Dynamic deformation behavior and microstructural evolution during high-speed rolling of Mg alloy having non-basal texture

doi:10.1016/j.jmst.2018.10.010

Abstract

Abstract:

The dynamic deformation behaviors and resultant microstructural variations during high-speed rolling (HSR) of a Mg alloy with a non-basal texture are investigated. To this end, AZ31 alloy samples in which the basal poles of most grains are predominantly aligned parallel to the transverse direction (TD) are subjected to hot rolling with different reductions at a rolling speed of 470 m/min. The initial grains with a TD texture are favorable for {10-12} twinning under compression along the normal direction (ND); as a result, {10-12} twins are extensively formed in the material during HSR, and this consequently results in a drastic evolution of texture from the TD texture to the ND texture and a reduction in the grain size. After the initial grains are completely twinned by the {10-12} twinning mechanism, {10-11} contraction twins and {10-11}-{10-12} double twins are formed in the {10-12} twinned grains by further deformation. Since the contraction twins and double twins have crystallographic orientations that are favorable for basal slip during HSR, dislocations easily accumulate in these twins and fine recrystallized grains nucleate in the twins to reduce the increased internal strain energy. Until a rolling reduction of 20%, {10-12} twinning is the main mechanism governing the microstructural change during HSR, and subsequently, the microstructural evolution is dominated by the formation of contraction twins and double twins and the dynamic recrystallization in these twins. With an increase in the rolling reduction, the average grain size and internal strain energy of the high-speed-rolled (HSRed) samples decrease and the basal texture evolves from the TD texture to the ND texture more effectively. As a result, the 80% HSRed sample, which is subjected to a large strain at a high strain rate in a single rolling pass, exhibits a fully recrystallized microstructure consisting of equiaxed fine grains and has an ND basal texture without a TD texture component.

Key words: Magnesium alloy, High-speed rolling, Texture, Dynamic recrystallization, Twinning

Sang-Hoon Kim, Jeong Hun Lee, Chong Soo Lee, Jonghun Yoon, Sung Hyuk Park. Dynamic deformation behavior and microstructural evolution during high-speed rolling of Mg alloy having non-basal texture[J]. J. Mater. Sci. Technol., 2019, 35(4): 473-482.

Figures/Tables 13

Fig. 1. (a) Schematic illustration depicting a sample for HSR machined from a hot-rolled plate and (b) its dimensions and sample coordinate system. (c) Optical micrograph and (d) XRD pole figure of initial sample for HSR. davg denotes the average grain size.

Table 1 Process parameters of high-speed rolling and microstructural characteristics of high-speed-rolled samples with different rolling reductions.

Rolling reduction	Process parameters		Microstructural characteristics
Rolling reduction	Strain	Strain rate (s^-1)	f_DRX (%)	d_avr (μm)	d_DRX (μm)	d_unDRX (μm)	I_max
20%	0.22	91	3.9	17.4	3.8	18.4	6.5
40%	0.51	128	35.4	9.4	3.7	13	4.9
60%	0.92	157	61.8	5.2	3.4	8.1	8.2
80%	1.61	181	89.9	4.7	4.5	6.4	6.6

Fig. 2. Optical micrographs of HSRed samples with rolling reductions of (a) 20%, (b) 40%, (c) 60%, and (d) 80%.

Fig. 3. Inverse pole figure maps of (a) initial sample and (b-e) HSRed samples with rolling reductions of (b) 20%, (c) 40%, (d) 60%, and (e) 80%. davg denotes the average grain size.

Fig. 4. Inverse pole figure maps of (a-d) DRXed and (e-h) unDRXed regions of HSRed samples with rolling reductions of (a, e) 20%, (b, f) 40%, (c, g) 60%, and (d, h) 80%. fDRX and funDRX denote the area fractions of DRXed and unDRXed regions, respectively.

Fig. 5. Variations in area fraction of DRXed grains and average grain sizes of total, unDRXed, and DRXed regions with imposed rolling reduction.

Fig. 6. Inverse pole figure maps and (0001) and (10-10) pole figures of (a) untwinned matrix region and (b) twinned region of 20% HSRed sample. (c) Misorientation map showing {10-12} twin boundaries and misorientation angle distribution of 20% HSRed sample.

Fig. 7. Inverse pole figure maps showing (a) partially and (c) completely twinned grains of 20% HSRed sample. (b, d) Point-to-point misorientation line profiles along directions indicated by arrows in (a) and (c). M, ET, CT, and DT denote the matrix, {10-12} extension twin, {10-11} contraction twin, and {10-11}-{10-12} twin, respectively.

Fig. 8. Inverse pole figure maps showing DRXed grains formed at {10-11} contraction and {10-11}-{10-12} double twins in 40% HSRed sample: (a) total region, (b) DRXed region, and (c) unDRXed region. The blue and yellow arrows in (c) indicate the {10-11} contraction twins and {10-11}-{10-12} double twins, respectively, which are formed in the {10-12} extension twin (ET).

Fig. 9. Inverse pole figure maps showing untwinned matrix and {10-12} twins remaining without occurrence of DRX in 60% HSRed sample: (a, d) total region, (b, e) DRXed region, and (c, f) unDRXed region. M and ET denote the matrix and {10-12} extension twin, respectively.

Fig. 10. Misorientation angle map showing high-angle grain boundaries and various twin boundaries in HSRed samples with rolling reductions of (a) 20%, (b) 40%, (c) 60%, and (d) 80%. (e) Variation in length per unit area of twin boundaries with imposed rolling reduction.

Fig. 11. (a-d) Kernel average misorientation (KAM) maps and (e-h) grain orientation spread (GOS) maps of HSRed samples with rolling reductions of (a, e) 20%, (b, f) 40%, (c, g) 60%, and (d, h) 80%. KAMavg and GOSavg denote the average KAM and GOS values, respectively.

Fig. 12. (0002) and (10-1) Pole figures of total, unDRXed, and DRXed regions of initial sample and HSRed samples. The numbers on the (0001) pole figures are the maximum intensity values of the basal texture.

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