Acta Metall Sin  2018, Vol. 54 Issue (7): 969-980    DOI: 10.11900/0412.1961.2017.00461
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Study on Formation Mechanism of Necklace Structure in Discontinuous Dynamic Recrystallization of Incoloy 028
Xiting ZHONG1, Lei WANG1,2, Feng LIU1()
1 State Key Lab of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China
2 Tubular Goods Research Institute of CNPC, Xi'an 710077, China
Abstract

During hot deformation, discontinuous dynamic recrystallization (DDRX) taking place by nucleation and growth in materials with low to medium stacking fault energies (SFEs), plays a crucial role in grain refinement, especially for the material with coarse grains. In order to study the formation mechanism of typical microstructure (necklace structure) during DDRX, the behavior of Incoloy 028 alloy at temperature range of 1000~1150 ℃ and the strain rates of 0.001~1 s-1 was investigated by means of thermodynamic simulation, EBSD and TEM. The results show that with the decrease of deformation temperature or the increase of strain rate, the mechanism of DDRX is transformed from the traditional type nucleating at triple junctions, into necklace structure which dominated by the multilayer nucleation mechanism. The first strand of recrystallized grain is nucleated through the bulging of serrated grain boundaries which is assisted by twinning at the back of the fluctuation. With the increase of the true strain, the large strain gradient in the deformation band develops rapidly resulting in the transformation of the subgrain boundary into a high angle grain boundary, and then the second/followed layer nucleation occurs by the rotation of subboundaries accompanied with nucleation at triple junction. Twin boundaries are formed by strain-induced grain boundaries migration and disappeared after nucleation to enhance the recrystallization grain boundary mobility, and then formed again during growth to lower the interfacial energy of the system.

 ZTFLH: TG146.4
Fund: Supported by National Natural Science Foundation of China (No.51431008), National Key Research and Development Program of China (Nos.2017YFB0703001 and 2017YFB0305100) and Research Fund of the State Key Laboratory of Solidification Processing (No.117-TZ-2015)
 Fig.1  Initial microstructure of Incoloy 028 alloy Fig.2  Flow stress curves of Incoloy 028 alloy at the strain rates $(ε˙)$ of 0.001 s-1 (a), 0.01 s-1 (b), 0.1 s-1 (c) and 1 s-1 (d) (εp—peak strain, σp—peak stress, εs—steady-state strain, σs—steady-state stress) Fig.3  Microstructures of Incoloy 028 alloy deformed at $ε˙$=0.1 s-1, ture strain of 0.916 and the temperatures of 1000 ℃ (a), 1050 ℃ (b), 1100 ℃ (c) and 1150 ℃ (d) (Ds—steady-state grain size) Fig.4  Microstructures of Incoloy 028 alloy deformed at 1100 ℃, ture strain of 0.916 and strain rates of 0.001 s-1 (a), 0.01 s-1 (b), 0.1 s-1 (c) and 1 s-1 (d) Fig.5  TEM images of Incoloy 028 alloy deformed to true strain of 0.916 under the deformation of 1050 ℃, 0.001 s-1 (a, b) and 1050 ℃, 0.1 s-1 (c, d) (Inset in Fig.5c shows the local enlarged image) Fig.6  Microstructural evolution of Incoloy 028 alloy at T=1150 ℃ and $ε˙$=0.01 s-1 with the true strains of ε=0.15 (a), ε=0.32 (b), ε=0.6 (c) and ε=0.916 (d) (The compression axis (C.A.) is shown in Fig.6d; the white, black and red lines represent grain boundaries with misorientation angles (θ): <15° (sub-boundaries), >15° and twin boundaries, respectively) Fig.7  Microstructural evolution of Incoloy 028 alloy at T=1100 ℃ and $ε˙$=1 s-1 with the true strains of ε=0.25 (a), ε=0.35 (b), ε=0.6 (c) and ε=0.916 (d) (The white, black and red lines represent grain boundaries with θ<15°, θ>15° and twin boundaries, respectively; arrows in Fig.7c indicate the nucleation sites) Fig.8  OIM maps (a, c) and corresponding orientation analyses along lines (b, d) of Incoloy 028 alloy deformed at T=1150 ℃, $ε˙$=0.01 s-1 and ε=0.15 (a, b) and ε=0.32 (c, d) Fig.9  OIM maps (a, c, e) and orientation analyses alone lines (b, d, f) of Incoloy 028 alloys deformed at T=1100 ℃, $ε˙$=1 s-1 and ε=0.25 (a, b), ε=0.35 (c, d) and ε=0.6 (e, f) Fig.10  Misorientation angle distributions (a, b) and twin boundary density as a function of true strain (c) of the Incoloy 028 alloy deformed at T=1150 ℃, $ε˙$=0.01 s-1 (a) and T=1100 ℃, $ε˙$=1 s-1 (b)