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.
Microstructures and Mechanical Properties of High‐Strength Fe‐Mn‐Al‐C Light‐Weight TRIPLEX Steels
High‐strength TRIPLEX light‐weight steels of the generic composition Fe‐xMn‐yAl‐zC contain 18 ‐ 28 % manganese, 9 ‐ 12 % aluminium, and 0.7 ‐ 1.2 % C (in mass %). The microstructure is composed of an austenitic γ‐Fe(Mn, Al, C) solid solution matrix possessing a fine dispersion of nano size κ‐carbides (Fe,Mn)3 AlC1‐x and α‐Fe(Al, Mn) ferrite of varying volume fractions. The calculated Gibbs free energy of the phase transformation γfcc → ∊hcp amounts to ΔGγ→∊ = 1757 J/mol and the stacking fault energy was determined to ΓSF = 110 mJ/m2. This indicates that the austenite is very stable and no strain induced ∊‐martensite will be formed. Mechanical twinning is almost inhibited during plastic deformation. The TRIPLEX steels exhibit low density of 6.5 to 7 g/cm3 and superior mechanical properties, such as high strength of 700 to 1100 MPa and total elongations up to 60 % and more. The specific energy absorption achieved at high strain rates of 103 s−1 is about 0.43 J/mm3. TEM investigations revealed clearly that homogeneous shear band formation accompanied by dislocation glide occurred in deformed tensile samples. The dominant deformation mechanism of these steels is shear band induced plasticity ‐SIP effect‐ sustained by the uniform arrangement of nano size κ‐carbides coherent to the austenitic matrix. The high flow stresses and tensile strengths are caused by effective solid solution hardening and superimposed dispersion strengthening.
Microstructure and mechanical properties of ductile Ni3AI-type compound in Fe-(Ni, Mn)-AI-C systems rapidly quenched from melts
By the rapid quenching technique, nonequilibrium Ni3Al-type compounds with high strength and hardness as well as large elongation have been found in Fe-Ni-Al-C and Fe-Mn-Al-C systems. This formation region is limited to about 7 to 55 wt pct Ni, 3 to 9 wt pct Al and 0.8 to 2.4 wt pct C for Fe-Ni-Al-C and to about 7 to 65 wt pct Mn, 3 to 9 wt pct Al and 0.8 to 2.4 wt pct C for Fe-Mn-Al-C. The Ni3Al-type compound has fine grains of about 1 to 10 μm in diam. Their Vickers hardness and yield strength increase with increase in the amounts of carbon, aluminum or nickel and the highest values attain about 665 DPN and 1690 MPa for Fe-Ni-Al-C and 600 DPN and 1740 MPa for Fe-Mn-Al-C. Elongation increases with decrease in carbon or aluminum and attains about 11 pct for Fe-20 wt pct Ni-6 wt pct Al-1.2 wt pct C and 28 pct for Fe-20 wt pct Mn-8 wt pct Al-1.6 wt pct C. The good strength and ductility of the Ni3Al-type compounds remain unchanged on tempering for 1 h until heated to about 750 K. Further, it has been found that the addition of chromium, molybdenum or cobalt is effective for the improvement of mechanical properties and thermal stability of the compounds. Thus, the use of materials containing Ni3Al-type compounds may be attractive for fine gage high-strength wire or plate applications.