Dynamic strain aging in multiphase steels

Mechanical properties and microstructures of a low carbon high aluminium manganese steel was investigated after intercritical annealing and quench and partition (Q&P) treatment. The Q&P heat treatments were conducted using three different intercritical annealing temperatures, two quench stop temperatures and three partitioning times.
Dynamic strain aging (DSA) was observed during tensile tests at room temperature in samples intercritically annealed close to A1 transformation temperature; DSA serrations of the type A and type D were observed in the samples. The onset of DSA has been studied in ferritic steels [1]–[4], aluminium alloys [5]–[7]; it is reported that DSA is due to the interaction between moving dislocations and interstitial solute atoms during plastic deformation [6]–[9]. But studies related to DSA in multiphase steels are limited [10], [11]. The main focus of this work was to investigate the occurrence of DSA serrations in low carbon manganese steels undergone intercritical annealing and Q&P heat treatment.
Images from Digital Image correlation (DIC) showed Luder’s bands at the start of plastic deformation while Portevin Le-Chatelier (PLC) bands were visible after yielding in the stress strain curve. EBSD micrographs (Fig 1) shows blocky retained austenite morphology in the samples intercritically annealed at low temperature, while the samples annealed at high temperature show lath type morphology.
Transmission electron microscopy (TEM) investigations were conducted to study the dislocation cells before and after tensile testing. TEM micrographs (Fig 2), obtained from the samples before and after tensile testing, show the presence of carbides at the grain boundaries of ferrite. The dislocation movement seen in Figure 2b, shows pinning of dislocations at the areas where there are no carbides. It is hypothesized that these areas are solute rich atmospheres and dislocations are pinned at the solute atoms in these areas.
Simulations from JMaTPro (version 12.4) shows that carbides inside ferrite are in the stage of dissolution between 640°C-750°C annealing temperature. The carbides are of the type M(C,N) and M2(C,N) and upon dissolution they release C and N interstitials which can remain in ferrite. It is therefore speculated that DSA serrations is due to interaction between dislocations and C, N interstitials inside ferrite phase.