High Temperature Titanium Alloy Research Articles

Simple formula learned via machine learning for creep rupture life prediction of high-temperature titanium alloys

Simple formula learned via machine learning for creep rupture life prediction of high-temperature titanium alloys

Effects of pre-tension on the microstructural stability and mechanical properties of a near-alpha high temperature titanium alloy

Effects of pre-tension on the microstructural stability and mechanical properties of a near-alpha high temperature titanium alloy

Unique grain refinement mechanism of graphene oxide reinforced high-temperature titanium alloy matrix composite

Grain refinement is an effective way to improve the comprehensive mechanical properties of titanium matrix composites. In this work, the strong grain refinement effect of GO on 600 ℃ high-temperature titanium alloy prepared by hot isostatic pressing was found. The refinement mechanism was studied by experimental analysis, first-principles calculation and cellular automaton simulation. The α grain size of the composite with 0.50 wt% GO decreased by 53.2 % compared with the matrix alloy. The direct effect of GO on grain refinement was weak. GO introduced C and O as α-stable elements to increase α phase content, resulting in the solid solubility of Si decreasing and fine (TiZr)6Si3 particles precipitating. (TiZr)6Si3 particles prevented dislocation from moving to accelerate continuous dynamic recrystallization nucleation and pinned grain boundary to inhibit grain growth. The results of cellular automaton simulation show that the inhibitory efficiency of (TiZr)6Si3 on grain growth was 4.7 times that of GO. The unique mechanism that GO promoted silicide particles precipitation and indirectly refined grain provided a design route for novel titanium matrix composites synergistically strengthened by two-dimensional nanosheets, in-situ nanoparticles and grain refinement.

Cooling rate effects on microstructure and diffusion behaviour in Ti65 alloy: Insights from a modified diffusion model

In the continuous cooling process, the growth of the equiaxed α-phase grains in near-α high-temperature titanium alloys is controlled by the diffusion of alloying elements. Establishing a specific connection between the cooling rate and the diffusion behaviour of alloying elements aids in the precise prediction of the evolution of equiaxed α-phase grain size. This study meticulously controlled the cooling rate during the two-phase region annealing treatment of the Ti65 alloy. Using EPMA technology, the diffusion behaviour of solute elements during cooling was accurately characterized. The study found that slowing the cooling rate promotes the coarsening of the lamellar secondary α-phase grains and the growth of the primary equiaxed α-phase grains. At higher annealing temperatures, the growth of equiaxed α-phase grains can occur at faster cooling rates, while coarse lamellar secondary α-phase grains can be retained at slower cooling rates. The diffusion behaviour of solute elements Al, Ta, Mo, and W between the α-phase and transformed β-phase matrix is significantly influenced by the cooling rate, thus they are considered as the controlling elements for the growth of the equiaxed α-phase grains. Based on the diffusion behaviours of these controlling elements, their single-element diffusion models were categorized and integrated for predicting the grain size of the equiaxed α-phase. The predictions from the revised diffusion model show an excellent agreement with the actual results, with an error margin of about 5%.

Evolution of microstructure and mechanical properties of Ti65 high-temperature titanium alloy after additive manufacturing and annealing

Ti65 was prepared by Laser Direct Energy Deposition (LDED) and subjected to Annealing Heat Treatment (AHT). The microstructure and mechanical properties of different specimens were evaluated. The results indicated that the prior β columnar crystals had a strong [0001] texture along the grain growth direction. The coarsening or spheroidization of α lath was attributed to the splitting of lamellar α, subsequent non-directional growth, element migration, and Ostwald ripening. The isotropy of the specimen annealed at 950 °C was caused by trimodal microstructure, discontinuous grain boundary α (αGB), precipitates dissolution, and low-angle grain boundary. A mathematical model was established to predict yield strength (YS) and microhardness based on the α lath thickness. The microhardness-strength correlation of LDED Ti65 followed an empirically derived model (Tabor's relationship). Improvements to the original models of the microhardness-strength relationship for Ti65, in conjunction with the strain-hardening effect and α orientation, made it possible to use these models to predict the strength of LDED Ti65 parts and improve the accuracy of the predicted values. Simultaneously, the model can provide data support for the industrial application of Ti65 or, where appropriate, provide a reference for other technical production (material applications).

Effects of Ta and Nb on high-temperature oxidation properties of Ti-6Al-3.5Sn-4Hf-0.4Si-X alloys

In previous studies, we identified Ta and Nb as the crucial elements affecting the high-temperature oxidation resistance of titanium alloys. To investigate their impact on microstructure evolution and oxidation resistance, we employed a constant temperature oxidation method at 700°C. The results demonstrate that Ta and Nb elements effectively inhibit oxygen diffusion and adsorption on the alloy surface, promote the formation of Al2O3 in the oxide film, and enhance its density. Notably, Ti-6Al-3.5Sn-4Hf-0.4Si-3Ta alloy exhibits exceptional antioxidation performance with an oxidation weight gain of only 0.4884 mg/cm2 after 100 hours of constant temperature oxidation at 700°C, an oxide film thickness of merely 1.047 μm, and a significant proportion (36.29 %) of O atoms combined with Al in the oxide film. The statement is in line with our previous computational findings and serves as a research foundation for the advancement of high-temperature titanium alloys.

Effect of thermal exposure on microstructure and mechanical properties of Ti65 high-temperature titanium alloy deposited by laser direct energy deposition

Industrial design and new product development benefit from the adaptable processing approach offered by laser direct energy deposition (LDED). In this paper, a novel Ti65 alloy was successfully prepared by LDED and subjected to thermal exposure at 650 °C and 750 °C, respectively. The results showed that the as-deposited microstructure varies in different regions due to the cooling rate. Silicide precipitation was observed at the α/β interface, and a coherent α2 phase was generated in the matrix after thermal exposure at 650 °C. Two-scale silicides were observed at the α/β interface and inside the α laths after thermal exposure at 750 °C. The as-deposited ultimate tensile strength (UTS) was 1058 MPa and 690 MPa at room temperature and 650 °C, respectively. After thermal exposure at 650 °C, the room temperature UTS was 1135 MPa, and the elongation (EL) at 650 °C was 27.5 %. After thermal exposure at 750 °C, the room temperature EL was 11.05 %, and both UTS and EL at 650 °C showed a decrease. The quantitative analysis showed that silicide precipitation was primarily responsible for the strengthening mechanism of the specimens thermally exposed at 650 °C at room temperature. The results provide scientific data for the manufacture of Ti65 alloy with excellent mechanical properties, laying the foundation for its application in aerospace.

Multi-scale dispersion strengthening for high-temperature titanium alloys: Strength preservation and softening mechanisms

The long-lasting expectation “the hotter the engine, the better” calls for the development of high-temperature metallic alloys. Although the high specific strengths of titanium alloys are compelling for such applications, their deleterious softening beyond 600 °C imposes serious limitations. Much has been known for decades regarding the phase metallurgy for precipitation strengthening design in titanium alloys, however, the other facile strength promotion mechanism, dispersion strengthening, remains comparatively less-explored and unutilized. The present research concerns the multi-scale dispersion strengthening in titanium alloys, with mechanistic emphases on the critical plasticity micro-events that affect strength preservation. Due to the simultaneous introduction of intragranular dispersoids and intergranular reinforcers, the current titanium alloys present superior engineering tensile strength of 519 MPa at 700 °C. Throughout the examined 25–800 °C temperature range, noticeable softening induced by the thermal activation occurs above 600 °C, accompanied by evident strength loss. The temperature-dependence transition of dominated softening mechanisms from dynamic recovery to dynamic recrystallization has been clarified by theoretical calculations. Furthermore, the strengthening effect of multi-scale architectures is underpinned as the enhanced dislocation strengthening owing to the introduction of thermally-stable heterointerfaces, which could generically guide the design of similar heat-resistant titanium alloys.

High-temperature “Inverse” Hall-Petch relationship and fracture behavior of TA15 alloy

The Hall-Petch relationship is important for material design at room temperature. However, it is not well studied at high temperatures. In this work, the influence of different microstructures on the high-temperature mechanical behavior and corresponding softening mechanisms of high-temperature titanium alloy TA15 (Ti-6.5Al-2Zr-1Mo-1 V, wt.%) were studied. Specimens with duplex, Widmanstätten, and coarse Widmanstätten microstructures were obtained through powder metallurgy. High-temperature tensile tests were carried out between 500 and 650°C. It was found that the fracture mode of the Widmanstätten microstructure was transgranular at 500 °C and 550 °C, but intergranular at 600 °C and 650 °C. EBSD analysis revealed that the high-temperature deformation was facilitated by several mechanisms including GB softening, GB migration, grains rotation, and activation of multiple slip systems. High temperature nanoindentation indicated sofenting of individual grains with increasing temperature. In-situ tensile testing under SEM revealed deformation was primarily in grain interior at room temperature, and GBs played a significant role at 650℃. The GB behavior at high temperatures is believed responsible for the inverse Hall-Petch relationship. These findings provide a new perspective for improving the high-temperature mechanical properties and microstructure control of titanium and its alloys.

The mechanism for annealing-induced ductile to brittle transition in a high-temperature titanium alloy and its mitigation

By investigating the relation between microstructure evolution and tensile responses of a BTi6431S alloy at room temperature and 700 °C, this study finds an annealing-induced ductile-to-brittle transition phenomenon in the near-α titanium alloy. The high strength of the as-received BTi6431S alloy is rooted in the high dislocation density and collective dislocation behavior in the α phase. However, after annealing at 650 °C, the alloy exhibits brittle fracture at room temperature. Using multiscale characterization methods, we attribute this phenomenon to the presence of brittle precipitates and the change of dislocation structures near the surface of BTi6431S titanium alloy. Silicides are generated in the vicinity of the surface region due to the relatively high dislocation density on the surface after the rolling process. The room temperature ductility can be restored by removing the near-surface area. When the alloy is tested at 700 °C, most of the near-surface brittle silicides and dislocations can be eliminated, because the deformation is mainly affected by dislocation behavior and dynamic recrystallization at high temperature. The findings in this study can advance the understanding of alloying effects on the mechanical behavior of high-temperature titanium alloy. The results suggest that inhibiting silicide formation and controlling the environmental temperature during service are potential approaches for maintaining the mechanical performance of this near-α titanium alloy.

Insights into microstructural stability and embrittlement of TC25 high temperature titanium alloy subjected to thermal exposure

High temperature titanium alloys are being urgently developed owing to the requirements of advanced aerospace industry. Performance of the alloys in service under high-temperature environments always attracts great attention. In this study, variations of microstructure and mechanical property in TC25 (Ti-6.5Al-2Mo-1Zr-1Sn-1 W-0.2Si) dual-phase high temperature titanium alloy subjected to thermal exposure have been systematically investigated by coupling microstructural characterization, compositional analyses and mechanical measurement. It is found that strength is significantly enhanced while ductility is sharply degraded when the alloy is thermally exposed at 550 °C for a long duration of 100 h. Microstructural observations manifest that ternary α-needles (αt) are drastically precipitated from β-ligaments inside transformed β-matrix (βtrans) whilst α2 (Ti3Al)-phase is produced from primary α-phase (αp) through ordered transformation. The precipitation of αt-needles is promoted by β-phase metastability resulting from insufficient element partitioning in the initial microstructure. The β-phase stability also renders αt-precipitation encounter size effect of β-ligaments, where the precipitation is suppressed when β-ligaments are thin in size. Both precipitation of αt-needles and α2-phase triggers precipitation-hardening effect, which results in strain localization and thereby regresses alloy ductility. However, further analysis indicates that compared with α2-phase, αt-precipitation plays more adverse role in embrittlement in the current alloy. This is different from thermal-exposed behavior of most of high temperature titanium alloys reported in literatures. These findings enrich our fundamental understanding on thermal stability of high temperature titanium alloys, and provide plausible implication to improve high temperature performance via controlling phase precipitation.

A novel magnetic field assisted powder arc additive manufacturing for Ti60 titanium alloy: Method, microstructure and mechanical properties

Due to the rapid heating-cooling process and unstable keyhole, pore defects are easily formed during the high energy beam-powder additive manufacturing (AM) process, posing great challenges to the high-performance manufacturing of aerospace components. Using an arc with lower energy density as a heat source can effectively avoid these defects, but its matching raw materials are limited to wire. In this paper, a novel magnetic field assisted powder arc additive manufacturing (MFA-PAAM) method has been proposed, which combines arc heat source and powder raw material to achieve dense metal parts. A transverse magnetic field (TMF) was introduced to solve the powder spattering problem which impacts the stability of the PAAM process, by regulating the direction of the arc plasma flow and the arc pressure. The unique melting behavior of the powder under arc heating was captured by a high-speed camera, which can be summarized into two steps: balling and adsorption by the molten pool. And the well-formed and fully dense Ti60 high-temperature titanium alloy wall components were successfully manufactured using the self-developed equipment. The microstructure of the thin-wall component is dominated by basket-weave distributed α laths and few β sheets remaining at their boundaries. A layered periodic distribution along the building direction was observed, which is mainly distinguished from the content and morphology of the β phase. The average ultimate tensile strength (UTS) and elongation to failure in the travelling direction are 1003.2 MPa and 15.1%, while the average UTS and elongation along the building direction are 1009.3 MPa and 16.6%, respectively. The fracture morphologies in both directions are characterized by ductile fracture. The MFA-PAAM technique shows great potential for fabricating materials that are difficult to be drawn into wires with high efficiency and performance.

Influence of pre-heating on TIG-welding thermal cycle of high-temperature titanium alloy of TI‒AL‒ZR‒SN‒MO‒NB‒SI system

Influence of pre-heating on TIG-welding thermal cycle of high-temperature titanium alloy of TI‒AL‒ZR‒SN‒MO‒NB‒SI system

Influence of quenching plus aging on microstructures and mechanical properties evolutions for Ti-Al-Sn-Zr-Mo-Si-Nb-Ta-Er-C near-α high temperature titanium alloys

The microstructures and mechanical properties of β-forged and as-heat treated high temperature titanium alloys were carefully investigated. Heat treatments with solution processes varying from 940℃～1010℃ and aging at 650℃ after water quenching were adopted. After forging, prior β grains were squashed and elongated. The content of the primary α phase decreased with increasing solution temperatures, while the content of the transformed β structures increased. After water quenching plus aging processes, super-fine secondary α lamellas with widths of 80～400 nm precipitated in transformed β structure. Secondary nano-scale silicides with a size of ～82 nm re-precipitated. More α2 phase particles with a mean size of 8.00 nm were dispersively and uniformly precipitated. After heat treatment at 1000℃, the room temperature ultimate strength and yielding strength were enhanced by 241 MPa and 228 MPa, reaching 1256 MPa and 1151 MPa, respectively. The high temperature ultimate strength and yielding strength were enhanced by 119 MPa and 128 MPa, reaching 756 MPa and 669 MPa, respectively. The high temperature elongation increased to 13.6%. The excellent mechanical properties compared to the previously studied were ascribed to the precipitation of super-fine secondary α lamellas in βt, dispersed nano-scale silicides and α2 particles, and the superior mixing proportion of Pα and βt acquired after optimization of heat treatments.

Microstructure and Mechanical Properties Evolution of High-Temperature Titanium Alloys with In Situ Synthesized TiB Whiskers

Microstructure and Mechanical Properties Evolution of High-Temperature Titanium Alloys with In Situ Synthesized TiB Whiskers

Enhancing high-temperature fretting wear resistance of TC21 titanium alloys by laser cladding self-lubricating composite coatings

Fretting wear is one of the main factors restricting the fastener service reliability for the high-temperature titanium alloys. Enhancing the high-temperature fretting wear resistance of titanium alloys with laser cladding coatings is an effective method. Thus, in this work, a self-lubricating composite coating was designed and fabricated on the surface of TC21 titanium alloy with laser cladding. Special attentions were made to the effect of the fractions of the hard and self-lubricating phases on high-temperature fretting resistance. In addition, the contribution of laser cladding power was also concerned. The primary phases of the self-lubricating composite coatings are α-Co, CaF2, WC, TiC and Cr23C6, and the reasons for the formation of TiC and Cr23C6 are investigated from a thermodynamic point. The prepared composite coatings are dense and uniform. The crystallographic characteristics and the volume fractions of the hard phases (WC, TiC and Cr23C6) and self-lubricating phase (CaF2) were revealed, and the effect of grain size on the coating properties was analyzed. The hardness of the coatings were all maintained around 700 HV0.2, which was significantly higher than that of the substrate. The composite coating significantly improves the friction and wear resistance at 500 ℃. The hard phase was also found to be the most resistant to friction and wear. The frictional performance of the coating gets better when the hard phase and the self-lubricating phase increase. However, the hard phase contributes more to the high-temperature friction and wear resistance than the self-lubricating phase and the wear volume is 1/40 that of substrate. For the laser cladding power, the higher power can lead to a higher fraction of CaF2 but a lower fraction of hard phase, causing a decline in COF but an increase in wear volume. The wear mechanisms of the coating and the substrate were analyzed, the subsurface stress distribution state of the wear scars were further analyzed. The current results present new insights on the anti-wear design strategy to the composite coatings consist of hard and self-lubricating phase.

Ignition mechanism of near α high temperature titanium alloy

The risk of titanium fire increases significantly with the development of future aero-engine, however, the burning mechanisms of titanium alloys remain uncertain. Therefore, the ignition behavior and mechanism of near α high-temperature titanium alloy are studied in this work by an integrated experiment method, including laser-oxygen concentration ignition method, infrared temperature measurement and observation of molten metal by high-speed camera. Based on this, the ignition boundary curve is determined and the ignition temperature of the alloy is found to decrease from 1595 to 1527 ℃ with the laser power increasing from 200 to 325 W and oxygen concentration increasing from 21% to 60%. The ignition microstructure is characterized by FIB and TEM to study the evolution of reaction products. Pores are found to form beneath the TiO<sub>2</sub> surface layer, which can be attributed to the instablity of TiO. The failure mechanism of protective oxide layer is further analyzed according to the thermal stress caused oxide layer damage model. When the temperature approaches the ignition temperature, which is below the melting point, the high vapor pressure of TiO leads to the formation of porous defects beneath the TiO<sub>2</sub> surface, thus accelerating the fracture and failure of the TiO<sub>2</sub> layer under thermal stress. It is revealed that critical conditions of temperature and instantaneous temperature change rate are needed to realize ignition. Based on this, an ignition model is further constructed to discuss the relationship among ignition temperature, laser power and oxgyen concentration. According to the experimental data fitting, the reaction activation energy of TA19 alloy during the ignition stage is calculated to be about 280 kJ/mol, and the function for calculating ignition temperature is given as follows: <inline-formula><tex-math id="M2">\begin{document}$ 1.2 \times {10^{10}}{{\mathrm{e}}^{\frac{{ - 280000}}{{R{T_{{\text{ig}}}}}}}}{c^{\frac{1}{2}}} + $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20240003_M2.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20240003_M2.png"/></alternatives></inline-formula><inline-formula><tex-math id="M2-1">\begin{document}$ 0.52{P_{\mathrm{L}}} - 315 = 0 $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20240003_M2-1.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20240003_M2-1.png"/></alternatives></inline-formula>. This provides a theoretical reference for predicting the ignition temperatures of near α high temperature titanium alloy and other types of titanium alloys under complex airflow conditions in aircraft engines.

Effect of thermal exposure on the microstructure and mechanical properties of Ti60 alloy

The effect of thermal exposure on the microstructure and mechanical properties of Ti60 alloy was investigated in the present study. Meanwhile, the fatigue fracture microscopic appearance characteristics at elevated temperature were analyzed compared, providing the basis to further improve the performance of the series of high temperature titanium alloy. The results show that the yield strength and tensile strength were basically stable under long-term thermal exposure below 600°C, while the elongation decreased with the increase of exposure temperature. Thermal exposure below 800°C did not change the microstructure type of Ti60 alloy, but at higher temperature local β coursing occurred at the grain boundary. When the alloy was exposed above 600°C, Si was obviously concentrated in the grain boundary region, and the maximum concentration was up to 2.5%. With the increase of thermal exposure temperature, the characteristics of high-temperature fatigue fracture of Ti60 alloy change from trans-granular toughness to intergranular brittleness.

Research Progress on the Creep Resistance of High-Temperature Titanium Alloys: A Review

High-temperature titanium alloys are one of the most important research directions in the field of high-temperature aerospace alloys. They are mainly used in high-temperature-resistant components, such as blade disks, blades, and casings of aero-engines, and are key materials in a new generation of high thrust-to-weight ratio aero-engines. In the service environment of engineering applications, the creep resistance of high-temperature titanium alloys is one of the most important characteristic indicators. This paper reviews and analyzes the research status and progress on the creep properties of typical high-temperature titanium alloys in service in recent years. The effects of the creep parameters, alloy composition, and microstructure on the creep behavior of high-temperature titanium alloys are discussed, and various possible mechanisms for increasing the creep resistance of high-temperature titanium alloys are summarized.

Improving thermal stability and creep resistance by Sc addition in near-α high-temperature titanium alloy

High-temperature titanium alloys’ thermal stability and creep resistance are significant during service in high temperatures. This study systematically investigated the thermal stability and mechanical properties of Ti-6.5A1–2.5Sn-9Zr-0.5Mo-1Nb-1W-0.3Si-xSc (x, 0–0.5 wt.%) at 650 °C. The lamellar secondary α phase is refined and the formation of Sc2O3 is increased with the increasing scandium (Sc) additions, which improves the strength of the alloy, while excessive Sc2O3 becomes the crack source and deteriorates the plasticity. The oxygen content in the matrix is reduced by the interaction between Sc and oxygen, inhibiting the growth of the Ti3Al phase and improving the thermal stability of the alloy. Meanwhile, Sc accelerates the dissolution of the residual β phase and precipitation of fine, diffusely distributed ellipsoidal silicides, which strongly prevents dislocation movement. The enhancement of creep resistance for the Sc-containing alloy is attributed to the refined lamellar secondary α phases, Sc2O3 particles, Ti3Al phase, and silicides, especially the precipitated silicides. Eventually, the 0.3Sc alloy shows optimal thermal stability (the plasticity loss rate 17.3%) and creep resistance (steady-state creep rate 4.4 × 10–7 s–1). The investigation results provide new insights into the mechanism and thermal stability improvement in high-temperature titanium alloys modified by rare earth (RE).