Deformation mechanisms in calcium containing magnesium alloys

Said, Mohammed ORCID: 0009-0008-4178-4020 (2025). Deformation mechanisms in calcium containing magnesium alloys. University of Birmingham. Ph.D.

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Abstract

This study investigates the micromechanical origins of enhanced formability in Ca-containing Mg alloys (ZXK: Mg-Zn-Ca and AZX: Mg-Al-Zn-Ca) compared to commercial AZ31, focusing on texture-mediated deformation mechanisms. Through in-situ synchrotron X-ray diffraction, HR-EBSD, and VPSC modelling, twinning, slip activation, lattice strains were explored. The study included strain rate sensitivity (SRS) under varying temperatures (25–250°C) and loading directions (RD/TD), with conditions chosen to guide the suitability for sheet forming processes.
Texture weakening in the Ca-containing alloys was strongly inferred to occur via a solute-driven mechanisms: The weaker basal texture in ZXK compared to AZX gives strong evidence for Ca segregation to grain boundaries during recrystallization, suppressing the basal texture intensity in ZXK, while in AZX, Al-Ca intermetallic formation likely reduced solute Ca availability, resulting in a stronger basal texture. This texture weakening promoted directional yield anisotropy, also evidenced in lattice strains, with ZXK exhibiting twinning-dominated plasticity under TD loading (evidenced by sigmoidal flow curves and lattice strain inflections in the (0002) plane). HR-EBSD confirmed twin nucleation at grain boundaries, driven by c-axis alignment and local stress concentrations.
Dynamic strain ageing (DSA) observed by serrated flow during tensile testing was seen at elevated temperatures (150–250°C). In AZ31, DSA was linked to Al/Zn so- lute pinning, while in ZXK (0 Al wt.%), Ca solutes likely mediated dislocation pinning. AZX showed attenuated DSA due to Al-Ca precipitation, producing weaker serration amplitudes, corroborating the Al-Ca precipitation effect on solute content in AZX.
Strain rate sensitivity analysis highlighted the anomalous behaviour of ZXK: At room temperature, its high SRS (primarily attributed to a better ratio/combination of basal/prismatic slip, but also noted differences in prismatic plane lattice strain redistribution) correlated with improved formability, while a sharp SRS reduction at 150°C aligned with reported warm-forming ductility minima. This was linked to temperature- dependent solute drag on dislocations, with dislocation recovery mechanisms dominating at 250°C.
VPSC modelling predicted enhanced basal slip activity in Ca-containing alloys at room temperature (versus prismatic slip dominant in AZ31), with ZXK exhibiting doubled twin volumes under TD loading validated by EBSD, while elevated temperatures activated pyramidal ⟨c+a⟩ slip and reduced twinning dependence. These results establish how Ca-induced texture modifications govern competing deformation modes, providing a microstructure-property framework for further investigation of forming relevant behaviour.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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