Miguel Oscar Prado

Martensitic transformation in Cu-Mn-Al alloys

The Cu-Mn-Al alloys exhibit, in a certain range of composition, an A2 (BCC) phase which is stable at high temperatures (around 1,050 K). Two ordering reactions take place on lowering the temperature A2 {yields} B2 and B2 {yields} L2{sub 1}, with a distribution of antiphase domains. Spinodal decomposition can be found in the as quenched specimen for some compositions, as shown by Sato et al. In addition a martensitic transformation can also occur, with an Ms strongly dependent on the composition. In this work the dependence of the Ms on composition is investigated, for alloy compositions along the Cu{sub 3} Al-Cu{sub 3}Mn{sub 2} line of the ternary phase diagram. The results, together with other data taken from the literature, are analyzed in terms of a model, based on the pair interchange energies, developed by Ahlers.

Magnetic properties of Cu-Mn-Al alloys with shape memory effect

The magnetization of Cu-Mn-Al alloys with a shape memory effect in the composition line Cu{sub 3} Al-Cu{sub 3}Mn{sub 2} was measured from 5 K to room temperature and at fixed temperatures up to (5 {times} 10{sup 7})/(4 {times} {pi}) A/m magnetic fields. It was found that from room temperature to about 90 K (depending upon composition) the samples behaved as superparamagnets. This behavior is described in terms of the magnetic cluster size, the cluster concentration and the interaction between clusters for different compositions and temperatures by fitting the results with the Brillouin function proposed by Beck. Evidence was also found that below this temperature, mictomagnetism (a mixture of ferromagnetism and antiferromagnetism) is present in these alloys. Careful measurement of the magnetization during the martensitic transformation, for a sample with 11.9 at.% Mn, was done in order to estimate the magnetic energy change between {beta} and martensite phases. No noticeable effect on the Ms martensitic transformation temperature, for applied magnetics fields up to (5 {times} 10{sup 7})/(4 {times} {pi}) A/m, was found.

The Influence of β Phase Aging on the Martensitic Transformation Temperatures of CuMnAl Alloys

Alloys with the composition along the line Cu{sub 3}Al-Cu{sub 3}Mn{sub 2} retain their high temperature {beta} phase when quenched into water at room temperature from the homogenization temperature 1,123 K. During the quenching the {beta} phase orders with the sequence A2 {leftrightarrow} B2 {leftrightarrow} L2{sub 1}. These alloys transform from {beta}(bcc) to 2H or 18R martensite as has been reported before. In this work it will be shown that the M{sub s} (temperature at which martensite starts to form) measured in the as quenched alloys are quite different from those corresponding to ordered alloys. Measured differences between the M{sub s} temperature after room temperature aging and after 373 K aging will be presented, and these values will be compared with calculated differences in terms of energies of reordering of atomic pairs in CuMnAl alloys.