Investigation of the Yield Anomaly and the Role of APBs in L21-structured Fe2AlX Compounds and Related Alloys

Dartmouth College, Hanover NH

Investigators

Abstract

NON-TECHNICAL EXPLANATION. The broader impacts of the proposed activity include the education of a Ph.D. student and a number of undergraduates, who will work on the project. The P.I. typically has 5-6 undergraduates/year working on research projects, in which women and minorities have been well represented. They will acquire skill in crystal growth, materials characterization techniques (optical microscopy, SEM, TEM, X-ray diffraction), mechanical testing and modeling material behavior. The work will lead to an understanding of whether two-phase alloy f.c.c./L21 alloys may be useful structural materials for elevated temperature applications. The work will involve collaborations with Los Alamos National Laboratory, and Oak Ridge National Laboratory. TECHNICAL EXPLANATION. Marked yield anomalies have been observed in the L21-structured compounds Fe2AlTi and Fe2AlMn. In addition, the latter compound was found to contain a complex, thermal anti-phase domain (APD) structure, to deform by <111> slip, and, as a single crystal, to show room-temperature ductility. The aims of this research are to determine and model the yield anomaly mechanism in L21-structured Fe2AlX compounds; elucidate the interactions between gliding dislocations and thermal anti-phase boundaries (APBs) in Fe2AlMn including modeling the resulting effects on mechanical properties; and to understand the mechanical behavior and deformation mechanisms of a new two-phase f.c.c./L21 alloy as a function of temperature. Investigation of the yield anomaly will involve tensile testing (including slip-line analysis) and transmission electron microscope (TEM) observations of single crystals. Tensile tests performed over a range of temperatures at prescribed strain rates will be used to assess the effects of strain rate on the yield anomaly. Tensile tests performed on a Gleeble test machine, which enables very rapid heating and quenching, will both allow yield strength measurements to be made at the peak yield stress temperature with different vacancy concentrations and enable dislocation structures to be studied that have not recovered significantly after high-temperature testing. The activation enthalpy for vacancy formation will be determined and compared with values obtained from the vacancy-hardening model. Additional mechanical testing and TEM observations will allow other yield anomaly mechanisms to be investigated. For example, iron will be partially substituted by nickel in Fe2AlTi in order to: 1) vary the order/disorder temperature and determine if the peak yield stress temperature varies correspondingly, and 2) alter the slip direction from <111> to <110>. This will test yield anomaly mechanisms that rely on short-range order and anti-phase boundaries between dislocations, respectively. Thin single-crystal tensile specimens oriented to deform plastically by predominately edge or screw dislocations will be tested as a function of temperature in order to determine which type of dislocation is responsible for the yield anomaly. Finally, TEM in-situ straining experiments will be performed at different temperatures to enable direct observations of dislocation behavior. This will help to determine the role of pinning, crossslip, dissociation and climb of dislocations on the yield anomaly. Tensile tests will be performed on Fe2AlMn single crystals to examine the effects of different APD sizes on strength and ductility.