Abstract
As a special hardenable α titanium alloy, Ti-2.5 Cu alloy was a candidate material for high temperature components requiring high strength and plasticity. The effect of prestrain on the precipitation behaviors was investigated in the present study. Tensile tests show that elongation up to 22 % can be obtained after solid solution (SS) treatment. Thereafter, prestrain in tension with 5 %, 10 %, 15 % and 20 % was carried out for the SS samples and then duplex aging was applied. Transmitting electron microscopy (TEM) investigations show that larger Ti2Cu particles were observed in the prestrained condition than free aging one, as prestrain significantly speeds up the precipitation kinetics. The strength firstly increases and then decreases for the prestrained samples after duplex aging, where the competition between precipitation hardening and recovery softening should be responsible. With the consideration of SS, precipitation and recovery, a strength model for duplex aging combined with prestrain was established, which is in well agreement with experiments. Present study may provide a promising way to obtain the strength of deformed hcp materials in industry application.
Introduction
Titanium alloys have widely been used in aerospace, biomedical and automotive industries because of their high specific strength, low density, nontoxicity against osteoblastic cells, excellent corrosion resistance and good performance at high temperature. According to the microstructure, titanium alloys can be divided into three categories, i. e., α, β and α+β. For the heat treatable titanium alloys, solution and subsequent aging is the essential way to achieve high strength.
In recent years, their precipitation behaviors during aging have been widely investigated, especially for α precipitation from β matrix. Based on the TEM observation, Song et al. revealed that high ductility and strength can be obtained through variant selection of α plates precipitated during the aging process with stress in Ti–10Mo–8V–1Fe–3.5Al alloy [1]. Different precipitation morphologies can also be obtained through the heat treatment routes. Xu et al. [2] found that needle-like and fine shuttle-like α precipitates can be observed for single and duplex aging treatments respectively, where the formation of ω phase assists the shape change for the duplex aging. Both high tensile strength and plasticity were achieved in Ti-15Mo-2.7Nb-3Al-0.2Si alloy for duplex aging [2]. At the same time, the length of the α precipitates also influences the mechanical behaviors of titanium alloys. Li et al. studied the microstructural evolution and aging hardening behavior of a new metastable β alloy and revealed that the strength decreased with the increase of α precipitates length [3].
Except for the routine heat treatments, cold deformation after solution treatment with subsequent aging process is another effective way to improve the mechanical properties of titanium alloys. After cold deformation, large amount of defects are introduced into the alloys [4, 5, 6, 7, 8], which may strongly affect the precipitation behavior and associated aging response. After cold rolling, Li et al. revealed that lath or platelet-shaped α nucleated at the sub-grain boundaries and the proportion of α phase dramatically increased after annealing treatments [5]. When subjected to sufficiently high strain by high-pressure torsion, Xu et al. indicated that the aging response of a Ti-20 wt.% Mo β alloy was greatly altered [6]. A complete ultrafine-duplex (α+β) structure was formed consisting of equiaxed instead of acicular α precipitation. Relatively coarse structure can also be obtained for heavily deformed alloy [8], since the deformation induced substructure enhances the atoms diffusion.
Substantial research effort has been put into investigating the role of the precipitation behaviors of α phase in metastable β matrix. However, as the only kind of aging hardenable α titanium alloy, little information is available on the fundamental precipitation strengthening mechanisms in aged Ti-2.5 %Cu alloy combined with prestrain. According to the Ti–Cu phase diagram, plate-shaped Ti2Cu particles can be obtained from the supersaturated solid solution of Ti–2.5(wt.)%Cu when applying aging treatments [9, 10, 11]. Based on the Orowan strengthening mechanism, Song et al. established a strength prediction model, exploring the variation of yield strength with aging [11]. Nevertheless, as one typical α titanium alloy, the significance of understanding the strengthening mechanism is that the microstructural factors needs to be further studied.
In order to investigate more systematically the effect of pre-deformation and artificial aging on the mechanical properties of titanium alloy. The present work explores the role of cold pre-deformation on the aging-hardening response and mechanical properties of Ti-2.5Cu alloy, aiming to further elevate the yield strength and thus broaden its application field.
Experiments
The chemical composition of the as-received Ti-2.5Cu alloy is in (wt%):Fe<0.06 %, Si<0.04 %, Cr<0.08 %, N<0.01 %, H<0.01 %, O<0.14 %, N=0.006 %, Cu≈2.5 %, bal. Ti. The alloy was hot-rolled into the rods with the diameter of 14 mm at the temperature of 1073 K. The rods were then solution treated at 1078 K for 1 h and cooled in water. Equiaxed grains were observed by optical microscopy. After solution treatment, the rods were machined into the tensile samples with the gauge length of 35 mm and diameter of 7.5 mm. Tensile tests were then performed on the Instron 1195 testing machine with the strain rate of 1×10‒4s‒1. Plastic strain (pre-strain) of 5 %, 10 %, 15 % and 20 % were subsequently obtained. After that, the prestrain samples were subjected to two-step aging treatment, i. e., at 673 K for 24 h and at 748 K for different holding time. After aging treatment, these rods were fabricated into tensile samples with 12 mm in gauge length and 5 mm in diameter. Subsequent tensile tests were carried out at a strain rate of 1×10‒4. TEM examinations were carried out on a JEM-200CX transmission electron microscope. Specimens for TEM examinations were cut along the cross-section of aged samples. Thin foils for TEM examinations were prepared by a twin jet electropolisher at a temperature lower than 253 K.
Experimental results and discussion
Microstructure observation
Figure 1(a) is a bright field TEM image showing arrangement of the precipitated Ti2Cu plates in duplex aged Ti-2.5Cu alloy without prestrain, i. e., aged at 673 K for 24 h and at 748 K for 8 h. It can be seen that three variants of precipitates appeared homogeneously. In reference [11, 12], we have confirmed that the habit plane of the precipitates of Ti2Cu was

Bright field TEM images of Ti-2.5Cu alloy after duplex aging.
Figure 2 presents the TEM microscopy of the Ti-2.5Cu alloys after duplex aging (400℃ 24 h + 475℃ 8 h) with the prestrain of 10 % and 20 %. It can be easily seen that no remarkable recrystallization occurs since the dislocation substructure retains after the aging process. At the same time, the size of the precipitates in the prestrain samples is much larger than that of free aged ones. As is known, prestrain can introduce high density of dislocations [5, 13, 14, 15], increasing the free energy of the alloy. In addition, dislocations act as the channel nets enhancing the diffusion of the solute atoms, speeding up the precipitation process. In this case, Ti2Cu particles first nucleated at the dislocation tangle sites and grow quickly than the free aging condition. Consequently, prestrain accelerates the aging process.

Bright field TEM images of Ti-2.5Cu alloy after duplex aging with the prestrain of (a) 10 % and (b) 20 %.
Mechanical behaviors
Figure 3 shows the engineering stress–strain curve for the solid solution Ti-2.5Cu alloy. The yield strength and elongation are 538 MPa and 22 %, respectively. Remarkable work hardening was observed during the deformation process.

Engineering stress-strain curve for the solid solution Ti-2.5Cu alloy.
The yield strength and elongation of Ti-2.5Cu alloy with different prestrain subjected to first (673 K~24 h) and duplex aging (673 K~24 h + 748 K~8 h) was shown in Figure 4. The strength monotonically increases with prestrain for first aging. However, the strength firstly increases and then decreases for the prestrain samples after duplex aging. As the prestrain exceeds 15 %, the strength after duplex aging becomes lower than first aging. Two reasons might be responsible for this phenomenon. First of all, large prestrain leading to high density of dislocation accelerates the diffusion and thus the kinetics of precipitation process [16, 17,18,19], promoting the occurrence of over-aged stage for the prestrain samples. On the other hand, as recrystallization was not observed (recrystallization temperature Tr≈780 K ℃ for pure titanium), it is believed that recovery reduces the density of dislocation during the aging resulting in the reduction of strength [19, 20]. The kinetics of recovery also fastens remarkably as the prestrain and temperature increase. In this case, the strength depends mainly on the competition between the precipitation hardening and recovery softening

Comparison of (a) yield strength and (b) elongation for first and duplex aging.
Figure 4(b) shows the variation of elongation with the prestrain. All the samples display slightly decline trend in elongation with the prestrain. No apparent degradation in elongation was observed for the prestrained samples after duplex aging.
The strength model for duplex aging
For the titanium alloys, much attention was paid to investigate the phasetransformation, as the α↔β was the dominant topic. However, as one kind of hardenable titanium alloy, Ti-2.5Cu has its unique properties, which is different from the other titanium alloy [9, 10, 11]. There is no phase transformation during the aging process, while the precipitation of Ti2Cu particles occurs. The Ti2Cu particles act as a hard phase, improving the strength of the alloy [9, 10, 11, 18],.
As is known, due to the complex slip systems in the geometry of hcp structure with precipitates, it is much harder to establish the strength prediction model according to the microstructure. In this part, we attempt to construct a numerical model to describe the microstructure with the strength to further understand the strengthening mechanisms of α titanium alloy.
Three strengthening contributions are considered in the present study responsible for the duplex aging treatments, (1) solid solution strengthening; (2) precipitation hardening; (3) deformation strengthening. Item (1) accounts for the substitutional solution strengthening, as Cu is the single element in Ti-2.5Cu alloy regardless of the minor elements introduced during casting and forming. According to TEM under room temperature, the primary plastic deformation mechanism was dislocation slip for aged Ti-2.5Cu alloy. Precipitation of non-deformable Ti2Cu particles impedes the dislocation glide. Therefore, item (2) should be established on the basis of Orowan strengthening mechanism for dislocation to bypass Ti2Cu particles. At the same time, prestrain raised the density of dislocation while dislocation density decreased as recovery occurs during aging. Item (3) should be based on the dislocation dynamics. Therefore, the yield strength of aged Ti-2.5Cu alloy can be given as
where
Solid solution strengthening
The substitutional solution strengthening can be expressed as [21]:
Here, C is the concentration of Cu dissolved in Ti matrix, β is the strengthening constant related to the lattice and modulus misfit of Cu with respect to Ti and is proposed to be 75 MPa [11]. C can be obtained from mass balance in the expression of precipitation solute concentration.
where
where d is the plate diameter, h is the foil thickness which can be obtained by grain boundary fringes technique [23], and A is the projected area fraction of precipitates, determined by the point count method.
Precipitation strengthening
For α titanium alloys,
where
The additional applied stress for dislocation to bypass Ti2Cu plates is described by Orowan-Ashby mechanism [21, 27], in the following form:
where G, b and
In order to obtain the value of

Schematic drawings of the crystallographic orientation relationships between Ti2Cu plates and slip planes of α-Ti matrix.
Considering the probability and area of intersection on three slip planes,
Deformation strengthening
Prestrain introduce large amount of dislocations, resulting in the working hardening of titanium alloys. The dislocation contribution can be given as [28]:
Here, α is the constant, ρ is the dislocation density.
As the aging proceeds, recovery occurs at the same time. During recovery the stored energy of the material is lowered by dislocation movement and annihilation. There are two primary processes, these being the annihilation and rearrangement of dislocations to lower the dislocation density. Thus, the cold deformation induced defect density decreases because of the recovery during aging in the alloys. The dislocation density change during recovery can be given as [29]
where
where K is a constant, relating to work hardening, which can be experimentally determined to be 560 MPa,
Application of the strength model
It is well known that microstructure of metal materials has a great influence on their mechanical properties. Based on the TEM observation, the averaging value of diameter and thickness can be estimated, which are shown in Figure 6(a) and (b). The fitting equations for diameter and thickness variation with aging time can also be acquired, indicating that the power law works, as is shown in Table 1. The volume fraction of the Ti2Cu precipitates was supposed to obey the Avrami law.

Experimental values and corresponding fitting lines of (a) diameter, (b) thickness and (c) volume fraction of precipitated Ti2Cu plates in Ti-2.5Cu alloy samples for duplex aging.
Fitting equations for the experimental values.
Prestrain (%) | Diameter (nm) | Thickness (nm) | Volume fraction |
---|---|---|---|
0 | 30+55t0.15 | 1.28+0.74t0.2 | 0.002+0.048(1-exp(‒0.15t1.5)) |
5 | 33+67t0.1 | 1.42+0.89t0.2 | 0.003+0.046(1-exp(‒0.2t1.5)) |
10 | 38+71t0.1 | 1.66+1.4t0.2 | 0.006+0.043(1-exp(‒0.25t1.5)) |
15 | 42+75t0.1 | 2+2.1t0.1 | 0.012+0.038(1-exp(‒0.3t1.5)) |
20 | 48+78t0.08 | 2.1+3t0.1 | 0.015+0.035(1-exp(‒0.45t1.5)) |
By substituting the fitting equations into eq. (1), the variation of the strength with aging time for prestrained Ti-2.5Cu alloy after duplex aging was obtained, as is shown in Figure 7 (solid lines). The resulting equations meet very well with the experimental values for different prestrain.

Calculated strength lines and experimental yield strength of Ti–2.5Cu alloy after duplex aging. All samples were first aged at 673 K for 24 h.
In the present study, the strength of the Ti-2.5Cu alloys mainly depends on the prestrain and aging time. Prestrain-induced work hardening noticeably improves the strength of Ti-2.5Cu alloy. The peak stress during duplex aging for the free aged samples is much lower than the prestrained ones, indicating that prestrain strengthening can be more efficient for Ti-2.5Cu alloy than precipitation strengthening. When the prestrain exceeds 15 %, no remarkable strengthening was observed during aging, while decrease in strength occurs since recovery became dominant. After duplex aging for 6 h, the prestrained samples display the uniform strength irrespective of the value of prestrain.
Conclusions
In this study, the influence of prestrain on the aging-hardening response and mechanical properties of Ti-2.5Cu alloy was systematically investigated. The following conclusions are drawn based on the results:
Prestrain brings about marked work hardening effect and rising in prestrain can significantly speeds up the precipitation kinetics.
Duplex aging can effectively improve the strength as the prestrain below 15 %. When the prestrain exceeds 15 %, no remarkable strengthening was observed, while decrease in strength occurs compared to first aging, which is distributed to the competition between the precipitation hardening and recovery softening.
A strength model for duplex aging with prestrain was established, incorporating solution strengthening, precipitation strengthening and recovery mechanisms. The results of the model meet well with the experimental data, providing a promising way to predict the strength of hcp materials in industry application.
Funding statement: Project of industry-university-research of Jiangsu Province (Grant / Award Number: ‘BY2016061-09’, ’BY2016061-20’) Natural Science Foundation of Jiangsu Province (Grant / Award Number: ‘BK20140458’, ’BK20150418’) National Natural Science Foundation of China (Grant / Award Number: ‘51501067 ‘,’51503072’).
Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 51501067 and 51503072), the Natural Science Foundation of Jiangsu Province (No. BK20140458 and BK20150418) and the project of industry-university-research of Jiangsu Province (BY2016061-09, BY2016061-20).
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- Short Communication
- Study on the Growth Mechanism of K2Ti4O9 Crystal
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Articles in the same Issue
- Frontmatter
- Research Article
- Effect of Trailing Intensive Cooling on Residual Stress and Welding Distortion of Friction Stir Welded 2060 Al-Li Alloy
- Short Communication
- Study on the Growth Mechanism of K2Ti4O9 Crystal
- Research Articles
- Artificial Neural Network-Based Three-dimensional Continuous Response Relationship Construction of 3Cr20Ni10W2 Heat-Resisting Alloy and Its Application in Finite Element Simulation
- Influence of Thermal Ageing on Microstructure and Tensile Properties of P92 Steel
- A Novel Process for Joining Ti Alloy and Al Alloy using Two-Stage Sintering Powder Metallurgy
- Modeling and Finite Element Analysis for the Dynamic Recrystallization Behavior of Ti-5Al-5Mo-5V-3Cr-1Zr Near β Titanium Alloy During Hot Deformation
- Study on Dynamic Development of Three-dimensional Weld Pool Surface in Stationary GTAW
- Influence of Heat Treatment on Fracture Toughness and Wear Resistance of Nicral-Zro2 Multilayered Thermal Barrier Coating
- Kinetic Study on Phosphate Enrichment Behavior in CaO–SiO2–FeO–Fe2O3–P2O5 Steelmaking Slags
- Effect of Prestrain on Precipitation Behaviors of Ti-2.5Cu Alloy
- Study on Gamma Prime and Carbides of Alloy A286 by Traditional Thermodynamic Calculation