Study of the Yield Anomaly of Fe2MnAl by In-Situ Straining in a TEMIan Baker, Dartmouth College, DMR 0505774

The yield anomaly observed in L21-ordered Fe2MnAl has been investigated using in-situ tensile testing in a TEM at elevated temperatures. It found that the slip vector is <111> with slip on {110} and {211} at 300-900 K. Cross slip between these planes was found to occur frequently. Fig.1a shows a thin foil loaded along [21-1] at 700 K. A [111] screw dislocation that initially glided on (01-1) cross-slipped onto (10-1). Fig.1b shows the trace of dislocation motion at 900 K. The dislocation on (01-1) traveled was a short distance on (10-1) before it cross-slipped back onto (01-1).

At 300 K, curved dislocations of mixed character glide steadily, in contrast to the screw dislocations, which move in a jerky manner. Around the temperature of the yield stress peak (700-800K), jerky motion is prevalent, while above the yield stress peak at 900 K dislocations glide smoothly.

Fig 1: Bright field TEM images of Fe2MnAl strained in situ in a TEM at (a) 700 K and (b) 900 K. Dislocations with a Burgers vector of <111> glide on multiple {110} and {211}. Cross slip takes place frequently at all temperatures tested.

a b

Dislocations are four-fold dissociated at 300 K with the outer partials separated by ~5 nm, and ~20 nm for the inner partials. The dissociated dislocations move together at 300 K, as shown in Fig.2a. (The outer partials are not resolved due to the very small separation). The dislocations of mixed character are bowed-out. At 700 K, pairs of partials with a total slip vector of a/2<111> are operative (see Fig.2b). Thse produce trailing a/2<111>-type APBs. Above 900 K, Fe2MnAl is B2 structure and the matrix is not disordered by the motion of two-fold dissociated partials.

Four Dartmouth undergraduates (Andrew S. Geffken, Stephen M. Ubnoske, Ayrat Safine and Benjamin J. Meigs) and a REU student (Jacob R. Grosek from the University of Utah) were actively involved in the research. They were trained to cast alloys and characterize them using XRD, DSC, and both compression and hardness tests.

The work involves cooperation with several researchers from Oak Ridge National Laboratory: Dr. Easo George provided ingots; and Dr. Andrew Payzant performed in-situ heating XRD measurements.

Fig 2: Weak-beam (g-3g) TEM images of dislocations gliding during in situ straining. (a) At 300 K, partial dislocations are bonded together during gliding. (b) At 700 K, partial dislocations glide independently. a/4<111>-type thermal APBs are present in both images, which show fringe contrast.

a b

Study of the Yield Anomaly of Fe2MnAl by In-Situ Straining in a TEMIan Baker, Dartmouth College, DMR 0505774

300 K 700 K