Martensite to austenite reversion in a high-Mn steel: Partitioning-dependent two-stage kinetics revealed by atom probe tomography, in-situ magnetic measurements and simulation

Abstract

Austenite () reversion in a cold-rolled 17.6 wt.% Mn steel was tracked by means of dilatometry and in-situ magnetic measurements during slow continuous annealing. A splitting of the -reversion into two stages was observed to be a result of strong elemental partitioning between  and '-martensite during the low temperature stage between 390 and 575ºC. Atom probe tomography (APT) results enable the characterization of the Mn-enriched reversed- and the Mn-depleted remaining 'martensite. Because of its lower Mn content, the reversion of the remaining 'martensite into austenite takes place at a higher temperature range between 600 and 685ºC. APT results agree with partitioning predictions made by thermo-kinetic simulations of the continuous annealing process. The critical composition for nucleation was predicted by thermodynamic calculations (Thermo-Calc) and a good agreement was found with the APT data. Additional thermo-kinetic simulations were conducted to evaluate partitioning-governed -growth during isothermal annealing at 500°C and 600°C. Si partitioning to  was predicted by DICTRA and confirmed by APT. Si accumulates near the moving interface during -growth and homogenizes over time. We used the chemical composition of the remaining '-martensite from APT data to calculate its Curie temperature (T Curie) and found good agreement with magnetic measurements. These results indicate that elemental partitioning strongly influences not only -reversion but also the T Curie of this steel. The results are important to better understand the thermodynamics and kinetics of austenite reversion for a wide range of Mn containing steels and its effect on magnetic properties.

The Effect of the In-Situ Heat Treatment on the Martensitic Transformation and Specific Properties of the Fe-Mn-Si-Cr Shape Memory Alloys Processed by HSHPT Severe Plastic Deformation

“…However, in the latter case, austenite reversion can be driven by both long-range and an interface-controlled mechanism. The same behavior was reported in [ 23 , 47 ]. At temperatures higher than 925 K, both the HTC and HC samples become paramagnetic.…”

“…Magnetic properties of Fe-Mn-Si-Cr alloys are sensitive to the Mn content [ 19 , 38 , 39 , 40 ], and also to the preparation route [ 18 , 19 , 23 , 24 , 41 ]. The alloys with high Mn content favour the ε-γ martensitic transformation and order antiferromagnetically below the Néel temperature [ 20 , 42 , 43 , 44 , 45 , 46 ].…”

“…As a result, the new δ phase did not have time to grow/stabilize homogenously in the sample, which might explain the presence of two Tc temperatures that might belong to two phases with slightly different compositions. The Curie temperatures are higher for the TC sample due to the lower Mn content in the newly formed cubic phase [ 23 , 46 , 47 ].…”

“…The MT of Fe-Mn-Si alloys shows a particular non-thermoelastic stress-induced γ‒ε martensitic transformation and its ε‒γ reverse transformation during subsequent heating [ 10 , 11 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 ]. Additionally, beside ε-martensite (hexagonal close packed—hcp—stress-induced structure), the α’-martensite (body-centred-tetragonal—bct—structure) may appear at the intersections of crystallographic defects induced by plastic deformation such as mechanical twins, stacking faults and shear bands [ 22 , 23 ]. In addition, according to [ 11 ], the α’-martensite can be formed in the stress-induced ε martensite, in a probable γ-ε -α’transformation, with the ε-martensite acting as an intermediate phase for α’-formation [ 23 ].…”

Microstructural Evolution in an Fe-10Ni-0.1C Steel During Heat Treatment and High Strain-Rate Deformation

“…THE presence of thermally stable but mechanically unstable austenite in martensitic steels increases the work-hardening response and damage tolerance through transformation-induced plasticity (TRIP). Considerable work has been done on Mn-and C-containing steels as both Mn and C are austenite stabilizers [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] ; these alloys can be heat-treated to produce thermally stable austenite either by austenite formation (and Mn enrichment) through intercritical annealing or C enrichment of residual austenite through quench and partitioning treatments. Development of these TRIP steels has required a multifaceted approach including fine tuning the alloy composition and designing novel thermo-mechanical processing techniques that together influence the morphology, [11,12,16] size, [11,16] volume fraction, [4,9] composition, [4,6,7,9,15,17] and hence the thermal [4] and mechanical [7,11,16] stability of the austenite phase.…”