Enhanced Magnetoelasticity in Fe-X alloys

Featuring Thomas A. Lograsso, Ph.D.
Ames Laboratory
Iowa State University

Location:  2111 Frederick Reines Hall

Abstract:
Since the discovery of the extraordinary increase of magnetostriction strain in Fe-X alloys through the addition of non- magnetic gallium, investigations have sought to understand the origin of the enhanced magnetoelasticity and to tune the chemistry and optimize the magnetoelastic behavior of Fe-based solid solution alloys.  Detail characterizations of Fe-Ga alloys suggest that a local clustering of gallium atoms may act as elastic dipoles, leading to enhanced magnetoelastic coupling. 

When combined with the dramatic softening of the shear modulus in this alloys, large increases in strain over pure Fe are found.  First principle calculations also confirm that the local atomic structure is an important parameter in determining not only the value of strain but also the sign of the strain.  In this presentation, Lograsso will review the development of the Fe-Ga (Galfenol) binary alloys for magnetostriction properties.  Most of this work has used single crystalline measurements as single crystals provide the ideal vehicle to assess the effectiveness of the addition on the magnetostrictive properties by eliminating grain boundary effects, orientation variations, and grain-to-grain interactions that occur when polycrystals respond to applied magnetic fields.  Results show that in the regions where monotonic increases in (3/2)_100 are exhibited, single phase A2 or D03 are found. For the alloys in the range of 18-21 at% Ga, quenching extends the single phase A2 to higher Ga concentrations thereby continuing to enhance magnetostriction. The sudden decrease in (3/2)_100 near Fe-19 at% Ga as well as Fe-29 at% Ga were associated with the formation of phase mixtures, either (A2+D03) or (D03 + unidentified secondary phases), respectively.

For quenched alloys between 25 and 29 at% Ga, a phase mixture of (A2+B2+D03) was found although the presence of this phase mixture does not seem to have a large effect on magnetostriction.  It has been found that ternary additions of transition metal elements have decreased magnetostriction from those of the binary Fe-Ga alloy.  Most of these ternary additions are known to stabilize D03-type chemical order that could be a primary contribution to the observed reduction in magnetostriction.  In contrast, both Sn and Al are found to substitute chemically for Ga, and for Sn additions, whose solubility is limited, no reduction in magnetostriction strains are observed when compared to the equivalent binary alloy composition. Aluminum additions, whose effect on the magnetoelastic coupling on Fe is similar to Ga, resulted in a rule of mixture relationship between the binary end members. Notably, addition of small amounts of C increases the magnetostriction of the slow cooled binary alloy to values comparable to the rapidly quenched alloy. We assume that small atoms (C, B, N) enter interstitially and inhibit chemical ordering, thus maximizing the magnetostriction.  Further, the addition of carbon was also found to extend the single-phase bcc to higher Ga contents, thereby allowing alloys of Fe-Ga beyond the binary solubility limit to exhibit even larger values ofl100 than previously reported.  The general conclusion these results support is that phase stabilization of the disordered bcc structure is one of the key components to optimize magnetoelasticity of Fe-Ga alloys.