Unusual Indentation Behavior of Alkali Metaphosphate Glass Above Glass Transition Temperature

The mixed alkali metaphosphate glass 12.5Li 2 O–12.5Na 2 O–12.5K 2 O–12.5Cs 2 O–50P 2 O 5 (mol%) compressed under uniaxial stress shows unusually large recovery of its shape during heating above its glass transition temperature ( T g ). To clarify the mechanism, the viscous, elastic, and plastic behavior of the metaphosphate glass was investigated with the depth‐sensing indentation method. From the load‐displacement ( P – h ) curves of the glass, a large recovery of the depth displacement was found during unloading above T g . By analyzing the P – h curves with a viscous–elastic–plastic model, the large recovery was concluded to be due to larger elastic deformation than viscous deformation even above T g . The phenomenon can be attributed to relaxation to the randomly oriented ‐P–O–P– chain structure from the oriented chain structure formed under stress in the metaphosphate glass .