|dc.description.abstract||Metastable beta titanium alloys have drawn great attention and became important structural materials in the aeronautical industry thanks to their overall physical and mechanical performance including ultra-high specific strength, excellent strength-ductility balance, outstanding corrosion resistance, wide service temperature range and great hardenability. However, the lack of enough understanding of the nature of their hot workability and deformation behaviour and insufficient data for optimising the respective hot processing conditions makes it difficult for the titanium manufacturing industry to implement the precise processing of metastable beta titanium alloys with required quality control. Furthermore, the exorbitant manufacturing cost limits their further widespread applications.
Our research group has initiated to fabricate titanium alloys from blended elemental powder mixtures by using the thermomechanical powder consolidation (TPC) approach, and it is demonstrated that the TPC process is a feasible method to cost-effectively produce titanium alloys from powder. In this research, a commercial metastable β titanium alloy, Ti-5Al-5V-5Mo-3Cr (Ti-5553), was fabricated via the TPC process from a blended elemental powder mixture, and its microstructure variation, mechanical properties, deformation behaviours and mechanisms (at both room temperature and elevated temperatures), workability, and post-heat treatment effects were systematically investigated. For comparison, similar research was undertaken for the ingot metallurgy (IM) counterpart Ti-5553 alloy that was prepared by conventional vacuum-arc melting and casting.
The as-consolidated power metallurgy (PM) Ti-5553 alloy has much finer grain size, higher β phase transformation temperature and higher interstitial element (oxygen and nitrogen) content than those of the IM counterpart. The PM alloy exhibits lower load-bearing capacity and tensile ductility than those of the IM alloy during the in-situ tensile test, and the coalesced grain boundary α (GB-α) and widened αʺ/β microcracks contribute to early brittle failure. The IM alloy has better compatible-slip-deformation capability than the PM alloy, and the serious cracking at the V-shape notch and the microcracks near α/β interfaces together lead to the gradual fracture of the IM alloy specimen. The residual pores and microvoids existed in the PM alloy have little effects on the alloy’s slip deformation and fracture behaviour.
The PM Ti-5553 alloy’s flow stress is increased with decreasing the deformation temperature and increasing the strain rate, and vice versa. The alloy’s deformation activation energy is 371.65 kJ/mol in the (α+β) region and 226.94 kJ/mol in the β region, respectively. The optimal processing window for PM Ti-5553 alloy is determined as: processing temperature of 900 °C-1050 °C combined with deformation strain rates below 1 s⁻¹ with a high deformation degree to at least 70% height reduction. Furthermore, the potential “best” processing condition is recommended as the medium deformation temperature (about 950 °C) and moderate-low strain rate (about 0.01 s⁻¹). Because of flow localization and external cracking, unstable deformation happens when the deformation temperature is lower than 1025 °C and the strain rate is higher than 1 s⁻¹, and this region should be avoided for processing the PM Ti-5553 alloy.
The PM Ti-5553 alloy exhibits lower flow stress, slighter discontinuous yielding phenomenon and less adiabatic temperature rising than its IM counterpart for the same processing condition. Comparing to the IM Ti-5553 alloy, the PM Ti-5553 alloy has lower average activation energy, larger optimal processing windows, smaller flow instability region and higher cracking resistance. Dynamic α globularization and coarsening are the dominated mechanisms for the PM Ti-5553 alloy deformed at low temperature (700 °C to 800 °C) and low strain rate (less than 0.1 s⁻¹), while the IM Ti-5553 alloy is governed by dynamic α precipitation at these conditions. The complete dynamic recrystallization temperature for the PM alloy is about 100 °C lower than that of the IM counterpart, and dynamic recrystallization (DRX) mechanism is controlled by discontinuous dynamic recrystallization (DDRX) for the PM alloy but continuous dynamic recrystallization (CDRX) for the IM counterpart.
Based on the guidance of the hot processing map, the as-consolidated PM Ti-5553 alloy is safely thermomechanically-processed (single uniaxial open-die forging) at the temperatures of 950 °C and 1050 °C, strain rates about 0.5 s⁻¹ and 0.01 s⁻¹ to the deformation degree of 75%. The alloy forged at the condition of 950 °C/~0.01 s-1 (FR-1 alloy) shows the highest mechanical properties (UTS: 1450.9 MPa, elongation: 3.23% and MH: 492.4 HV), comparing to those processed at either higher temperature or higher strain rate.
Various heat treatments were carried out for the forged PM Ti-5553 alloy to tailor the alloy’s microstructure for achieving desired strength-ductility balance. Attributed to the harmonious concurrence of hierarchical α precipitation and heterogeneous grain structure, superior strength-ductility combinations are achieved for the FR-1 alloy after the heat treatment at 700 °C and 750 °C, with the UTS and elongation values of 1386.5 MPa/6.76% and 1252.3 MPa/8.64%, respectively. These strength-ductility combinations are comparable and/or even better than other IM metastable β titanium alloys.||