Flow behavior of Ti-46.2Al-2.5V-1.0Cr-0.3Ni alloy in

secondary hot deformation

SI Jia-yong(司家勇), HAN Peng-biao(韩鹏彪), CHANG Xia(昌 霞), ZHANG Ji(张 继)

High Temperature Material Research Division, Central Iron and Steel Research Institute, Beijing 100081, China

Received 28 July 2006; accepted 15 September 2006

Abstract: The flow behavior of already forged Ti-46.2Al-2.5V-1.0Cr-0.3Ni alloy was investigated by the isothermal compression experiments. The direction of secondary hot deformation was taken to be vertical to the former forging axis. And the deformation activation energy was calculated. Specimens have three kinds of starting microstructures, i.e. as-forged, relief annealed and duplex. The true strain—stress curves show that the duplex microstructure has the lowest flow resistance, better steady-state flow behavior compared with other two microstructures. It is found that obtaining duplex microstructure makes the work hardening rate and the strain rate sensitivity increase. The duplex microstructure alloy has the lowest value.

Key words: gamma TiAl; flow behavior; secondary hot deformation

1 Introduction

Two-phase gamma TiAl alloys are a prospective light material for high temperature structural applications such as automotive and aerospace engine components, and had been investigated extensively in the last decades[1-3]. Thermomechanical treatment has been used for the microstructure homogenization and grain refinement in order to improve the mechanical properties of TiAl alloys[4-8].

It has been reported[9-10] that the hot-deformation behavior and the associated mechanisms of two-phase gamma TiAl alloys depend mainly on the alloy composition and the starting microstructure. For example, the orientation of the lamellar structure, which exists in most TiAl alloys, especially cast ingots, strongly influences their deformation condition[11]. It is difficult for the lamellar colony to deform when its orientation is perpendicular or parallel to the pressure direction (hard orientation); while it will be easy when the orientation angle is around 45?(soft orientation). Therefore remnant lamellas are usually found in the forged alloy because of the lamellar orientation’s dispersity[12]. It will form the non-uniform microstructure and reduce the mechanical properties of wrought TiAl alloys. Thus, in many cases, secondary hot deformation is needed to broken down those residual lamellar structures further.

In this study, a series of isothermal compression tests were conducted to reveal the flow behavior of already forged gamma TiAl alloy in secondary hot deformation. The influence of the starting microstructures was also discussed. Work hardening rate, strain rate sensitivity and activation energy of deformation were calculated from the true strain—stress curves.

2 Experimental

Ti-46.2Al-2.5V1.0Cr-0.3Ni alloy was prepared by cold crucible induction levitation melting and cast as a cylinder ingot of d100 mm×140 mm. Ingot was hot isostatic pressed at 1 320 ℃, 150 MPa in argon to eliminate cast pores. Then ingot was isothermally forged to 70% height reduction at 1 100℃.

Cylindrical compression specimens with d8 mm×12 mm, were taken from the already forged TiAl alloy with their axis vertical to the former forging direction. The specimens were divided into three groups according to the microstructures after heat treatment. All the tested samples were coated with glass as lubricant, put inside a radiant heater, and equilibrated at the test temperature for 10 min before the testing commenced. The specimens were compressed to a final engineering strain of 0.7 (approximately 150% of true compressive strain). The surfaces of metallographic specimens were taken in accordance with the length direction. Table 1 lists the heat treatment condition and starting microstructure for the hot deformation. The test conditions are listed in Table 2.

Table 1 Starting microstructure of test alloys

Table 2 Compression test condition

3 Results and discussion

3.1 Starting microstructure

Remnant lamellas are still found in the 70%-forged pancake (shown in Fig.1(a)). Specimens annealed at 950 ℃ for 6 h and air-cooled show a similar microstructure to the as-forged. Both of the microstructures consist of severe distortion, including bending and kinking and some fine, equiaxed grains precipitated along the colony boundaries. More equiaxed γ grains appears at the boundaries of the lamellar colonies as shown in Fig.1(b). The microstructure transforms to duplex containing equiaxed γ grains and lamellar colonies after annealed at 1 260 ℃ for 24 h and air cooled (Fig.1(c)).

3.2 Flow behavior

The true stress-strain curves of three kinds of starting microstructures were put together corresponding to the same test conditions for comparison. Typical results are shown in Fig.2. The flow behavior data of the tested alloys, σ_P and the correspondent strain ε_P, as well as the steady-state flow stress σ_1.0 were measured and listed in Table 3. The values of σ_P-σ_1.0 were also calculated and included in Table 3 to show the amount of flow softening in quantity.

The dynamic recrystallized stress—train curves were characterized by a distinct peak at the beginning stage of the curve followed by a gradual smoothing until a flat line appears (flow softening). The true stress— strain curves in Fig.2 show that TAC-2N_D exhibits much lower peak flow stress (σ_P) and less flow softening than TAC-2N and TAC-2N_M. It can be concluded that TAC-2N_M and TAC-2N_D have better steady-state flow behavior than TAC-2N. Furthermore, the results of each alloy have manifested the same rule that the degree of softening is high at elevated strain rates and low temperatures[13].

Fig.1 Optical microstructures of alloy TAC-2N, TAC-2N_M and TAC-2N_D: (a) Already forged; (b) Annealed at 950 ℃ for 6 h and air cooled; (c) Annealed at 1 260 ℃ for 24 h and air cooled

Table 3 Flow behavior data of the tested alloys

Fig.2 True stress—strain curves of alloy TAC-2N, TAC-2N_M and TAC-2N_D

In addition, ε_P of TAC-2N_D and TAC-2N_M is smaller than that of TAC-2N especially at the higher tempera- tures and lower strain rates. But when comparing TAC-2N_D with TAC-2N_M, some values of TAC-2N_D are lower than those of TAC-2N_M and others are close to or little higher than those of TAC-2N_M. Because ε_P is the reflection of the balance of work hardening and dynamic recrystallization, these indicate that already forged TiAl alloy heat treated to duplex microstructure is easy to dynamic softening during the successive hot deformation. The duplex microstructure shows the lowest flow resistance, better steady-state flow behavior.

3.3 Work hardening rate

The work hardening rates of the test alloys can be calculated by Eqn.(1) at constant strain rate and temperature.

θ=?σ/?ε                                     (1)

The working hardening rate-strain curves of 1 000 ℃ and 0.01 s^-1 test conditions are shown in Fig.3. It can be seen that TAC-2N_D exhibits the highest θ than TAC-2N and TAC-2N_M before the sharp fall to zero. This indicates that microstructure modification can increase the work hardening rate of the already forged gamma alloy, which is good for the homogenous deformation and grain refinement in secondary hot deformation.

During the hot deformation process the mechanisms controlling the flow stress are hardening and recovery. The mechanical properties of the alloy during hot deformation vary as a result of a balance between the work hardening(strain hardening) and dynamic softening (dynamic recovery and dynamic recrystallization) processes[14]. And in this study, the θ of the tested alloys is nearly constant before dropping down sharply after certain amounts of deformation. It suggests that no obvious dynamic recovery occurs after arriving at the stress peak.

Fig.3 Working hardening rate—strain curves at 1 000 ℃ and 0.01 s^-1

3.4 Strain rate sensitivity

The strain rate sensitivity m can be calculated from the peak flow stress σ_P and the correspondent strain ε_P by the following equation.

                      (2)

The results are shown in Table 4. These figures indicate that m of TAC-2N and TAC-2N_M samples is roughly at the same level but obviously lower than that of the duplex TAC-2N_D alloy.

Table 4 Values of m of test alloys

The research on Ti-48Al-2Nb-2Cr indicates that the absence of lamellar-γ/α₂ phase morphology in the microstructure is of benefit to the gamma alloy in terms of exhibiting high strain rate sensitivity [10]. In this study, the duplex microstructure has been proved to increase m about 0.022, while the relief annealed microstructure just raises the value about 0.003. Thus the duplex microstructure appears to have a higher positive influence on the strain rate sensitivity of already forged gamma TiAl alloys.

3.5 Activation energy of deformation

It has been suggested that the flow stress can be expressed with a hyperbolic function as in Eqn.(3), where Z is related to the Zenear-Hollomon parameter Z given by Eqn.(4)[16],

Z=A{sinh(ασ)}ⁿ                               (3)

                           (4)

Eqn.(4) can be transformed into Eqn.(5) or Eqn.(6) when taking  or T as constant, respectively.

Q=βR(?σ/?(1/T))                              (5)

                               (6)

The activation energy of deformation in the studied alloy was calculated using Eqns.(5) and (6). All the data were obtained from Table 3. The result was put together with the activation energy values of other forged TiAl alloys consisting of normal lamellar microstructures or near gamma microstructure with equiaxed fine grains, it is shown in Table 5. The already forged alloy TAC-2N_D has the lowest activation energy of deformation among the listed alloys at this temperature range. This indicates that the deformation mechanisms, such as dislocation sliding and climbing, can be easily started in the duplex microstructure alloy TAC-2N_D.

Table 5 Activation energy of deformation in TiAl alloys

4 Conclusions

1) Already forged TiAl alloy with duplex microstructure shows the lowest flow resistance, better steady-state flow behavior.

2) As for the work hardening rate, the value of duplex microstructure is the highest. It is good for the homogenous deformation and grain refinement in successive secondary hot deformation.

3) Compared to other two microstructures, the strain rate sensitivity of duplex microstructure is obviously increased.

4) The deformation activation energy of already forged alloy with duplex microstructure is the lowest. It means that dislocation sliding and climbing can be easily started in the duplex alloy.

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(Edited by CHEN Can-hua)

Foundation item: Project(2002AA305209) supported by the High Tech Research and Development Program of China

Corresponding author: SI Jia-yong; Tel: +86-10-62181009; E-mail: sjy98106@163.com