Enhanced nanohardness and new insights into texture evolution and phase transformation of TiAl/TiB 2 in-situ metal matrix composites prepared via selective laser melting

As a crucial branch for titanium industry, high-strength titanium alloys (HS-TAs, with UTS ≥ 1100 MPa) are indispensable structural materials for advanced engineering applications such as aerospace and marine fields. Along with the expansion of HS-TAs’ market, achieving satisfying synergies of high strength, high ductility (elongation ≥ 6%) and high toughness (KIC ≥ 50 MPa⋅m1/2) has been identified as the uppermost technical bottleneck for their research and development. To overcome the challenge, two primary strategies have been initiated by the titanium community, developing novel alloys and innovating processing technologies. For the former, a dozen of newly-developed alloys were reported to exhibit excellent strength-ductility-toughness combinations, including Ti-5553, BT22, TC21 and Ti-1300, for which the ideal mechanical performances were based on specific microstructures realized by low impurity rate (e.g. oxygen content ≤ 0.15 wt%), complicated processing and complex heat treatment. For the latter, several innovatory forging and heat treatment technologies were originated for the mature alloys to optimize their balanced property by extraordinary microstructural characteristics. In this review, we provide a comprehensive overview over the research status, processing and heat treatment technologies, phase transformation, processing-microstructure-property correlation and strengthening-toughening mechanism of HS-TAs for aerospace engineering applications manufactured via melting-forging process. Finally, the prospects and recommendations for further investigation and development are proposed based on this review. 

High-strength titanium alloys for aerospace engineering applications: A review on melting-forging process

The interior porous defects formed during the layer-by-layer fabrication process have attracted increasing attention for different additive manufacturing (AM) techniques and are regarded as a crucial factor affecting the overall performance. In this work, aiming at the cold spray Ti6Al4V bulk materials, the hot isostatic pressing (HIP) treatment is adopted to reduce the interior defects, adjust the microstructure, and improve the mechanical properties. To characterize the pore morphologies and porosity evolution, the CS Ti6Al4V sample is characterized by optical microscopy and X-ray computed tomography (XCT). The 3D reconstructions by XCT show that the fully dense Ti6Al4V alloy can be obtained through the high temperature diffusion and high pressure compacting of the HIP sample. After the HIP treatment, the severely deformed grains experience an obvious growth with the uniformly distributed β precipitates around equiaxed α grains. The tensile test shows that the strength of CS Ti6Al4V alloys can be largely improved by the enhanced diffusion and resultant metallurgical bonding. With the HIP treatment, the CS samples exhibit highly densified morphology and adjusted microstructure that can benefit the improvement of mechanical properties.

Effect of hot isostatic pressing (HIP) on microstructure and mechanical properties of Ti6Al4V alloy fabricated by cold spray additive manufacturing

Metastable β titanium alloys are widely used in the biomedical, automotive, and aerospace industry, due to their excellent corrosion resistance, fatigue strength, biocompatibility, and easy formability. Besides all these use areas, the suitable microstructure of the alloys for heat treatment increases the efficient usability day by day. In literature research, it has been found that heat treatment types such as cryogenic treatment and precipitation hardening can be applied efficiently to the alloys. Optimum strength/ductility, wear resistance, creep strength, and fatigue strength can be obtained with these heat treatments. For this reason, it has become important to understand the effect of heat treatments on the microstructural and mechanical properties of the alloys and parameters affecting this heat treatment efficiency. Precipitation hardening includes solution and aging treatment steps. The solution treatment can be applied at temperatures below and above the β transition temperature. While the aging treatment can be applied in four different ways, in the review article, the effects of single step and duplex aging treatment, which are applied with high efficiency, are emphasized. Precipitation hardening efficiency affects the chemical composition of the alloys, heat treatment steps, treatment temperature and times, and heating/cooling rate. The cryogenic treatment provides the formation of martensite α phases in metastable β titanium alloys cooled below the martensitic transformation temperature. Higher-strength and hardness have been obtained in the studies where aging treatment was applied after cryogenic treatment. Cryogenic treatment efficiency determines chemical composition of the alloys, treatment temperature and time, the heating/cooling rates, and heat treatments applied after cryogenic treatment.

A review on heat treatment efficiency in metastable β titanium alloys: the role of treatment process and parameters

Experimental study and numerical simulation of dynamic recrystallization behavior of TiAl-based alloy

Hot pack rolling nearly lamellar Ti-44Al-8Nb-(W, B, Y) alloy with different rolling reductions: Lamellar colonies evolution and tensile properties

Hot deformation behavior and dynamic recrystallization (DRX) of a β-solidifying TiAl alloy Ti–43Al–2Cr–2Mn–0.2Y (at%) were investigated by means of isothermal compression tests in a temperature range of 1100–1225 °C and strain rate range of 0.01–0.05 s−1. A processing map at a true strain 0.8 was developed based on the dynamic materials model. The current alloy possesses a wide processing window, and the optimum deformation condition is 1175–1225 °C/0.05 s−1. The main softening mechanism is DRX of γ grains. The DRX proceeded preferentially at triangle boundaries, and subsequently in the lamellae. Both deformation temperature and strain rate have an obvious impact on the microstructural evolution. The partial DRX occurred when the alloy deformed at low temperature and high strain rate. Some remnant lamellar structures existed due to the plastic anisotropy of lamellar colony. With the increase of temperature and /or the decrease of strain rate, the lamellae were effectively broken down into fine grains, and a relatively complete DRX occurred. Dynamic recovery is the main deformation mechanism for β0 phase, which promotes the compatible deformation between γ and α2 phases and prevents premature cracks. Additionally, the DRX mechanism of γ phase was investigated through TEM observations. The DRX of γ grains begins with the formation of sub-boundaries, followed by a process of rearrangement of sub-boundaries to form new grains.

Hot deformation behavior and dynamic recrystallization of a β-solidifying TiAl alloy

Effect of beam current on microstructure, phase, grain characteristic and mechanical properties of Ti-47Al-2Cr-2Nb alloy fabricated by selective electron beam melting

The effect of scanning speed on microstructure, phase composition, texture and tensile property of Ti-47Al-2Cr-2Nb alloy fabricated by selective electron beam melting (SEBM) was investigated systematically in this paper. With the scanning speed increasing from 1500 mm/s to 2400 mm/s, the microstructure transformed from near γ microstructure with average grain size of 5.11 µm into fine duplex microstructure with average grain size of 2.98 µm and the content of α 2 and B2 phase decreased from 5.0% to 1.5% and 11.6–1.2%, respectively, while the volume fraction of γ phase increased from 83.4% to 97.3%. The crystallographic texture showing an obvious α 2 (0001) and γ (111) orientation weakened with enhancing the scanning speed. Additionally, the tensile property was tested and the result showed that the ultimate tensile strength (UTS) increased from 626.07MPa to 684.98 MPa with scanning speed increasing from 1500 mm/s to 2100 mm/s, while when the scanning speed reached to 2400 mm/s, the UTS decreased to 556.9 MPa. The fracture strain (δ) had the same tendency as the tensile strength with increasing the scanning speed. The increasing porosity was responsible for the degradation of tensile property with the scanning velocity of 2400 mm/s. Finally, the mechanisms of microstructure refinement and texture weakening with increasing scanning speed were discussed detailedly.

Xiaopeng Wang

Papers

Hot pack rolling nearly lamellar Ti-44Al-8Nb-(W, B, Y) alloy with different rolling reductions: Lamellar colonies evolution and tensile properties

Hot deformation behavior and dynamic recrystallization of a β-solidifying TiAl alloy

Effect of beam current on microstructure, phase, grain characteristic and mechanical properties of Ti-47Al-2Cr-2Nb alloy fabricated by selective electron beam melting

Microstructure, texture and tensile property as a function of scanning speed of Ti-47Al-2Cr-2Nb alloy fabricated by selective electron beam melting

Grain refinement by trace TiB2 addition in conventional cast TiAl-based alloy