STRUCTURE OF TWINS IN FE-NI MARTENSITE

It has previously been assumed that twins observed by TEM in twinned FeNi martensites are planar defects defined by the twinning plane. {211}b.c.c. and by the twinning direction. 〈111〉b.c.c.. It is shown here that the twins are very thin ribbons limited by two long parallel sides and two short edges, at most 1 or 2 nm thick, closely stacked. Both edges of each twin correspond to a {211}b.c.c. twinning plane which contains the 〈111〉b.c.c. twinning direction. As regards the twin sides, they are defined by the intersections of the plane of the twin (or composition plane) with both surfaces of the thin foil. Trace analysis confirms these deductions and show that the plane of the twin is one of the two other {211}b.c.c. planes which also contains the 〈111〉b.c.c. twinning direction. Thus the 'twins' are sheet-like objects whose plane (the plane of the twin, or the plane of the sheet) does not coincide with the crystallographic twinning plane. As a consequence it can be thought that the 'twins' result in fact not from a shear, but from the splitting of 〈111〉b.c.c. screw dislocations into three-dimensional arrays of partial dislocations. A model is proposed which accounts for the crystallographic and geometrical features of the twinned martensite. The model shows that the twins can form by the progressive aggregation of screw dislocations. Thus it appears that the distinction between 'twinned' and 'dislocated' martensite may be one of dislocation arrangement only. © 1979.