Characterization of hot deformation behavior of a P/M nickel-base superalloy using processing map and activation energy

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Abstract

The hot deformation behavior of a nickel-base superalloy prepared by powder metallurgy (P/M) was investigated by using isothermal constant strain rate compressing tests conducted in deformation temperature from 1000 °C to 1100 °C and strain rate from 0.001 s−1 to 1 s−1. The true stress–true strain curves show that the alloy has higher strength at the temperature of 1000 °C. The processing maps for hot working were established based on the variations of efficiency of powder dissipation with deformation temperature and strain rate, interpreted using dynamic material model (DMM). The processing maps demonstrate that strain markedly affected the instability regions. The efficiencies of powder dissipation obtained in the strain range from 0.1 to 0.7 are essentially similar, which indicates that strain has a slight influence on it. The alloy exhibits a better workability in deformation temperature range from 1060 °C to 1100 °C and strain rate range from 1 × 10−1.5 s−1 to 1 × 10−0.5 s−1 owing to lower activation energy. Moreover, the fine recrystallized grain region can be identified on the basis of Zener–Hollomon parameter map at strain of 0.6.

Research highlights

▶ Strain has significant influence on the instability regions. ▶ Activation energy is closely related to dislocation density. ▶ Strengthening phases γ′ promote activation energy. ▶ Dislocation density increases with the increasing of strain rate. ▶ The sensitivity of activation energy to temperature decreases with the increase of temperature.

Introduction

The P/M nickel-base superalloy (nominal composition Ni–15Cr–4W–3Mo–2Al–3Ti), which is produced by powder metallurgy (P/M) process, has uniform microstructure, high working temperature and low crack growth rate. In aviation industry, the excellent working and mechanical properties make it widely applicable for gas turbine application as high-temperature attaching parts. For this superalloy, many products are usually fabricated by hot forging and extrusion. However, the flaws may be generated during deformation owing to narrow forging temperature range and unstable forged microstructure. Controlling the forging processes to attain a uniform microstructure seems to be very important. Thus, a comprehensive understanding of the hot deformation behavior of this alloy is essential.

It has been widely accepted that processing map is very useful for optimizing hot working processes and controlling microstructure in this class of material. With the help of processing map, it is possible to find the optimum parameters for designing a hot working process without resorting to expensive and time-consuming trial-and-error methods. Liu et al. [1] obtained an optimum condition for the hot working of Haynes230 alloy. Their results showed that the alloy exhibited a good workability at the deformation temperature of 1150 °C and the strain rate of 0.001 s−1. Krishna et al. [2] developed the processing maps of near-α titanium alloy 685 on the basis of dynamic material model. The optimum conditions for α–β working were at the deformation temperature of 975 °C and the strain rates of lower than 0.02 s−1. Prasad et al. [3] investigated the hot working of a P/M iron-aluminide alloy with the help of processing map, and obtained the superplasticity domain (1000–1050 °C, 1 × 10−3–1 × 10−1 s−1). Reddy et al. [4] investigated the hot working of Al–Li alloy, and pointed out two domains with peak efficiencies were associated with cracking processes due to grain boundary cavitation and flow localization. Wang et al. [5] pointed out the fraction of dynamic recrystallized grains in superalloy 718 increased with the increase of deformation temperature and the decrease of strain rate. The results showed that dynamic recrystallization was strongly dependent on the value of Zener–Hollomon parameter. Cai et al. [6] investigated a nickel-base superalloy in deformation temperature from 1050 °C to 1180 °C and strain rate from 0.01 s−1 to 10 s−1, pointing out strain had a slight effect on the processing maps. Zhang et al. [7] developed the processing maps of a P/M titanium-aluminide alloy and pointed out that superplastic deformation occurred at the deformation temperature of 1100 °C and the strain rate of 0.001 s−1 with peak efficiency of 60%. Sivakesavam and Prasad [8] found that the processing maps obtained at various strains were essentially similar. Wang et al. [9] found that the instability region of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy increased with the increasing of strain. Li and Zhang [10] investigated the hot deformation behavior of Ti–6Al–4V alloy basing on processing maps at various strains and found that hydrogen content affected the instability parameter.

Processing map is developed on the basis of the dynamic material model (DMM), which aims to correlate the constitutive behavior with microstructure evolution, flow instability and hot workability [11], [12], [13]. According to dynamic material model, the total power P consists of two complementary parts: G content representing the power dissipated by plastic work, and J co-content related to the microstructural evolution. The total power is described as follows [14], [15]:P=σε˙=G+J=0ε˙σdε˙+0σε˙dσwhere ε˙ is the strain rate. The dissipated power J is obtained by:J=0σε˙dσ=mm+1σε˙

The value of J for a non-linear dissipater is normalized with that of a linear dissipater (m = 1) to obtain an efficiency of power dissipation (EPD), given by:η=JJmax=2mm+1where m is the strain rate sensitivity exponent and can be described as follows:m=dJdGT,ε=lnσlnσε˙T,ε

The instability criterion ξ(ε˙) is developed based on the extremum principles of irreversible thermodynamic as applied to large plastic flow [16]. This criterion represents the maximum rate of energy production in material system, and when ξ(ε˙)<0, the material may generate flow instabilities. The instability criterion ξ(ε˙) is given by:ξ(ε˙)=log(m/(m+1))logε˙+m<0

The variation of dimensionless parameters ξ(ε˙) and η with deformation temperature and strain rate constitutes processing map, which exhibits various domains that may be correlated with specific microstructural mechanisms.

In this study, the hot deformation behavior of a P/M nickel-base superalloy was systematically investigated by isothermal constant strain rate compressing tests, which were conducted in deformation temperature range from 1000 °C to 1100 °C and strain rate range from 0.001 s−1 to 1 s−1. The dependence of flow behavior and micorsturctural evolution on deformation temperature and strain rate was established by processing map. The hot workability was studied based on activation energy and Zener–Hollomon parameter. The effects of strain rate and deformation temperature on activation energy were investigated by dislocation density. Further, the ranges of optimal forming parameters were developed.

Section snippets

Experimental procedures

The main chemical components of the P/M nickel-base superalloy used for isothermal constant strain rate compressing test are shown in Table 1. The nickel-base superalloy powder was prepared by plasma rotation electronic pole (PREP) with average diameter of 115 μm (Fig. 1(a)). It can be seen that the powder particles were nearly spherical in shape (Fig. 1(b)). The initial specimens were obtained by hot isostatic pressing (HIP) conducted at the deformation temperature of 1200 °C and the pressure of

Flow stress–strain behavior

The typical true stress–true strain curves of the P/M nickel-base superalloy obtained at different deformation temperatures from 1000 °C to 1100 °C with various strain rates from 0.001 s−1 to 1 s−1 are shown in Fig. 3(a)–(c). The flow stress decreases with the increase of deformation temperature and the decrease of strain rate. At the strain rate of 0.1 s−1, the stress decreases from 198.51 MPa to 83.90 MPa as the deformation temperature increases from 1000 °C to 1100 °C (Fig. 3(d)). However, it is

Conclusions

  • (1)

    At higher strain rates (>0.1 s−1), this P/M nickel-base superalloy exhibits higher strength at deformation temperatures of 1000 °C, 1050 °C and 1100 °C. However, after peak stress, it shows obvious softening phenomenon with the increasing of strain. The flow stress fluctuating with lower amplitude at lower strain rates (<0.01 s−1) indicates DRV and DRX are dominant deformation mechanisms in this condition.

  • (2)

    It can be seen from processing maps that two instability regions are as follows: one is in

References (30)

  • Y. Liu et al.

    J. Mater. Process. Technol.

    (2009)
  • V.G. Krishna et al.

    J. Mater. Process. Technol.

    (1997)
  • Y.V.R.K. Prasad et al.

    Intermetallics

    (2000)
  • Y. Wang et al.

    Mater. Sci. Eng. A

    (2008)
  • D.Y. Cai et al.

    Mater. Charact.

    (2007)
  • W. Zhang et al.

    J. Mater. Process. Technol.

    (2009)
  • O. Sivakesavam et al.

    Mater. Sci. Eng. A

    (2003)
  • K. Wang et al.

    Mater. Charact.

    (2009)
  • M.Q. Li et al.

    Mater. Sci. Eng. A

    (2009)
  • M.C. Somani et al.

    Mater. Sci. Eng. A

    (1998)
  • M.C. Somani et al.

    J. Mater. Process. Technol.

    (1995)
  • Y.V.R.K. Prasad et al.

    Mater. Sci. Eng. A

    (1998)
  • J. Luo et al.

    Mater. Sci. Eng. A

    (2009)
  • G. Meng et al.

    Mater. Sci. Eng. A

    (2009)
  • S.A. Sajjadi et al.

    Mater. Sci. Eng. A

    (2001)
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