Thermal properties of γ-Mn (abstract)

Abstract

Mn can exist in a fcc environment in many alloys with other elements of the 3d-transition series. Some of these alloys, which can contain more than 50 at. % Mn, are classified as Invar. They show large magnetovolume effects and at the same time have ferromagnetic, antiferromagnetic, or mixed magnetic ground states. In order to gain a better understanding of Invar systems incorporating Mn, a closer examination of this element in a fcc environment is necessary. The γ state of Mn (fcc or fct Mn) is relatively little known, since its stability range lies only between 1364 and 1410 K. However, it is possible to stabilize γ-Mn in a temperature interval ranging from low temperatures up to about 600 K by the addition of small amounts of Cu. We have measured the specific heat (300<T<600 K) and thermal expansion (4<T<600 K and 1100<T<1300 K) of three MnCu alloys containing 4.42, 6.75, and 8.55 at. % Cu. The specific heat shows a first-order transition at Tt, which is the temperature below which the lattice undergoes a tetragonal distortion. This distortion is accompanied by an abrupt appearance of antiferromagnetism. The same effect is seen in the thermal expansion which shows a volume increase associated with the lattice distortion and the magnetic ordering. The high-temperature value of the thermal expansion coefficient is the largest so far observed in any fcc 3d system (45×10−6 1/K). From this data we estimate the thermal expansion behavior of pure γ-Mn. We obtain the atomic volume in the nonmagnetic and antiferromagnetic state and compare it to theoretical calculations.