Alloys based on austenitic Fe-Mn-Al-C system have been received considerable attentions for cryogenic structural applications due to their high strength, good toughness, and low cost. The mechanical properties of these alloys have been reported to be enhanced by the formation of strain-induced deformation twinning or martensitic transformation during deformation.
Kim et al. Reported the development of a high strength and excellent toughness cryogenic alloy, Fe-30Mn-5Al-0.3C-0.1Nb (referred to as "CAM-1"). A particularly interesting behavior of this alloy was the increase in elongation with decreasing temperature, from 29% at room temperature to 57% at 77K. Studies of deformed microstructure and work hardening in the alloy showed that the gradual formation of strain-induced mechanical twinning during deformation was responsible for the high elongation at the cryogenic temperatures. Besides mechanical twins, strain-induced ε(hcp) and/or α′(bct) martensites may be formed during deformation, depending on chemical composition and test temperatures.
The mechanical properties of meta-stable austenitic alloys depend largely on the characteristics of deformation modes such as slip, deformation twinning, transformation to ε martensite, or transformation to α′martensite, which are known to be closely related to the stacking fault energy(SFE) of austenites. It has been reported that, in Fe-Mn-Cr-C and Co-Ni-Cr systems, the dominant deformation mode shifts from $\gamma$→ε martensitic transformation to deformation twinning, and to slip as SFE increases. SFE is a function of chemical composition and temperature. A minimum in SFE occurs near at manganese content of 22% in Fe-Mn-C alloys. Kim et al. have found that aluminium additions to Fe-Mn-C alloys decreased the tendency to form martensite during deformation. Zuidema et al. also observed the similar effect in Hadfield steel. They suggested that the aluminium addition to Hadfield steel would presumably increase the SFE of austenite, but no experimental evidence was presented.
The purpose of this work was to examine the effect of aluminium content on the deformation modes and tensile properties in Fe-19Mn-5Cr-(0~5.5)Al-0.25C alloys. Aluminium content varied from 0 to 5.5 wt%. Measurement of SFE has been performed using the dissociated dislocation node method. The temperature dependence of deformation modes and tensile properties was also investigated. The effect of sequential formation of strain-induced deformation twins and α′ martensite during deformation on tensile elongation was studied.
The results obtained in this work are as follows:
(1) In Fe-19Mn-5Cr-(0∼5.5)Al-0.25C system, aluminium acts as an austenite stabilizer against ε martensite formation, but it becomes a ferrite former when added excessively(4wt%).
(2) The deformation mode varied in the order of (ε+α′)→(deformation twinning)→(slip) with increasing Al content and temperature.
(3) Aluminium increased the stacking fault energy(SFE) at the rate of 10mJ/㎟/wt%, which was so high as to give birth to the large variation of deformation mode.
(4) The formation of bct α′ martensite was preceded by the prior formation of hcp ε martensite or deformation twin bands.
(5) The sequential formation of deformation twins and α′ martensites enhanced the tensile elongations.
(6) The 3.5Al alloy(Fe-19Mn-5Cr-3.5Al-0.25C) exhibited excellent elongation exceeding 65% from room temperature to 77K. The high elongation at room temperature was attributed to the formation of deformation twins, and the high elongation at 77K was due to the sequential formation of deformation twins and α′ martensites.