The anomalous yield strength behavior of CoTi and CoHf, commonly referred to as a yield strength anomaly (YSA), in which the yield stress increases with increasing temperature at intermediate temperatures, was investigated. Compression tests performed at temperatures from 300–1100 K and at strain rates from 5 × 10−6 s−1 to 1 s−1 revealed that in the temperature range of the YSA, serrated yielding occurred, and increasing strain rate caused the YSA to move to higher temperatures. These alloys also show a negative strain rate sensitivity in the temperature range of the YSA, and are subject to static strain aging. Transmission electron microscopy performed on material deformed at 300 K, 600 K and 800 K showed that deformation occurred by a100> dislocations at all temperatures. There was an increased density of edge dislocations observed at 800 K. These observations can be explained by the interaction of impurity atoms with dislocation strain fields. It is concluded that the yield strength anomaly in these alloys is due to dynamic strain-aging.; Compression tests of L21-structured Fe2MnAl performed from 300 K to 1000 K revealed that this alloy also displays an YSA. Scanning calorimetry revealed that the order-disorder reaction from a L21-structure to a B2-structure occurs at 897 K, and that quenched specimens retain excess enthalpy not associated with the order-disorder reaction. Specimens quenched from temperatures above 700 K showed increasing room temperature yield stress with increasing quench temperature. Transmission electron microscopy of material deformed at 300 K and 800 K showed that deformation occurs by 111> dislocations at both temperatures Based on the available mechanical data for Fe2MnAl possible strengthening mechanisms responsible for the YSA are discussed.
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