Needle-like LPSO Phase Tailoring for Ultra-low Thermal Expansion and Enhanced Stability in Mg-12Y-1Al Alloy

A low coefficient of thermal expansion (CTE) and good thermal cycling stability are critical for expanding the applications of Mg alloys. In this study, thermal cycling experiments were conducted on three Mg-12Y-1Al (WA121) alloys containing distinct second phases to systematically evaluate the influence of the long-period stacking ordered (LPSO) phase on dimensional stability and thermal expansion behavior. The results indicate that the WA121 alloy with the highest LPSO phase content exhibits the lowest CTE (minimum value: 14.61 × 10⁻⁶ K⁻¹) and superior dimensional stability. These findings confirm that the significant reduction in CTE originates from the incorporation of a substantial volume fraction of the LPSO phase. The enhanced dimensional stability is attributed to two mechanisms: First, the atomic density near the interface between the needle-like LPSO phase and the matrix decreases during thermal cycling, enabling compensation for thermally induced expansion. Second, stress concentration and localized strength reduction in regions adjacent to LPSO phases induce plastic deformation; the resulting strain hardening in these areas increases resistance to subsequent plastic deformation under equivalent stress conditions.