Cyclic Martensitic Transformations Influence on the Diffusion Of Carbon Atoms in Fe-18 wt.%Mn-2 wt.%Si alloy.

A significant carbon diffusion mobility acceleration as a result of cyclic γ↔ε martensitic transformations in iron-manganese alloy is determined by one- and two-dimensional structure defects of ε-martensite with face-centered close-packed lattice. Such defects (dislocations, low angle sub-boundaries of dislocations, chaotic stacking faults) were formed during cyclic γ↔ε martensitic transformations. Peak carbon diffusion coefficient increase was observed under thermocycling when maximum quantity of lattice defects increase was fixed.

The Formation of Crystal Defects in a Fe-Mn-Si Alloy Under Cyclic Martensitic Transformations.

Formation of crystalline defects due to cyclic martensitic transformations (CMT) in the iron-manganese Fe-18 wt.% Mn-2 wt.% Si alloy was investigated using X-ray diffractometry. Conditions for accumulation of fragment sub-boundaries with low-angle misorientations and chaotic stacking faults in crystal lattice of austenite and ε-martensite were analyzed.

The effect of cyclic martensitic transformations on diffusion of cobalt atoms in Fe-18wt.%Mn-2wt.%Si alloy.

Diffusion characteristics of cobalt atoms were investigated using radioactive isotope method in phase-hardened Fe-18wt.%Mn-2wt.%Si alloy. The observed significant increase of diffusion coefficient of cobalt atoms under the cyclic γ-ε-γ martensitic transformations was due to the action of two independent mechanisms - an athermal one and a thermally activated one. The first one arose from the direct γ-ε and the reverse ε-γ transformations with corresponding direct and reverse lattice shears during alternating stresses and simultaneous lattice restructuring. Another mechanism arose under the diffusion annealing of the phase-hardened alloy.

The effect of multiple martensitic transformations on diffusion of Fe and Ni atoms in Fe-31.7%Ni-0.06%C alloy.

Diffusion characteristics of iron and nickel atoms were investigated using radioactive isotopes method in phase-hardened metastable iron-nickel Fe-31.7%Ni-0.06%C alloy with nanofragmented structure. It has been found that diffusion mobility of nickel and iron atoms in reverted austenite of Fe-31.7%Ni-0.06%C alloy significantly increases as the result of multiple γ-α-γ martensitic transformations. The diffusion coefficients of nickel and iron in the austenite at 400°C corresponded to the stationary diffusion coefficients at the temperatures above 900°C. The revealed diffusion acceleration at low temperatures is caused by high-density dislocations and additional low-angle subboundaries of disoriented nanofragments of reverted austenite and deformation twin subboundaries formed during multiple γ-α-γ cycles.