J. Mater. Sci. Technol. ›› 2018, Vol. 34 ›› Issue (2): 265-276.DOI: 10.1016/j.jmst.2017.11.019
Special Issue: 2017-2018年Mg合金专题
• Orginal Article • Previous Articles Next Articles
Sang-HoonKima, Jong UnLeea, Ye JinKima, Jun HoBaeb, Bong SunYoub, Sung HyukParka(
)
Received:2017-08-03
Revised:2017-08-25
Accepted:2017-08-25
Online:2018-02-10
Published:2018-02-10
Sang-HoonKim, Jong UnLee, Ye JinKim, Jun HoBae, Bong SunYou, Sung HyukPark. Accelerated precipitation behavior of cast Mg-Al-Zn alloy by grain refinement[J]. J. Mater. Sci. Technol., 2018, 34(2): 265-276.
Fig. 1. (a), (b) Optical micrographs and (c), (d) SEM micrographs of as-homogenized alloys: (a), (c) normally fabricated AZ92 and (b), (d) grain-refined AZ92. davg denotes the average grain size.
Fig. 2. Age-hardening curves of normally fabricated AZ92 and grain-refined AZ92. Region A: Hardness of both alloys increases. Region B: Hardness of AZ92 increases whereas that of GR-AZ92 decreases. Region C: Hardness of both alloys decreases.
Fig. 3. Optical micrographs of (a) normally fabricated AZ92 and (b) grain-refined AZ92 after aging for 1 h.
Fig. 4. Variation in (a) area fraction and (b) thickness of discontinuous precipitates with aging time for normally fabricated AZ92 and grain-refined AZ92.
Fig. 5. SEM micrographs of (a), (c) normally fabricated AZ92 and (b), (d) grain-refined AZ92 after aging for 4 h. (c), (d) Enlarged images of red-colored rectangular areas in (a) and (b), respectively.
Fig. 6. SEM micrographs of (a), (c) normally fabricated AZ92 and (b), (d) grain-refined AZ92 after peak aging. (c), (d) Enlarged images of red-colored rectangular areas in (a) and (b), respectively. The areas marked with red circles indicate the matrix regions where continuous precipitates are formed to a lesser extent. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 7. Tensile stress-strain curves of homogenized and peak-aged alloys. GR indicates the grain-refined sample.
| Heat treatment condition | Alloy | Aging properties | Tensile properties | |||
|---|---|---|---|---|---|---|
| Peak-aging time (h) | Hardness (Hv) | YSa (MPa) | UTSa (MPa) | ELa (%) | ||
| Homogenized | AZ92b | - | 67.7 (±7) | 105 (±5) | 213 (±4) | 7.6 (±0.4) |
| GR-AZ92b | - | 72.4 (±4) | 125 (±1) | 281 (±5) | 12.1 (±1.5) | |
| Peak-aged | AZ92 | 24 | 96.2 (±3) | 143 (±3) | 245 (±6) | 3.2 (±0.4) |
| GR-AZ92 | 8 | 100.4 (±6) | 161 (±2) | 256 (±9) | 2.5 (±1.1) | |
Table 1 Aging and tensile properties of homogenized and peak-aged alloys.
| Heat treatment condition | Alloy | Aging properties | Tensile properties | |||
|---|---|---|---|---|---|---|
| Peak-aging time (h) | Hardness (Hv) | YSa (MPa) | UTSa (MPa) | ELa (%) | ||
| Homogenized | AZ92b | - | 67.7 (±7) | 105 (±5) | 213 (±4) | 7.6 (±0.4) |
| GR-AZ92b | - | 72.4 (±4) | 125 (±1) | 281 (±5) | 12.1 (±1.5) | |
| Peak-aged | AZ92 | 24 | 96.2 (±3) | 143 (±3) | 245 (±6) | 3.2 (±0.4) |
| GR-AZ92 | 8 | 100.4 (±6) | 161 (±2) | 256 (±9) | 2.5 (±1.1) | |
Fig. 8. Optical micrographs of tensile-fractured samples of (a), (b) homogenized and (c), (d) peak-aged alloys: (a), (c) normally fabricated AZ92 and (b), (d) grain-refined AZ92. The fracture mechanism mode changes from twinning-induced cracking in the homogenized state to precipitate-induced cracking in the peak-aged state.
Fig. 9. Variation in (a) age-hardening rate of normally fabricated AZ92 and grain-refined AZ92 and (b) difference in total hardness between normally fabricated AZ92 and grain-refined AZ92 during aging treatment.
Fig. 10. Hardness curves of discontinuous precipitates and matrix in normally fabricated AZ92 and grain-refined AZ92. PAT denotes the peak-aging time.
Fig. 11. SEM micrographs of discontinuous precipitates after aging treatment for (a), (d) 1 h; (b), (e) 4 h; and (c), (f) 64 h: (a)-(c) normally fabricated AZ92 and (d)-(e) grain-refined AZ92.
Fig. 12. Schematic illustration showing difference in precipitation behavior of continuous and discontinuous precipitates of Mg17Al12 between normally fabricated AZ92 with coarse grains (420 μm) and grain-refined AZ92 with fine grains (91 μm).
Fig. 13. Variation in (a) difference in area fraction of discontinuous precipitates, (b) hardness difference between discontinuous precipitates and matrix, and (c) difference in matrix hardness with aging time. (a), (b), and (c) represent the first, second, and third terms, respectively, of Eq. (6), which express the hardness difference between normally fabricated AZ92 and grain-refined AZ92.
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