• Directionally solidified pure Mg with columnar crystals achieves a high elongation of 53% at room temperature, demonstrating exceptional tensile plasticity.
• The growth orientation concentrated in <022ˉ5> ensures high Schmid factors (>0.43) for basal <a> slip, enhancing deformation capability.
• A geometric compatibility factor greater than 0.7 across grain boundaries indicates outstanding strain coordination and reduced boundary cracking.
• This work provides a strategy to overcome the inherent low plasticity of cast Mg by optimizing grain morphology and orientation via directional solidification.