Phase transformations in Mn-Al permanent magnet alloys
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Phase transformation studies have been made of the Mn-Al alloys with compositions near the equiatomic range with or without small amounts of carbon, copper and nickel, using differential thermal analysis, X-ray diffraction and optical and electron microscopy. The high temperature hexagonalɛ phase obtained by quenching, transforms to the ferromagneticτ phase between 500 and 550° C and on further heating transforms back to the hexagonal phase between 750 and 950° C. Also, on controlled cooling of theɛ phase from about 900° C, the ferromagneticτ phase is formed between 800 and 670° C. TEM studies have shown the presence of the B19 ordered phase, ferromagneticτ phase and Mn5Al8 precipitates even in quenched alloys.
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