Effects of magnetic and structural changes on elastic moduli of iron-manganese alloys

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Abstract
Measurements of ultrasonic wave velocities have been carried out on a sample of Fe-22–6.at.° Mn between 300 and 500 K. Above 470 K the sample is a single crystal of f.c.c. structure, but below 350 K it contains platelets of h.c.p. martensite within the matrix single crystal. The longitudinal wave velocities measured in the [110], [001] and [111] directions show considerable hysteresis arising from the h.c.p. b.c.c. structural transition near 450 K with increasing temperature and from the reverse transition below 390 K. In the [110] and [001] directions the longitudinal wave velocity shows a smoothed step-like anomaly at the Néel point TN≈ 370 K of the f.c.c. crystal. These measurements, together with the measurements of shear wave velocity in the [110] direction, indicate that there is a softening of about 60° in the shear modulus C = (c11 — cl2) /2 arising from antiferromagnetism, but no detectable anomalies in the shear modulus C = c44 nor in the bulk modulus .B = (c11 + 2c12) /3. A comparison with C of other Fe-Mn single crystals and the calculated moduli based on Young's modulus measurements on polycrystalline alloys shows that the anomaly in C increases with decreasing manganese content. This could lead to complete softening of the f.c.c. lattice near 10 at.° Mn.

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