Research on the Semi-Solid Compressive Deformation Behavior of Ti-7Cu Alloy

Introduction

The extensive applications of titanium and its alloys, which are important structural materials with high strength and excellent corrosion resistance, are limited by their poor formability and high processing cost in solid state [1]. Most titanium alloy products are processed in a solid state, and these processes such as rolling, forging are difficult to machine in such state for their poor formability, i.e. high strength, relatively low modulus of elasticity, low thermal conductivity and high chemical reactivity. Many researches have been carried out to improve the formability and reduce the processing cost of Ti alloys [2–5]. Semi-solid processing is an effective net-shape forming process, which combines the elements of both casting and forging. The semi-solid deformation and processing behavior of Mg alloy [6, 7], Al alloy [8, 9] and steel [10] has been investigated for many years, and general results show that compared with conventional forming methods, semi-solid forming can easily process to accomplish as compared to established forging, with low deformation resistance, and it could reduce energy and machine costs for low processing load. In order to lucubrate the knowledge of compressive deformation characteristics of titanium alloy with different semi-solid deformation parameters, in this work, an comprehensive study was carried out to investigate the effect of deformation parameters including temperature and strain rate on compression behavior of Ti-7Cu alloy (Ti-Al-Cu-Si, the Cu content is 7%), which provides bases for the application of titanium semi-solid processing technology.

Experimental procedure

Materials used in this study were obtained from a Ti-7Cu (α + Ti₂Cu) burn resistant billet, the composition of which is shown in Table 1. The melting point of Ti₂Cu is 1,263 K. If the deformation or testing temperature rises above 1,263 K, Ti-7Cu alloy changes to a semi-solid state.

Semi-solid compression tests were conducted on Gleeble-3500 thermal and mechanical simulator using cylindrical samples with 12 mm in height and 8 mm in diameter. All the compression samples were machined by electron discharge machining and parallel to the billet. Semi-solid compression experiments were performed under isothermal condition at constant strain rates of 0.005, 0.05, 0.5 and 5 s⁻¹ and at temperatures of 1,223 K, 1,273 K, 1,373 K and 1,473 K, respectively. The temperature was controlled in the range of ±1 K and the adiabatic temperature rise was recorded with the help of a transient recorder. The specimens were heated to processing temperatures at a rate of 30 K/s and then held for 2 min before the deformation to ensure homogeneous temperature fields. All these specimens were compressed in a vacuum environment and the compression ratio was 50%. At the end of compression procedure, each specimen was water quenched immediately to retain the deformed microstructures for microstructural investigation. In order to reduce friction and maintain uniform deformation, two pieces of thin tantalum sheets were placed between the compression specimen and die.

Central regions of the specimens where exhibited the largest deformation were selected as the investigation area of interest. The specimens were etched by Keller’s reagent (10 ml HF +25 ml HNO₃ +15 ml HCl +500 ml H₂O) and the microstructure examination was conducted at an OLYMPUS/GX71 optical microscope. For transmission electron microscope (TEM) study, the specimens were prepared by ion milling method after being thinned to 50 μm by mechanical grinding, and then examined in a JEM-200CX transmission electron microscopy.

Results

Deformation at different temperature

Typical true stress–strain curves of Ti-7Cu alloy at different semi-solid temperatures are illustrated in Figure 1. In the analysis of the flow curves, it can be seen that temperature has a significant influence on flow stress at the same strain rate, and flow stress decreases with the increase of deformation temperature at a given strain rate. The amount of temperature rising and subsequent softening is usually more obvious at higher strain rates (for example, 0.5 s⁻¹ and higher). All the flow curves exhibit limited work hardening and significant flow softening occurs after yielding drop followed by a steady-state flow for higher strain rates. Compared with conventional solid deformation (1,223 K), flow stress changed smaller with increasing of semi-solid deformation temperature and reached to steady state immediately with the increase of strain rate, which suggests that Ti-7Cu alloy also has an obvious soften characteristic in semi-solid state.

Figure 1

Stress–strain curves of Ti-7Cu alloy with different strain rate (a) 0.05 s⁻¹;(b) 0.5 s⁻¹

Deformation with different strain rate

Figure 2 describes stress–strain curves of isothermally compressed Ti-7Cu alloy at strain rates range from 0.005 s⁻¹ to 5 s⁻¹. These curves show that the flow stress increases with the increase of strain rate at a given temperature. It is obvious that the stress–strain curves at strain rate of 0.5 and 5 s⁻¹ exhibit a sharp initial peak stress (discontinuous yielding) followed by a little work hardening and then soften to a steady state. Such discontinuous yielding phenomenon was also reported in semi-solid deformation behavior of Ti14 alloy [11–13], which may ascribe to the rapid response time and liquid flow rate during semi-solid deformation. Figure 3 shows the peak stresses and steady stresses of Ti-7Cu alloy during the compression with the different temperature and strain rate, respectively. It can be seen that peak stress and steady stress are sensitive to semi-solid deformation temperature and strain rate, peak stress and steady stress decrease significantly with increasing the deformation temperature and strain rate.

Figure 2

Stress–strain curves of Ti-7Cu alloy (a) 1,273 K; (b) 1,473 K

Figure 3

Steady stress and peak stress curves of Ti-7Cu alloy with different deformation temperature and strain rate during semi-solid deformation (ε = 0.6) (a) Steady stress; (b) Peak stress

Constitutive equation of Ti-7Cu alloy for semi-solid deformation

The flow stress σ depending on the temperature T and the strain rate ε exists a balance between dynamic softening and hardening, as similar to the relationships of creep. J.K. Fan and H.C. Kou [14] proposed using include deformation activation energy Q and temperature T in the form of amendments to the hyperbolic sine Arrhenius equation to describe the rheological behavior of thermally activated:

(1)ε˙=A[sin(ασ)]nexp−QRT

where A, α and n are constants, R is the gas constant, Q is the deformation activation energy, ε is strain rate and T is deformation temperature, which reflects the difficult degree of thermal deformation, whose size depends on the morphology of the material.

The studies of different thermal processing show that, the relationship between flow stress and strain rate in low stress levels and high stress level can be respectively described with exponential relationship:

(2)ε˙=A1σn1stress

(3)ε˙=A2exp (βσ)stress

The A₁, A₂, K, and β are constants in the above formulas. These relationships described the dynamic equilibrium of strain hardening and dynamic softening. When the low stress level is close to formula (2) and the high stress level is close to formula (3), stress can be applied to the entire range, and the constants α, β, and n are described as the following relationship:

(4)α=βn

In terms of the high temperature deformation theory, the apparent activation energy for deformation represents the magnitude of the energy barrier which the atomic transition needs to overcome. Thus, the apparent activation energy for deformation has been regarded as an important phenomenological parameter to reflect the workability of metals. When deformation activation energy Q is assumed to be independent of deformation temperature T, eqs (1)–(3) can be obtained as following:

(5)Inε˙=InA−Q/(RT)+nIn[sinh(ασ)]

where A and α are constants, Q is the activation energy, R is gas constant, and n is the stress exponent. Moreover, according to the investigation of Zener and Hollomon [14, 15], the strain rate of metal materials at high temperature is controlled by the thermal activation. The relationship between strain rate ε and temperature T can be expressed by the parameter Z:

(6)Z=εexp[Q/RT]

where Z is a Zener–Hollomon parameter, which means the compensation of temperature on strain rate. Q reflects difficulty or ease of hot deformation of materials and Z is match below equation:

(7)Z=A[sinh(ασ)]

The maximum flow stresses (σ) of Ti-7Cu alloy at different temperature (T) and strain rate (έ) are adopted as shown in Figure 4. The results of Figure 4(a) and (b) showed that, a linear relationship between stresses and strains was determined according to the slope of the value β and α can be obtained based on eq. (4). The inverse of slope gives the stress exponent n in eq. (5). It can be seen that n is strain rate dependent. Over a limited range of strain rates, it could be considered as a constant. The plot of ln[sin h(σα)] vs 1/T for different stain rate is shown in Figure 3(d). The slope of the latter is referred to as the temperature sensitivity parameter. Taking into logarithm form eq. (7), the relationship of lnZ and ln[sinh(ασ)] is shown in eq. (8):

(8)lnz=lnA+nln[sinh(ασ)]

Figure 4

Compressing stress–strain curves of Ti-7Cu alloy (a) lnσ–lnε; (b) σ–lnε; (c) ln[sinh(ασ)]–lnε; (d) lnsinh(σα)–1/T

Figure 5 shows the relationship of lnZ and ln[sinh(ασ)], the calculated correlation coefficient (R² = 0.967) exhibited high accuracy, which indicates that the eqs (1) and (5) could illustrate the relationship between σ, ε and T. It can be found that both groups were presented linear curve, respectively, and α= 0.046, Q= 92.96 KJ/mol, A= e4.93can be obtained on the above basis. The constitutive relationship equation for Ti-7Cu alloy during semi-solid deformation at 1,223 K–1,473 K can be obtained as following:

(9)ε˙=e60.882[sinh(0.031σ)]11.612exp(−38.375/RT)

Figure 5

Relationship between flow stress and Z parameter

Discussion

Generally, the stress–strain curve articulates the intrinsic relationship of flow stress with flow behavior. However, during the semi-solid deformation of Ti-7Cu alloy, the flow curves exhibit various continuous softening, steady-state flow and work hardening under different processing conditions. Thus, it is quite difficult to determine the deformation mechanism only by the shape of the stress–stain curves, because several similar flow behaviors may result in different microstructural mechanisms during deformation. Therefore, the flow behavior of Ti-7Cu alloy needs to be more deeply analyzed, and the deformation mechanism will be further discussed by microstructures. The typical microstructures of Ti-7Cu alloy after conventional and semi-solid compression are shown in Figure 6. The effect of temperature on the evolution of the microstructures can be clearly seen from these optical micrographs. In Figure 6(a), the globular recrystallized α grains are obtained and almost no Ti₂Cu precipitates is observed. However, while in semi-solid state, it is found that more plate-shaped Ti₂Cu precipitated at grain boundary and on obviously recrystallized grains is observed, as shown in 6(b) and (c), respectively. Finally, a precipitates zone was formed along the grain boundary (Figure 6(d)). In addition, high strain rate reduces the amount of response time required by microstructural deformation [16], the effects of temperature and strain rate on the flow rate can be demonstrated by Ringeval [17]:

Figure 6

Micrographs of the Ti-7Cu alloy during semi-solid compression (a) 950°C, 0.05 s⁻¹; (b) 1,100°C, 0.05 s⁻¹ (c) 1,200°C, 0.05 s⁻¹; (d) 1,000°C, 0.005 s⁻¹; (e) 1,000°C, 0.5 s⁻¹; (f) 1,000°C, 5 s⁻¹

where v is flow rate, V is strain rate, f_l is liquid fraction, R is radius and H is height of the cylindrical material. It is obviously that the increasing of temperature induces more liquid to overcome the restriction of the solid particle movement, which could reduce the dislocation pile-up on the surface of Ti₂Cu/α–Ti, that is coincident with the observation result by transmission electron microscope (TEM) shown above (Figure 7). The effect of the strain rate on the flow stress may also be explained by the dislocation kinetics [16]:

Figure 7

The dislocation configuration of Ti-7Cu alloy after compressive deformation (a) 950°C, 0.05 s⁻¹; (b) 1,200°C, 0.05 s⁻¹

where ε is the strain rate, p is the mobile dislocation density, b is the Burgers vector of a perfect dislocation and m is the average dislocation velocity. According to the equations, once ε increases, the velocity of mobile dislocations m is required to increase, and then the applied stress σ also increases.

The above results indicate that semi-solid deformation at higher temperature and lower strain rate seems to promote more flowing liquid and lower velocity of dislocation for plastic deformation. By the analysis of the effects of deformation parameters on flow curves and microstructural evolution, it further indicates that the optimum condition for semi-solid processing of Ti-7Cu alloy is high temperature (about 1,323 K–1,373 K) and low strain rate (about 0.005-0.05 s⁻¹), which implies that the workability will be improved in this domain. This can be attributed to the following reasons: one is the lower resistance and better plasticity is good for the alloy, and the other is the lower segregated liquid and less Ti₂Cu precipitates on grain boundary, which is beneficial for homogeneous microstructure.

Conclusions

1. The deformation of Ti-7Cu alloy in semi-solid state is sensitive to temperature and strain rate, and the characteristics of rheological of the alloy during semi-solid deformation are similar to that of conventional deformation. The flow stress decreases with temperature increase, and increasing with strain rate increase. It belongs to the sensitive materials of positive strain rate.

2. Liquid plays a significant role in lubrication for the deformation of alloy Ti-7Cu, which reduces the dislocation pile-up of the interface, and forms a Ti₂Cu precipitates zone among the grain boundary during the subsequent solidification. Strain rate affects the microstructure response time of alloy during deformation, and larger strain increases the liquid flow rate, and cause high flow stress.

3. According to hyperbolic sine model, the deformation constitutive equation for Ti-7Cu alloy in semi-solid state is demonstrated as follows:

   ε˙=e4.93[sinh(0.046σ)]2.747exp(−92.96/RT)