Precipitation hardening in AZ91 magnesium alloy

Zeng, Ruilin (2013). Precipitation hardening in AZ91 magnesium alloy. University of Birmingham. Ph.D.

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Abstract

The microstructure evolution of a sand cast AZ91 magnesium alloy during heat treatment (solution treatment and subsequent ageing) were characterized quantitatively using a combination of optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The area fraction of discontinuous precipitates and number density of continuous precipitates (N\(_V\)) in the AZ91 alloys with and without pre-deformation were measured using OM and TEM, respectively. Based on these metallurgical evidences, the existing precipitation strengthening mode for AZ91 was modified and the effect of pre-deformation on the precipitation strengthening of AZ91 was investigated.
Al-Mn-(Mg) particles in the size range of 20-200 nm have been found in the as-cast AZ91. Their morphologies, chemical composition and structures were investigated using TEM. It was found that these particles have a chemical composition of (Al \(_6\)\(_.\)\(_5\)\(_2\)Mn) \(_1\)\(_-\)\(_x\)Mg\(_x\) (x < 0.13) and a decagonal quasi-crystalline structure. These particles were stable during a solution treatment and acted as preferential nucleation sites for continuous Mg\(_1\)\(_7\)Al\(_1\)\(_2\) precipitates during the subsequent ageing.
The results obtained using two electron tomography (ET) approaches were also summarized in this thesis. One uses HAADF-STEM for Mg\(_1\)\(_7\)Al\(_1\)\(_2\) precipitates on Al-Mn-(Mg) particles. The other technique is BF-STEM applied to study Mg\(_1\)\(_7\)Al\(_1\)\(_2\) precipitates on the dislocations.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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