Abstract
Magnesium aluminosilicate (MAS) glasses are attractive candidates for high-modulus (HM) reinforcing fibres but are prone to crystallisation, which restricts the usable forming window. Four MAS compositions modified with Y2O3 (T9, V9) or a combined Y
2
O
3
/ZnO addition (T10, V10) were investigated on a single experimental platform to define the window for crystallisation-free fibre drawing. Two base glasses were compared: an MgO-rich, Al
2
O
3
-lean variant (T series) and an Al
2
O
3
-rich, MgO-lean variant (V series). Compositions were verified by ICP-OES/MS, and characterised by dilatometry, DSC, rotational viscometry with Vogel–Fulcher–Tammann fits, gradient-furnace liquidus measurement and X-ray diffraction. The V series shows a glass transition 30–40 K above the T series and a 20 % lower thermal expansion coefficient. Substituting half of the Y
2
O
3
by ZnO lowers the glass transition by 13–30 K and raises the liquidus of the Al
2
O
3
-rich base from 1311 to 1340 °C. The DSC crystallisation onset lies a near-constant 33–37 K above the glass transition across all four glasses, indicating a kinetic barrier set by the MAS network rather than by the modifiers. All four were drawn into fully amorphous continuous fibres from nozzle temperatures only 40–69 K above their liquidus, and remain calorimetrically indistinguishable from the parent glass; V10, drawn 40 K above the highest liquidus, is the most stringent case. Crystallisation-free forming is therefore controlled by the rapid quench inherent to filament drawing rather than by a positive thermodynamic margin. The results support Y
2
O
3
-rich, Al
2
O
3
-rich compositions for HM-MAS fibre scale-up.