Variation of hardness with temperature in sintered NdFeB magnets

Abstract
The variation of hardness of a sintered Nd15Fe78B7 magnet has been measured in the temperature range from room temperature to 750 °C. Relatively little change in hardness is observed below 200° or above 700°. A sharp drop in hardness begins at about 300°, with hardness decreasing to half its original value at 450° and to 10% of its original value at 650°. Melting of a Nd‐rich grain‐boundary phase begins at about 600 °C. The observed hardness behavior is consistent with reports of compression‐induced crystallographic alignment of NdFeB magnets at temperatures near 700 °C. High‐temperature microscopy and differential scanning calorimeter measurements on cast magnets of similar composition show that melting of a Nd‐rich grain‐boundary phase begins at about 600 °C.

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