• PDF / 838,305 Bytes
• 17 Pages / 420.48 x 639 pts Page_size
• 36 Downloads / 197 Views

DOWNLOAD

REPORT

---

MAGNETIC EIAXATICN AND STRUCTURAL TRANSFOR4ATICN IN METALLIC GLASSES R.C. O'HANDLEY Massachusetts Institute of Technology, Departmnnt of Materials Science and Engineering, Cambridge, MA 02139 ABSTRACT Current understanding of the local atomic structure of amorphous materials is reviewed. Sane results of probing short-range order by selected techniques are cited to illustrate the degree of uniformity that exists on a local scale. Observations of phase separation and phase changes are described with particular emphasis on a temperature driven, reversible transformation of the local structure observed magnetically in several cobalt-base glasses. The manifestations and implications of such transformations within the glassy state are examined. Several examples pointing to quasi-crystalline and to non-crystalline (non-space filling) local structures are given.

INTRODUCTION The most direct characterization of atomic arrangements in glasses have cane from scattering studies (X-ray, neutron, electron). Such studies voint to the absence of long-range order and to the existence of a certain statistical regularity in local coordination of metallic glasses. However, local atamic coordinates or even the angular arrangements of atcms (local bond orientational order) in amorphous solids cannot be unicuely determined from these experiments because of the randcm orientation of the local structural units. Scattering measurements provide statistical descriptions of the atcnic arrangements,

and thus must be supplemented by three-dimensional structural models and by experiments sensitive to local symmetry. A variety of magnetic measurements are effective synmetry-sensitive probes. Here we focus on two of these, magnetic relaxation and magnetic studies of structural transformations, and consider what they tell us about local atomic order in metallic glasses. IXCAL ATOMIC ORDER Egami et al (11 have developed a model of local structural defects in metallic glasses based upon two unique ways in which atomic configurations can fluctuate or deviate from the average disorder characteristic of the glassy state. One fluctuation defines a local shear defect and the other a local density fluctuation.

The former defects have a directional propertv

and can be associated with localized strain dinoles. Fig. 1 shows

shematic 2-D representations of possible atomic confiq-

urations involved in a) the reorientation

Mat. Res. Soc. Symp. Proc. Vol. 58. 1986 Materials Research Society

of a local shear stress by metal-

142

loid hopping between two local minima (double well potential) and b) short-range translation of a positive density fluctuation about a metalloid atom as it hops between two local minima.

These processes involve

changes in local atom configurations that a) have a directional shear character and can therefore couple directly to an applied uniaxial stress or couple indirectly through magnetostriction to changes in magnetization orientation or b) have no directional character and move only to lower their total energy. Presumably, l