Microstructure evolution during dynamic recrystallization of hot deformed superalloy 718

doi:10.1016/j.msea.2007.09.008

Materials Science and Engineering: A

Volume 486, Issues 1–2, 15 July 2008, Pages 321-332

https://doi.org/10.1016/j.msea.2007.09.008 Get rights and content

Abstract

Microstructure evolution during dynamic recrystallization (DRX) of superalloy 718 was studied by optical microscope and electron backscatter diffraction (EBSD) technique. Compression tests were performed at different strains at temperatures from 950 °C to 1120 °C with a strain rate of 10⁻¹ s⁻¹. Microstructure observations show that the recrystallized grain size as well as the fraction of new grains increases with the increasing temperature. A power exponent relationship is obtained between the dynamically recrystallized grain size and the peak stress. It is found that different nucleation mechanisms for DRX are operated in hot deformed superalloy 718, which is closely related to deformation temperatures. DRX nucleation and development are discussed in consideration of subgrain rotation or twinning taking place near the original grain boundaries. Particular attention is also paid to the role of continuous dynamic recrystallization (CDRX) at both higher and lower temperatures.

Introduction

Superalloy 718 is a nickel-base alloy strengthened predominately by ordered gamma double prime (γ″-Ni₃Nb) precipitates. Because of its good mechanical properties at elevated temperatures up to 650 °C, superalloy 718 has been widely used in gas turbine and related high temperature applications. Better control of the thermomechanical processing is of great importance for superalloy 718 to obtain a superior performance. It has been reported that dynamic recrystallization (DRX) is the responsible mechanism for hot restoration during high temperature deformation of superalloy 718 [1], [2], [3]. The microstructural characterization and recrystallization behavior in hot deformed superalloy 718 have been extensively studied using hot torsion or hot compression experiments [4], [5], [6], [7], [8], [9]. Mathematical models to predict the recrystallized grain size and recrystallization fraction as well as the processing maps were well developed in the past several decades. However, informations on the micromechanisms of DRX during the hot deformation in superalloy 718 were still limited [10].

As we know, a fundamental understanding of DRX is essential for modeling the microstructure evolution and analyzing its effect on flow behavior. The key to the comprehension of DRX lies on the physical knowledge of the nucleation mechanisms which are closely related to the microstructure evolution during deformation. Researches on single-phase polycrystalline materials have demonstrated the existence of different nucleation mechamisms of DRX, which was suggested to be strongly dependent on deformation variables. Meanwhile, different nucleation mechamisms of DRX may be happened simultaneously in a certain deformed material. Therefore, systematical investigation on the microstructure evolution under different deformation conditions, especially the nucleation mechanism of DRX for superalloy 718, is quite needed. Electron backscatter diffraction (EBSD) technique is a powerful method to obtain the detailed information on the orientations of grains and subgrains. Many researchers have studied the deformation microstructures and the nucleation mechanisms of DRX for different alloys by EBSD [11], [12], [13], [14], and it was also in application to the orientation analysis of superalloy 718 in the last few years [10], [15], [16]. The aim of the present study was to investigate the evolution of DRX microstructure in hot deformed superalloy 718. Misorientation measurements were systematically carried out to examine the nucleation as well as the progress of DRX. Particular attention was paid to the role of continuous subgrain rotation and the twin boundaries evolution with regard to the nucleation of DRX during hot deformation of superalloy 718.

Section snippets

Experimental procedure

The chemical compositions (wt.%) of superalloy 718 used in this investigation are as follows: Cr, 18.09; Fe, 17.69; Nb + Ta, 5.43; Mo, 3.07; Ti, 0.97; Al, 0.46; Co, 0.18; Si, 0.078; Mn, 0.065; Cu, 0.065; C, 0.040; S < 0.001; P < 0.007; Ni, balance. A 12.5 mm thick sheet was cut from an as-received wrought billet (252 mm in diameter and 152 mm in thickness) in the direction of cross section. Cylindrical specimens, with a diameter of 8 mm and a height of 12 mm, were machined from the center part of the

Flow characteristic and microstructures

True stress–true strain curves of superalloy 718 obtained at strain rate of 10⁻¹ s⁻¹ and at various temperatures from 950 °C to 1120 °C are shown in Fig. 2. The flow stress curves exhibit the similar features, i.e. a single peak at a critical strain followed by a strain softening stage and then sometimes a steady stage at high strain zone. The characteristics of the flow stress curves are the typical ones observed in many alloys, which implies the happening of DRX phenomenon during hot deformation

Flow softening and new grain evolution

The flow stress curves of superalloy 718 under hot deformation conditions, as shown in Fig. 2, are similar to that of superalloy 718 reported by other investigators [4], [5], [6], [7], [8], [9]. The increase of deformation temperature brings about an increase in mobility of grain boundaries and dislocations, which leads to the increase in the fraction of DRX grains (Fig. 3). A power exponent relationship between σ_p and D_R is obtained for superalloy 718, which was reported to derive from the

Conclusions

Superalloy 718 was hot deformed to different strains at temperatures from 950 °C to 1120 °C with a true strain rate of 10⁻¹ s⁻¹. The relationship between the stable dynamically recrystallized grain size and the peak stress can be expressed by a power law function with an exponent value of −0.75. Microstructure analysis substantiate that the nucleation of DRX in superalloy 718 can hardly be considered to only one mechanism. At higher temperature (T = 1100 °C), new grains were formed by bulging of

References (31)

L.X. Zhou et al.
Mater. Sci. Eng. A
(1995)
S.C. Medeiros et al.
Mater. Sci. Eng. A
(2000)
Y.-S. Na et al.
J. Mater. Proc. Technol.
(2003)
A.M. Wusatowska-Sarnek et al.
Mater. Sci. Eng. A
(2002)
P.J. Hurley et al.
Acta Mater.
(2003)
D. Ponge et al.
Acta Mater.
(1998)
M. Qian et al.
Mater. Sci. Eng. A
(2007)
Q. Li
Mater. Sci. Eng. A
(2007)
T. Sakai
J. Mater. Proc. Technol.
(1995)
S.I. Kim et al.
Mater. Sci. Eng. A
(2001)

B. Bay et al.

Acta Metall.

(1992)

D. Jorge-Badiola et al.

Mater. Sci. Eng. A

(2005)

A. Belyakov et al.

Acta Mater.

(2003)

H.J. Mcqueen

Mater. Sci. Eng. A

(2004)

O. Sitdikov et al.

Mater. Sci. Eng. A

(2002)

Cited by (206)

Microstructure characterization and dynamic recrystallization behavior of Ni–Cu alloy during hot deformation
2024, Mechanics of Materials
Flow curve and microstructure of Ni–30Cu alloy were studied after hot compression at 1150 °C and strain rate of 0.01 s⁻¹. The stress-strain and work hardening rate curves showed a weak peak at strain of 0.32, followed by a slight dynamic softening. The dominant microstructural mechanisms in low (0.1–0.23) and medium strains (0.23–0.32) were dynamic recovery and dynamic recrystallization, respectively. Microstructural characterizations by electron back scattered diffraction (EBSD) showed that continuous dynamic recrystallization brings about after the peak strain (ε = 0.32) and leads to grain refinement. It was found that the twin boundaries help to activate a grain dissociation mechanism which works as a new variant of continuous dynamic recrystallization. The grain boundary maps developed by EBSD showed that new twin boundaries form through the evolution of low angle grain boundaries. This step, named as “recovery twinning”, was identified as the origin for the observed twinning-assisted continuous dynamic recrystallization (TCDRX). The Kocks-Mecking dislocation evolution model was modified in order to take the dynamic softening by TCDRX into account and describe the variation in the frequency of high angle grain boundaries with strain. Model predictions were validated and confirmed by the experimental results.
Thermo-mechanical process variables driven microstructure evolution during additive friction stir deposition of IN625
2024, Additive Manufacturing
Additive friction stir deposition is an innovative technique for solid-state additive manufacturing involving complex thermo-mechanical processes such as frictional heating, plastic deformation, and viscoplastic material flow, which repeats layer by layer during material deposition. The present study is an attempt to understand the relationship between the complex thermo-mechanical process attributes and the resultant microstructure in additive friction stir deposition of IN625. The estimation of thermo-mechanical process attributes, primarily temperature and strain rate during multi-layer deposition was based on the already established thermo-pseudo-mechanical description of friction stir based processes. The complex thermo-mechanical interaction associated with multi-layer material deformation, deposition, and flash formation can be attributed to variations in the average deposition temperature and the strain rate. Microstructural examination of the additive friction stir deposited sample was performed using electron backscatter diffraction. As a result of the additive friction stir deposition associated thermo-mechanical conditions, the microstructure exhibited fine equiaxed grains in the plane along the build direction along with bands of very fine grains within the interface regions between deposited layers. The observation of fine equiaxed grains and a lower degree of in-grain misorientation in the additively fabricated sample pointed to dynamic recrystallization as the governing restoration mechanism during the deposition process. The grain size refinement during additive friction stir deposition of IN625 resulted in substantial augmentation in the yield strength compared to IN625 feed material and as-cast IN625.
Fabrication and performance evaluation of CICC jacket based on modified N50 austenitic steel for CFETR magnet
2024, Journal of Materials Research and Technology
The hot deformation behavior of modified N50 austenitic stainless steel in the temperature range 950∼1250 °C and strain rate range of 0.01–50/s was studied by uniaxial compression experiment. The processing map and the constitutive equation for the finite element simulation were established, and the recrystallization mechanism was discussed. The results indicated that the dominant deformation mode DDRX (discontinuous dynamic recrystallization) increased with the rise of deformation temperature while the CDRX (continuous dynamic recrystallization) was on the contrary. There is a decrease in global recrystallization with increasing strain rate, up to a threshold 1/s beyond which the global recrystallization began significantly promotion owing to not only the adiabatic heat and higher stored energy, but also quick Post-DRX mechanism induced by delay quenching. The reasonable hot working interval was determined by a hot working map, then the most suitable hot extrusion parameters (1180°C-100 mm/s) were further obtained with the help of finite element simulation, and ultimately, the industrial hot extrusion trials of N50 pipes with Post-DRX features were further completed. The tensile performance of final jacket pipes was far beyond the current mainstream low-temperature austenitic structural steel and met the requirements of the new generation of superconducting coil jacket materials.
Effect of deformation and annealing process on microstructure and properties of Inconel 718 foil
2023, Materials Characterization
In this study, the Inconel 718 foil with a thickness of 0.05 mm and an average grain size of 1.78 ⁓ 2.68 μm was prepared by multi-pass rolling and annealing. In the annealing process after rolling, two kinds of multistage heat treatments were designed to improve the plasticity and strength of the foil respectively by regulating precipitation phase. The RT process for plasticity enhancement (900 °C × 4 h + 960 °C × 1 h) provides a better elongation of 12.8% and a tensile strength of 1305 MPa. The sufficient pre-precipitation of the fine δ phase promotes nucleation and growth of recrystallized grain to refine the equiaxed grains and improve the strength of the alloy. Fine grains inhibit crack initiation and delay crack propagation to improve the toughness of the foil. Furthermore, the dispersed δ phase balances the stress at the grain boundaries and within the grain, resulting in uniform deformation of the grains to enhance the plasticity of the foil. Based on RT process, strength enhancement process (RT + DA process, RT process +720 °C × 8 h + 620 °C × 8 h) increases the tensile strength of the foil to 1480 MPa and reaches a suitable elongation of 8.52%. The improved tensile strength indicates a significant contribution of intensive γ” precipitation to impede the movement of dislocations. This paper is anticipated to provide as a reference for foil preparation regarding microstructure and property adjustment.
Subgrains of γ′ phase in a single-crystal superalloy induced by ultrahigh temperature creep
2023, Scripta Materialia
A new type of γ′ phase subgrain has been found in a single-crystal superalloy after exposure to 360 hours of creep at 1200 °C and 60 MPa. This signifies the capacity of dynamic recrystallization (DRX) of γ′ phase, overturning the widely accepted idea that it is incapable of undergoing DRX. The remarkable plasticity of the γ′-precipitate, activated at ultrahigh temperature, is likely due to the slip/rotation and cracking of the γ′-subgrains. Microstructural evidence of the subgrain boundary originating from γ′-shearing dislocations and their associated walls was obtained, which explains the subgrain formation dependence on plastic deformation accumulation. These findings help us understand the γ′-deformation mechanism and guide to improve creep resistance.
Mechanical properties and microstructure evolution during dynamic compression and cutting of nickel-based superalloys
2023, Journal of Manufacturing Processes
Superalloy GH4169 has been widely used on account of its excellent property. However, the microstructure characteristics of the machined surface will be changed significantly because of mechanical-thermal coupling effect after the cutting process, which affects the fatigue performance of key components. To clearly understand the microstructure development of surface layer during machining process, the mechanical behavior and microstructure evolution of GH4169 under high strain rate and high temperature conditions are studied in detail by dynamic compression tests. The peak stress can be improved by rising strain rate and decreasing temperature. Cutting experiments show that the depth of low angle grain boundaries distribution and plastic deformation in the machined surface layer are improved by rising cutting parameters. The degree of twin boundaries deviation from the ideal misorientation increases by improving cutting parameters. In addition, the geometrically necessary dislocation density increases first and then decreases by increasing distance from the machined surface. Furthermore, the cutting simulation model based on dynamic compression tests are established.

View all citing articles on Scopus

View full text

Microstructure evolution during dynamic recrystallization of hot deformed superalloy 718

Abstract

Introduction

Section snippets

Experimental procedure

Flow characteristic and microstructures

Flow softening and new grain evolution

Conclusions

Mater. Sci. Eng. A

Mater. Sci. Eng. A

J. Mater. Proc. Technol.

Mater. Sci. Eng. A

Acta Mater.

Acta Mater.

Mater. Sci. Eng. A

Mater. Sci. Eng. A

J. Mater. Proc. Technol.

Mater. Sci. Eng. A

Acta Metall.

Mater. Sci. Eng. A

Acta Mater.

Mater. Sci. Eng. A

Mater. Sci. Eng. A