K-CARBIDES PRECIPITATION IN TWO HIGH MANGANESE ALLOYS
Two high Mn alloys marked A (24Mn-1.2C-12Al) and B (28Mn-0.7C-8.6Al) were laboratory hot rolled. After rolling in material A coarser k-carbides were detected, sporadically on the austenite grains boundaries and more frequently on the ferrite ones. Reason was un-balanced Mn a C contents in superposition with the rolling deformations and finishing rolling temperature of 950°C. Hardness corresponded to 432HV30 on average. Using light microscope no carbides on grains boundaries were observed in B both after rolling and followed aging at 500°C/6´. After 6 minutes of aging the hardness of B was approx. by 9 % higher unlike the rolled state. The X-ray analysis nucleation of nano-size kcarbides confirmed. After aging of B at 500°C/30´ and/or 60´some coarser k-carbides were detected on ferrite-austenite boundaries and/or austenite-austenite interface. The hardness went up and corresponded to 343 and/or 350HV30. From the X-ray and microstructure analyses follows the 30´ and/or 60´ dwell supported both very fine and coarser k-carbides precipitation. The calculated stacking fault energies (SFE) of both materials complete all solution. The A showed by 9 mJ.m -2 higher SFE than the B. In the A alloy lower Mn content and higher Al one resulted to 2.8 times higher ferrite volume fraction in comparison with the B alloy.
Phase constitution and lattice parameter relationships in rapidly solidified (Fe0.65Mn0.35)0.83 Al0.17-xC and Fe3Al-xC pseudo-binary alloys
A quasi-equilibrium temperaturevs carbon-concentration phase diagram of rapidly solidified pseudo-binary (Fe0.65Mn0.35)0.83Al0.17-xC alloys was determined after heat treatment in the 823 to 1323 K range. Lattice parameter relationships of rapidly solidified (Fe0.65Mn0.35)0.83Al0.17-xC and Fe3Al-xC alloys in the ferrite, austenite, and perovskite carbide phases were established as a function of the carbon concentration. This study shows that when a high concentration of carbon is present in the alloys a perovskiteL′l2 carbide is formed directly from the rapid solidification process. It is established also in this study that the carbon atom contribution to the lattice parameter increase in the fcc-based cubic crystal is greater in the disorderedγ-phase than in the ordered (L′l2 structure)κ-phase.
The role of deformation twinning on mechanical properties of an austenitic Fe30Mn1.2Al0.3C alloy
Abstract An austenitic Fe30Mn1.2Al0.3C alloy has been investigated under tensile and total-strain controlled fatigue tests between room temperature and 77K. The alloy exhibits an elongation peak between room temperature and 77K. It was found that tensile elongation is not controlled by the total amount of deformation twinning, but rather the rate and temperature sensitivity of deformation twin formation. Total-strain-controlled fatigue tests show that the fatigue resistance of the Fe30Mn1.2Al0.3C alloy at 77K is superior to that at room temperature in the 102–104 cycles fatigue life range, although the monotonic tensile elongation at 77K is lower than that at room temperature. The enhancement of fatigue resistance at 77K is due both to an increased fatigue ductility coefficient, because of strain-induced deformation twins, and to increased strength. Increased formation of deformation twins during the fatigue test is responsible for the increased fatigue resistance at 77K.