 grains accommodate the sliding strains to reduce the propensity of cavitation at grain triple junctions and thus delay the cavitation and fracture process.

The role of ledges in creep of TiAl alloys with fine lamellar structures

Creep experiments have been conducted on two powder-metallurgy TiAl alloys with fine grains, fine lamellar structures, and compositions of Ti–47Al+2Cr+2Nb and Ti–47Al+2Cr+1Nb+1Ta. Results show that the stress exponent n and the activation energy Q decrease with decreasing stress ( n∼7–9 to 1.5, Q∼300 kJ/mol to ∼150 kJ/mol). Stress reduction led to the occurrence of negative, zero and positive creep at low stresses but only positive creep at high stresses. Thinning and dissolving of α 2 lamellae and continuous coarsening of γ lamellae were observed at both low and high stresses with cross-twinning being a microstructure exclusive at high stresses. To understand the underlying deformation processes, a ledge creep mechanism is proposed. The mechanism involves bunching and escaping of unit-height ledges into or from multiple-height ledges at interfacial boundaries ( α 2/ γ and γ/ γ). It is assumed that the ledge motion, which is diffusion-controlled, causes phase transformation and thus deformation of the sample. Using this mechanism, the experimental results including the stress exponents, activation energies and transient creep behaviors after stress reduction at different stresses are reasonably interpreted.