Acta Metall Sin  2019, Vol. 55 Issue (2): 213-222    DOI: 10.11900/0412.1961.2018.00179
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Dynamic Softening Mechanisms of GH4720Li AlloyDuring Hot Deformation
Zhipeng WAN1,2(), Tao WANG1, Yu SUN2, Lianxi HU2, Zhao LI1, Peihuan LI1, Yong ZHANG1
1 Science and Technology on Advanced High Temperature Structural Materials Laboratory, AEEC Beijing Institute of Aeronautical Materials, Beijing 100095, China
2 National Key Laboratory for Precision Hot Processing of Metals, Harbin Institute of Technology, Harbin 150001, China
Abstract

GH4720Li alloy is a precipitation strengthened Ni-based superalloy and widely applied in high performance applications such as disks and blades of either aircraft engines or land-based gas turbines attributing to its excellent properties including resistance to creep and fatigue, together with corrosion, fracture and microstructural stability for the intended applications. Hot working is an effective way for shaping metals and alloys as well as changing the microstructure and mechanical properties. Lots of typical metallurgical behaviors such as dynamic recovery (DRV), discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) occur, which are related to the hot working parameters, including deformation temperature, strain rate and strain. In order to investigate the effect of deformation parameters on dynamic softening behavior and evolution of twinning for GH4720Li alloy, the hot deformation behavior of as-forged GH4720Li alloy was studied by isothermal compression tests. OM, SEM, EBSD and TEM techniques were employed to investigate systematically the dynamic softening mechanisms, formation of DRX grains and evolution of substructure in grains under different deformation parameters. The results showed that DDRX can take place at all studied deformation conditions. The boundary bulging and nucleation of DDRX grains were restrained as a result of decrease of dislocation substructures and subgrain boundaries density consumed by continuous original boundary migration (COBM) in deformed grains at low strain rates and high temperatures, and then the occurrence of DDRX was suppressed. DDRX was promoted as the strain rate was increased and uniform microstructures composed of fine equiaxed grains can be readily obtained as well. The microstructural changes showed that the pinning effect of fine undissolved γ' precipitates was able to hinder the dislocation movement and promote the formation of high density of dislocation substructures and subgrain boundaries in deformed grains. The increase in sub-boundary misorientation brought about by continuous accumulation of the dislocations was introduced by the deformation, and fine DRX grains formed by particle-induced continuous dynamic recrystallization (PI-CDRX). According to the evolution of twinning under various deformation conditions, the effect of deformation temperature and strain rate on the evolution of twinning was characterized by the occurrence of DRX behavior.

 Fig.1  OM (a) and SEM (b) images of initial microstructures of as-forged GH4720Li alloy Fig.2  Curves of softening stress-temperature of the GH4720Li alloy under various strain rates Fig.3  SEM images of the GH4720Li alloy deformed at 10 s-1 and a strain of 0.8 with temperatures of 1060 ℃ (a), 1080 ℃ (b), 1100 ℃ (c) and 1120 ℃ (d) Fig.4  OM images of the GH4720Li alloy deformed at 1 s-1 and a strain of 0.8 with temperatures of 1060 ℃ (a), 1080 ℃ (b), 1100 ℃ (c) and 1120 ℃ (d) Fig.5  OM images of the GH4720Li alloy deformed at 1100 ℃ and a strain of 0.8 with strain rates of 0.001 s-1 (a), 0.01 s-1 (b), 0.1 s-1 (c) and 10 s-1 (d) Fig.6  TEM images of the GH4720Li alloy deformed at 1060 ℃ and a strain of 0.35 with strain rate of 0.1 s-1 (a) and 1080 ℃ and a strain of 0.8 with strain rate of 0.001 s-1 (b) Fig.7  EBSD images of the GH4720Li alloy deformed at 1060 ℃, 1 s-1 and strains of 0.35 (a) and 0.8 (b) (>15°, 5°~15° and 2°~5° boundaries are indicated by thick-black, thin-green and thin-orange lines, respectively. Σ3, Σ9 and Σ27 twins are displayed by thick red, thick blue and thick yellow lines, respectively. DDRX—discontinuous dynamic recrystallization) Fig.8  Misorientations measured along the lines A1 (a) and A2 (b) marked in Fig.7a Fig.9  EBSD images of the GH4720Li alloy deformed at 1100 ℃ and a strain of 0.8 with strain rates of 0.001 s-1 (a), 0.1 s-1 (b), 1 s-1 (c) and 10 s-1 (d) (>15°, 5°~15° and 2°~5° boundaries are indicated by thick-black, thin-green and thin-orange lines, respectively. Σ3, Σ9 and Σ27 twins are displayed by thick red, thick blue and thick yellow lines, respectively. GOS—grain orientation spread, HGBs—high angle grain boundaries) Fig.10  EBSD images of the GH4720Li alloy deformed at strain rate of 0.1 s-1 and a strain of 0.8 at temperatures of 1060 ℃ (a), 1080 ℃ (b) and 1120 ℃ (c) (>15°, 5°~15° and 2°~5° boundaries are indicated by thick-black, thin-green and thin-orange lines, respectively. Σ3, Σ9 and Σ27 twins are displayed by thick red, thick blue and thick yellow lines, respectively. α—the angle between ’grain boundary’ and ’tangent to the γ’ phase’) Fig.11  Schematics of softening mechanisms evolution under different deformation conditions (PI-CDRX—particle-induced continuous dynamic recrystallization, COBM—continuous original boundary migration, ε—strain) Fig.12  Effect of strain rate $(ε˙)$ and temperature on the formation of twinning boundaries of the GH4720Li alloy