Effect of heat treatment and silicon addition on the microstructure development of Ti–6Al–2Cr–2Mo–2Sn–2Zr alloy

doi:10.1016/S0921-5093(02)00381-7

Materials Science and Engineering: A

Volume 343, Issues 1–2, 25 February 2003, Pages 210-226

https://doi.org/10.1016/S0921-5093(02)00381-7 Get rights and content

Abstract

Microstructural development of Ti–6–22–22–0.22Si and Ti–6–22–22–0.02Si after beta, alpha/beta, and aging heat treatment was investigated. Primary alpha morphology was determined following beta heat treatment. Cooling rate from beta treatment was found significantly influence the HCP transformation products, ranging from principally alpha-prime martensite and acicular alpha at rates exceeding 5.5 °C s⁻¹, to mixtures of Widmanstatten and colony alpha at intermediate cooling rate, to colony alpha at the slowest rates of 0.27 and 0.055 °C s⁻¹. Varying cooling rates from the alpha/beta heat treatment significantly influenced the volume fraction of retained beta phase and secondary alpha in the microstructure. Higher cooling rate resulted in greater retention of the beta phase and transformation of the phase into secondary alpha upon aging. This higher amount of fine, secondary alpha phase promoted higher strength and lower toughness. Alpha/beta heat treatment temperature significantly influenced the heat treatment response of the microstructure. High heat treatment temperatures promoted greater amounts of retained beta with fine, transformed alpha in the final microstructures. Lower alpha/beta heat treatment temperatures promoted retained beta microstructures which were less responsive to aging treatment. Aging treatment promoted decomposition of retained beta phase, particularly in larger retained-beta regions that exhibited lower stability. Microstructural characteristics of heat treated high and low Si alloys appears identical in most cases. However, very fine silicides were observed in the high Si alloy after aging at 593 °C, which may have a significant effect on fracture toughness.

Introduction

Within the family of (α+β) two phase titanium alloys, high-strength Ti–6Al–2Cr–2Mo–2Sn–2Zr–0.15Si (Ti–6–22–22–Si) has recently attracted attention for a number of aerospace applications, such as F-22 jet fighter, X-33 Demonstrator/Reusable Launch Vehicle and the Joint Strike Fighter. Compared with conventional Ti–6Al–4V (Ti–6–4) alloy, Ti–6–22–22 has a higher strength and better damage tolerance [1], [2], [3], [4], especially at elevated temperatures and on thick sections.

In recent years, numerous investigations have been performed to optimize the mechanical properties and silicon chemistry of Ti–6–22–22–Si [2], [3], [4]. This work has resulted in the development of the TRIPLEX heat treatment, which involves beta solutionizing at 1004 °C for 60 min, fan cooling to 482 °C at 0.5 °C s⁻¹, air cooling to 93 °C; alpha/beta solutionizing at 927 °C for 60 min, fan cooling to 482 °C at 0.5 °C s⁻¹, air cooling to 93 °C; and aging at 538 °C for 8 h followed by air cooling [2].

Many uncertainties still remain regarding the relationships between Ti–6–22–22–Si alloy chemistry (particularly Si content), TRIPLEX heat treatment conditions, and the resulting alloy microstructure and mechanical properties. To date, a comprehensive study of the effects of Si content and TRIPLEX heat treatment conditions on the nature of phase transformations, microstructure evolution, and correspondingly on mechanical properties and fracture behavior, has not been performed. The objective of the present investigation was to perform such a study. Anticipated outcomes of this investigation would include an improved understanding of the effects of alpha/beta heat treatment temperatures, cooling rates and silicon chemistry on the alpha phase morphology and on the potential formation of intermetallic precipitates. In addition to developing a more complete understanding of these phenomena, a secondary objective of this study was to propose and assess alternate TRIPLEX type heat treatments that may improve mechanical properties of Ti–6–22–22–Si.

Section snippets

Outline of experiment

This study examined principle components of the TRIPLEX heat treatment via their systematic variation to understand their influence on microstructure evolution, and ultimately on mechanical properties and fracture behavior. These heat treatment parameters included:

1
cooling rate following beta heat treatment;
2
alpha/beta heat treatment temperature;
3
cooling rate during alpha/beta heat treatment;
4
aging temperature;
5
silicon content.

It is important to note that in order to fully assess the aforementioned

TRIPLEX microstructures

In order to better control the microstructure development, beta transus temperature was determined for the current Ti–6–22–22Si alloy. As-received samples were heated to 920–1000 °C for 1 h and then water quenched. Following standard metallography procedure, samples were examined on an optical microscope and SEM. The beta transus temperature for Ti–6–22–22–0.22Si alloy was determined as 965±5 °C. This is consistent with the previous CCT work and differential thermal analysis (DTA) data on

General discussion

It has been shown in the present study that cooling rate after beta heat treatment generally determines primary alpha morphology. Under normal cooling conditions (between 0.55 and 0.055 °C s⁻¹), primary alpha morphology exhibits mixed Widmanstatten/colony alpha structures, albeit in differing proportions. Such morphology is not expected to have significant effect on mechanical properties as it would have been if fully colony and fully Widmanstatten primary alpha structures were compared. However

Summary of the results

Primary alpha phase morphology is determined by the beta heat treatment. Cooling rate after beta heat treatment has significant influence on the beta transformation products that range from alpha prime martensite and acicular alpha at rates exceeding 5.5 °C s⁻¹, to mixtures of Widmanstatten and colony alpha at intermediate cooling rates, to colony alpha at rates as slow as 0.27 and 0.55 °C s⁻¹. Slow cooling rate promotes diffusion during the beta to alpha transformation, resulting in greater

Acknowledgements

This work was conducted at the Ohio State University supported by US Air Force through Lockheed Martin Aeronautical Systems. Special thanks should go to S. Meng for sample preparation.

References (11)

D.J. Evans et al.
Mater. Sci. Eng.
(1996)
X.D. Zhang et al.
Scr. Mater.
(1999)
X.D. Zhang et al.
Acta Mater.
(1998)
A.P. Woodfield et al.
Acta Metall.
(1988)
S.R. Seagle et al.

There are more references available in the full text version of this article.

Cited by (44)

Enhancing the elevated temperature strength of titanium matrix composites through a novel (α + β) TRIPLEX heat treatment
2024, Materials Science and Engineering: A
In this work, an (α + β) TRIPLEX heat treatment process was designed, as well as its impact on the microstructures and high–temperature mechanical properties of 2.5 vol% (TiB + TiC)/Ti–6Al–4Sn–7Zr–1Nb–1Mo–1W–0.2Si (wt.%) was carefully investigated. The results show that the microstructures of heat–treated TMCs consist of the primary α phase (α_p) and the transformed β (β_t) with a large number of nano–scale secondary α phases (α_s). The TRIPLEX heat treatment encourages the effect of structural heredity, causing the residual α_p lamellae to develop adequately and even mature into lamellae clusters with the same orientation, which induces a visible strengthening of the variant selection for the α phase. After the TRIPLEX heat treatment, the increase in content and size of silicides is mainly due to the process providing favorable sites and sufficient time for nucleation and growth. The longitudinal axes of some silicides exhibit an approximate 40° angle or parallel arrangement with the β/α_p interfaces, which means that the silicides will grow preferentially along a specific orientation to minimize the strain energy. The (α + β) TRIPLEX heat treatment significantly increases the high–temperature strength of the TMCs with no appreciable elongation loss (as–cast TMCs: UTS = 487.6 MPa, EL = 17.4%; HT–4#: UTS = 558.1 MPa, EL = 11.5%). The strength improvement of HT–4 # is mainly related to the combined effects of α_p content reduction, α_s precipitation, obstruction of dislocations by the β_t/α_p interface, hetero–deformation strengthening, silicides precipitation strengthening, and α₂ dispersion strengthening.
Enhanced processing map of Ti–6Al–2Sn–2Zr–2Mo–2Cr–0.15Si aided by extreme gradient boosting
2022, Heliyon
A processing map is required for Ti alloys to find processing parameters securing a high formability. This study adopted the extreme gradient boosting (XGB) approach of machine learning to predict a flow curve and plot a processing map with less experiments for the first time. The optimum XGB model predicted flow curves of Ti–6Al–2Sn–2Zr–2Mo–2Cr–0.15Si at 1073–1273 K and 10 s⁻¹. The predicted data were used to plot a processing map, which showed a higher accuracy in the instability map as compared with the map without XGB. The XGB model also anticipated the power dissipation map at low strain rates. The low accuracy at high strain rates would be improved by alleviating the bias towards a flow hardening. This work has successfully proven the potential usefulness of XGB for plotting an enhanced processing map in light of a higher accuracy with less experiments.
Effect of prior β grain size on the martensitic transformation of titanium alloys
2021, Materials Characterization
Various titanium alloys (Ti-Al-Fe-Si, VT3-1, Ti-62222S, and Ti-64) were prepared to investigate the effect of the prior β grain size on the kinetics of α'-martensitic transformation. The prior β grain size was controlled using various solution treatments (740–960 °C). Electron backscattering diffraction, energy dispersive spectroscopy, and field-emission transmission electron microscopy techniques were used to determine the volume fraction, composition, and grain size of the phase constituents. The grain refinement of prior β inhibited the α'-martensitic transformation. Moreover, the modified Mo equivalent and transformation kinetics model including the prior β grain size were formulated.
Precipitation behavior of α<inf>2</inf> phase and its influence on mechanical properties of binary Ti-8Al alloy
2021, Journal of Alloys and Compounds
Citation Excerpt :
The size and volume fraction of α2 precipitates were considered to obviously affect the plastic deformation and mechanical properties of titanium alloys. Several studies investigated the effect of α2 on mechanical properties and found an increase in the yield strength and Young's modulus [19–24]. The decrease of fracture toughness induced by the α2 precipitation was reported [16,21,22,25].
The precipitation behavior of Ti₃Al (α₂) and its effect on the tensile properties and fracture toughness of the binary Ti-8Al alloy were investigated. Ti-8Al alloy was firstly solution-treated to obtain Widmanstatten structure and then aged for different aging times to promote α₂ precipitation. Transmission electron microscopy and atom probe tomography analyses were employed to characterize the presence of α₂ phase. The results showed that the size and volume fraction of α₂ precipitates increased as aging time prolonged. The relationship between the mechanical properties and α₂ precipitates was evaluated. The increase in the volume fraction of α₂ enhanced the alloy strength, meanwhile, induced the long-rang planar slipping and inhibited the twinning activity. This could intensify strain localization and decrease fatigue crack tip plastic zone size (CTPZ), leading to the reduction of ductility and fracture toughness. The in-situ tensile test results revealed that the activation of slipping and colony rotations both occurred in response to deformation. Also, it was found that the increase of the plastic strain enhanced the intensity of strain localization, and the misorientation across the shearing band in the large α colony enhanced the stress concentration area which would promote the crack nucleation. In light of these findings, the tailoring of α₂ precipitation in titanium alloys are required in the design of high-performance alloys for structural applications.
Precipitation behavior of silicide and synergetic strengthening mechanisms in TiB-reinforced high-temperature titanium matrix composites during multi-directional forging
2021, Journal of Alloys and Compounds
To investigate the precipitation behavior of silicide and evaluate the contribution of the reinforcement phase to the properties of titanium matrix composites, a TiB-reinforced high-temperature titanium matrix composite was prepared by vacuum induction melting and multi-directional forging. The microstructure of the resultant composite after multi-directional forging was composed of a spheroidized α-phase, uniformly-distributed TiB and dual-scale silicides. The precipitation of silicides was attributed to the acceleration of elemental diffusion due to dislocations during forging. The strength of the composite after multi-directional forging at room temperature increased by about 151.9 MPa due to the fine-grain strengthening of the α-phase and the precipitation strengthening of the silicides, whereas the load-bearing efficiency of the TiB whiskers was reduced with the decrease in the aspect ratio. The strength of the multi-directional-forged titanium matrix composite at 700 °C was reduced by about 28.3 MPa. However, its plasticity was significantly improved, and the elongation reached 116.8%. The main reasons for the decrease in the strength were the easy initiation of cracks the grain boundaries and the weakening of the precipitation strengthening of the silicides.
Effect of temperature on microstructure and mechanical properties of ECAPed (TiB + La<inf>2</inf>O<inf>3</inf>)/Ti-6Al-4V composites
2018, Materials Characterization
TiB short fibers and La₂O₃ particles reinforced titanium matrix composites (TMCs) were successfully equal-channel angular pressed (ECAPed) at 600 °C–900 °C. The effect of ECAP temperature on the microstructure and mechanical properties of the TMCs was investigated. The results indicated that dislocation tangling formed cell structures in matrix at lower ECAP temperature. In contrast, continuous dynamic recrystallization occurred at higher ECAP temperature, and promoted the formation of numerous new ultrafine grains. The average aspect ratio of TiB short fibers decreased with increasing ECAP temperature, which induced interfacial debonding. The tensile strength of TMCs ECAPed at 800 °C was up to 1128 MPa, which was 18% higher than the as-received composites. Interfacial debonding led TiB short fibers in low aspect ratios disabled to bear load, while the formation of ultrafine grains increased the strength of TMCs. The coupling of these two factors resulted in little effect of ECAP temperature on the tensile strength. However, the elongation of ECAPed TMCs generally decreased due to the loss of strain hardening capacity. Stress concentration tended to occur around the tips of TiB short fibers due to the interfacial debonding and cause crack propagation. It led to the premature fracture and low ductility.

View all citing articles on Scopus

¹: Present address: Rensselaer Polytechnic Institute, Troy, NY 12180, USA.

²: Present address: General Electric Power Systems, Atlanta, GA, USA.

³: Present address: Allvac-Allegheny Technologies., Monroe, NC, USA.

View full text

Effect of heat treatment and silicon addition on the microstructure development of Ti–6Al–2Cr–2Mo–2Sn–2Zr alloy

Abstract

Introduction

Section snippets

Outline of experiment

TRIPLEX microstructures

General discussion

Summary of the results

Acknowledgements

Mater. Sci. Eng.

Scr. Mater.

Acta Mater.

Acta Metall.