Microstructure Modification of Powder Compact of Al-Zn-Mg Nanostructured Alloy by a Semisolid Thermomechanical Processing

doi:10.1007/s40195-015-0360-3

Abstract

Abstract:

A thermomechanical process (TMP) consisting of three cycles of cold pressing at 154 MPa and liquid-phase sintering at 600°C for 30 min in each cycle was applied to modify the microstructure of nanostructured Al-Zn-Mg alloy. The alloy powders were produced by mechanical alloying. Also, solid-state sintering at 550°C for 90 min was done to compare the results with those obtained from the TMP. The powders and the thermomechanically (TM) processed samples were analyzed by XRD to reveal the present phases in addition to calculating the crystallite size changes by the Williamson-Hall method. Moreover, scanning electron microscope was employed to observe the morphology of the powder and the microstructures of the sintered and the TM processed samples. The results revealed that the TMP affected the microstructure noticeably as well as the microhardness by removing the continuous grain boundary porosities and uniform distribution of the intermetallic phase particles as well as obtaining a near globular microstructure after the second cycle. Also, the average grain sizes in the first and the second cycles of the TMP were lower than those of the sintered sample. Furthermore, nanocrystalline grains were stable up to the second cycle of the TMP.

Key words: Thermomechanical processing, Al-Zn-Mg alloy, Intermetallics, Sintering, Grain growth

Mohsen Hajizamani, Mostafa Alizadeh, Seyed Ahmad Jenabali-Jahromi. Microstructure Modification of Powder Compact of Al-Zn-Mg Nanostructured Alloy by a Semisolid Thermomechanical Processing[J]. Acta Metallurgica Sinica (English Letters), 2016, 29(1): 39-45.

Figures/Tables 11

Fig. 1 Solid fraction versus temperature for the synthetized alloy from DSC analysis

Table 1 Nomenclatures used for the samples in this research

Nomenclature	Process
MAA-40	Al-Zn-Mg powder mechanically alloyed for 40 h
MAA-s	Al-Zn-Mg powder cold-pressed under 308 MPa + solid-state sintering at 550°C for 90 min
MAA-TM1	Al-Zn-Mg powder cold-pressed under 154 MPa + modification process by LPS at 600°C for 30 min
MAA-TM2	Al-Zn-Mg powder cold-pressed under 154 MPa + modification process by LPS at 600°C for 30 min + cold-pressed under 154 MPa + LPS at 600°C for 30 min
MAA-TM3	Al-Zn-Mg powder cold-pressed under 154 MPa + modification process by LPS at 600°C for 30 min + cold-pressed under 154 MPa + LPS at 600°C for 30 min + cold-pressed under 154 MPa + LPS at 600°C for 30 min

Fig. 2 XRD patterns of the solid-state sintered and the thermomechanically processed samples

Fig. 3 Changes in the Al main peak from the solid-state sintering to the third cycle of the TMP

Fig. 4 Crystallite sizes of the solid-state sintered and the thermomechanically processed samples calculated by the W-H method

Fig. 5 The SEM images of the samples perpendicular and parallel to the pressure direction: a MAA-s, perpendicular; b MAA-s, parallel; c MAA-TM1, perpendicular; d MAA-TM1, parallel; e MAA-TM2, perpendicular; f MAA-TM2, parallel; g MAA-TM3, perpendicular; h MAA-TM3, parallel

Fig. 6 Morphology of the mechanically alloyed powder

Table 2 Quantitative results of the EDS analysis (at%)

Point in Fig. 5	Al	Zn	Mg
1	0	64	36
2	0	83	17
3	30	33	37
4	24	21	55

Fig. 7 EDS analysis results for points 1-4 in Fig. 5

Fig. 8 Average grain sizes of the solid-state sintered and the thermomechanically processed samples

Fig. 9 Results of microhardness measurements

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