Jian Bao

Bio: Jian Bao is an academic researcher from Henan University of Science and Technology. The author has contributed to research in topics: Deformation (engineering) & Dynamic recrystallization. The author has an hindex of 2, co-authored 10 publications receiving 6 citations.

Dynamic precipitation and recrystallization mechanism during hot compression of Mg-Gd-Y-Zr alloy

Abstract: Hot compression tests of Mg-Gd-Y-Zr alloy at different deformation temperatures and strains were carried out with Gleeble-1500 simulator at strain rate of 0.002 s−1. The microstructure evolution of 400 °C/0.002 s−1 sample under different strain was analyzed emphatically by transmission electron microscopy and electron backscatter diffraction technology. Dynamic precipitation characteristics and nucleation-expansion mechanism of dynamic recrystallization (DRX) were discussed in detail. The results indicated that the dynamic precipitation takes place prior to DRX, and the morphology, size and distribution of precipitates change with increasing strain. At the later stage of deformation, the large-size hard phase β-Mg5(Gd,Y) can induce the particle-stimulated nucleation (PSN) mechanism under large strain and act as an effective nucleation site for DRX. In addition, we constructed a schematic diagram of the DRX nucleation-expansion mechanism of Mg-Gd-Y-Zr alloy under high temperature deformation. The first layer of DRX grains is formed by discontinuous dynamic recrystallization (DDRX) mechanism characterized by grain boundary bulge out, and the precipitates distributed at the original grain boundary promote DDRX nucleation; The expansion of necklace structure depends on continuous dynamic recrystallization (CDRX) mechanism; Subgrain division can further refine DRX grains.

Effect of Sn addition on the deformation behavior and microstructural evolution of Mg-Gd-Y-Zr alloy during hot compression

Abstract: We have studied the microstructure evolution and deformation behavior of Mg-5Gd-3Y-(1Sn)-0.5Zr alloys during hot compression (T = 350 °C, 400 °C, 450 °C and 500 °C, e ˙ = 0.002 s−1, 0.01 s−1, 0.1 s−1 and 1 s−1) by transmission electron microscopy and electron backscattering technology. Sn can promote dynamic precipitation to activate the particle-stimulated nucleation (PSN) mechanism induced by the cluster precipitates and promote and dynamic recrystallization (DRX); in addition, Sn can inhibit the formation of high-angle grain boundaries (HAGBs) by reducing the activation of pyramidal and slip and delaying DRX. The two processes are in a competitive relationship with each other in the hot deformation of Mg-Gd-Y-Sn-Zr alloys. At low temperatures (350 °C–400 °C) and high strain rates, the former dominates: DRX is promoted, accompanied by a decrease in flow stress. At high temperatures (450 °C–500 °C) and low strain rates, the latter is dominant due to the absence of dynamic precipitation: DRX is delayed, and flow stress is increased accordingly. Flow stress between the two extreme deformation conditions is determined by the competitive relationship between them. We also found that the addition of Sn could increase the thermal deformation activation energy of Mg-Gd-Y-Zr alloys, weaken the texture and inhibit twin growth. Finally, we constructed a schematic diagram of the DRX mechanism during the thermal deformation process to illustrate the effects of PSN, CDRX, and DDRX on the evolution of the microstructure in detail.

Deformation behavior characterized by reticular shear bands and long chain twins in Mg-Gd-Nd(-Zn)-Zr alloys

Abstract: We analyzed the deformation behavior of Mg-Gd-Nd(-Zn)-Zr alloys under high temperature uniaxial compression, and reported two special nucleation mechanisms of dynamic recrystallization (DRX): Nucleation and expansion of DRX based on reticular shear bands (Shear band dynamic recrystallization, SBDRX) and long chain twins (Twin dynamic recrystallization, TDRX). The reticular shear bands in Mg-Gd-Nd-Zr alloy are mainly caused by the crossing of strain gradients from different directions, which is related to the high activity of pyramidal slip at high temperature. The addition of Zn makes the DRX mechanism mutate, and TDRX is dominant in Mg-Gd-Nd-Zn-Zr alloy. The nucleation of a large number of {10–12} twins is abnormal, which may be related to the decrease of I 2 stacking fault energy caused by Gd/Nd–Zn compound addition. Based on the mechanical properties and microstructure, it can be concluded that the addition of Zn can promote the DRX, reduce the deformation resistance and improve the workability.

Hot deformation behavior and dynamic precipitation characteristics of Mg-Gd-Nd(-Sm)-Zr alloys

Abstract: We deeply analyze the deformation behavior of Mg-Gd-Nd(-Sm)-Zr alloys under high temperature uniaxial compression, and explore the influence mechanism of Sm on the microstructure evolution of Mg-Gd-Nd-Zr alloy. We can draw the following conclusions: the addition of Sm delays the dynamic recrystallization of Mg-Gd-Nd-Zr alloy, resulting in the increase of stress and the decrease of machinability. There is no dynamic precipitation in Mg-Gd-Nd-Zr alloy at 400 °C/0.01 s-1/0.7, which is related to the high solute-vacancy binding energy of Nd. Under the same deformation condition, two kinds of dynamic precipitates with different size and shape appear in Mg-Gd-Nd-Sm-Zr alloy: fine particle phase and coarse lamellar phase. The two precipitates are Mg(Gd, Nd, Sm) with body-centered cubic structure, and are incoherent with the matrix. The coarse lamellar phase effectively hinders dislocation rearrangement, which explains the delay of dynamic recrystallization.

Effect of Nd on the microstructure and corrosion behavior of Mg-Gd-Nd-Zr alloys

Abstract: The corrosion behavior of Mg-12.5Gd-xNd-0.3Zr (x = 1, 2, 3 wt%) alloys were investigated by weight loss tests and electrochemical measurements in 3.5% NaCl solution. The results show that the increase of Nd content refine the grains gradually and promote the precipitates. Mg-12.5Gd-xNd-0.3Zr alloy is composed of α-Mg and Mg5Gd and Nd41Mg5 phase, and the semi-continuous phase is formed with addition of 3% Nd. The corrosion resistance of Mg-12.5Gd-xNd-0.3Zr alloys sharply increases with the increase of Nd element by weight loss tests and the tendency is proven by means of electrochemical measurements. The corrosion resistance is improved when Nd content increases from 1% to 3% owning to the diminishing grain size and the change of distribution for second phase. The second phase play a dual effect on corrosion which depend on its distribution.

Deformation behavior characterized by reticular shear bands and long chain twins in Mg-Gd-Nd(-Zn)-Zr alloys

Abstract: We analyzed the deformation behavior of Mg-Gd-Nd(-Zn)-Zr alloys under high temperature uniaxial compression, and reported two special nucleation mechanisms of dynamic recrystallization (DRX): Nucleation and expansion of DRX based on reticular shear bands (Shear band dynamic recrystallization, SBDRX) and long chain twins (Twin dynamic recrystallization, TDRX). The reticular shear bands in Mg-Gd-Nd-Zr alloy are mainly caused by the crossing of strain gradients from different directions, which is related to the high activity of pyramidal slip at high temperature. The addition of Zn makes the DRX mechanism mutate, and TDRX is dominant in Mg-Gd-Nd-Zn-Zr alloy. The nucleation of a large number of {10–12} twins is abnormal, which may be related to the decrease of I 2 stacking fault energy caused by Gd/Nd–Zn compound addition. Based on the mechanical properties and microstructure, it can be concluded that the addition of Zn can promote the DRX, reduce the deformation resistance and improve the workability.