Mechanical properties of Mg-based materials fabricated by mechanical milling and spark plasma sintering

Abstract

In this work, magnesium powders of different grain sizes were synthesized by mechanical milling for periods ranging from 0.5 to 30 h. Subsequent to milling, powders were consolidated by spark plasma sintering at 550 ℃ for 10 min. Before and after sintering, microstructural changes were investigated by analytical methods including X-ray diffraction (XRD), X-ray spectrometer, optical and electron microscopy. Analyses showed that nanocrystalline sizes were achieved by mechanical milling for milling times exceeding 5 h. Additionally, it was recognized that grain growth occurred during sintering, but to a limited extent. Mechanical test results displayed reasonable improvements in both compressive yield strength and hardness values with increasing milling times up to 5 h, where these reached their maximum values (245.5 MPa and 75.9 HV). The enhancement in these properties with increased milling time up to 5 h was attributed to both the extent of grain refinement and the formation of MgO together with incorporation of Fe particles, originating from the milling process, into the matrix. On the other hand, a substantial decrease in yield strength and hardness values in the samples milled in excess of 5 h were recorded, which in turn was related to the accompanying decline in bulk density of the samples. Microstructural analysis of the deformed samples revealed that grain size reduction suppressed twin formation, which elucidates the enhancement in ductility with decreasing grain size.

Effect of Short Attritor-Milling of Magnesium Alloy Powder Prior to Spark Plasma Sintering

“…The reports focusing on the room temperature milling showed that long milling times are necessary to achieve saturation in the powders’ microstructure refinement. The grain size of ~80 nm (determined by transmission electron microscope) was achieved after 20 h of high-energy ball-milling [ 13 ], and the crystallite size of 40 nm (determined by XRD), which is comparable with the results shown in this study, were achieved after 10 h of shaker-milling [ 12 ] and after 5 h of planetary ball-milling [ 24 ]. Therefore, a relatively high effectivity of the attritor-milling in the microstructure refinement of magnesium alloys is in good agreement with our initial assumption.…”

“…For example, in cryo-milled AZ80, YCS of 442 MPa was achieved [ 20 ], and in cryo-milled AZ31, YCS of 400 MPa was reported [ 22 ], but in both studies, the average grain size was at least one order of magnitude lower. On the other hand, sintering of a heavily milled pure Mg resulted in coarser microstructure and, consequently, comparable grain size and mechanical strength were achieved—SPS of the cryo-milled pure Mg resulted in YCS of 179 MPa [ 28 ] and in the case of the planetary ball-milled pure Mg at room temperature, YCS was 236 MPa [ 24 ]. Nevertheless, comparable grain size (~3 μm) was also achieved after the sintering of the gas-atomized AZ91 alloy, and the measured YCS was 230 MPa [ 7 ].…”

Insights on Spark Plasma Sintering of Magnesium Composites: A Review

“…On the other hand, the relative density of the SPS-processed Mg composite was reported as 99.7%. It is possible to attain a relative density of greater than 99% in the SPS process, which is not valid in the case of conventional sintering [84]. The attainment of high density at lower processing time is possible in SPS, as the heating rate is high, which is correlated from the details given in Tables 1 and 2.…”

“…The Mg with a smaller particle size has a higher reaction rate than powder with a larger particle size. It leads to the formation of the MgO phase on the surface and may enhance the corrosion rate due to the formation of the oxide layer [84]. The SPS processing technique helps obtain better corrosion resistance of the material by enhancing the uniformity of the microstructure.…”

Investigation on deformation behavior of magnesium alloy sheet AZ31B in pulsating hydroforming

“…Magnesium alloys, as the lightest metal alloys in industrial applications, are widely used in many industries, like aerospace and automotive manufacturing. 1,2 Based on its close-packed hexagonal structure, the elongation of magnesium alloys is usually less than 20% at room temperature, and thus, its plastic deformation capacity is poor. 3–6 In the previous studies, most of the scholars improve the forming performance of magnesium alloys by increasing the forming temperature or refining the grains.…”