Micromechanical study of the role of precipitates on the plastic deformation of AZ91 magnesium alloy

Si, Shanshan (2020). Micromechanical study of the role of precipitates on the plastic deformation of AZ91 magnesium alloy. University of Birmingham. Ph.D.

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Abstract

AZ91 magnesium alloy micro-pillars have been studied in order to study the role of precipitates on plastic deformation.
Micro-pillars with a loading direction of [55 10̅̅̅̅ 9] were prepared in both the solution treated and peak-aged AZ91 conditions to study the influence of precipitates on basal (a) dislocation slip. Precipitation has been found to have a very limited strengthening effect with the critical resolved shear stress (CRSS) of the basal (a) slip increased by only 4 MPa. No precipitate shearing or Orowan looping has been observed in post-mortem TEM study.
Micro-pillars with a loading direction of [011̅0] were prepared in both the solution treated and peak-aged AZ91 conditions to study the influence of precipitates on tension twinning. The 0.2% proof stress was increased by about 60 MPa by the precipitates. In the peak-aged sample, more and smaller twins were observed than in the solution treated sample. The precipitates were engulfed by the tension twins during deformation. The back stress generated due to the presence of the precipitates is expected to provide a considerable contribution to strengthening which potentially slows the twin growth. The working hardening rate is slightly higher in the aged sample than in the solution treated sample. A higher density of stacking faults was observed in the aged sample than in the solution treated sample, which may influence the dislocation transmission upon further deformation resulting higher work hardening rate.
Submicron-pillars with a loading direction of [011̅0] were prepared in the solution treated and under-aged AZ91 conditions and subjected to in-situ TEM compression, in order to study the influence of precipitates on the tension twin nucleation and growth. In-situ TEM shows that the high stress concentration locations were preferential sites for tension twin nucleation while the precipitates had no obvious influence on twin nucleation. Twin growth, however, was obviously inhibited by the presence of precipitates, which would require an increased applied stress to overcome.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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