Explanation of the PLC Effect in Advanced High-Strength Medium-Mn Steels. A Review

Abstract

The paper reviews the recent works concerning the Portevin-Le Chatelier (PLC) effect in Advanced High-Strength Steels (AHSSs) with a special attention to high-strength medium-manganese steels. Theories explaining the mechanism of the plastic instability phenomenon in steels with medium-and high-Mn contents were discussed. The relationships between microstructural effects such as TRIP (Transformation-Induced Plasticity), TWIP (Twinning-Induced Plasticity) and the PLC effect were characterized. The effects of processing conditions including a deformation state (hot-rolled and cold-rolled) and strain parameters (deformation temperature, strain rate) were addressed. Factors affecting the value of critical strain for the activation of serrated flow behavior in particular in medium-manganese steels were described.The explanation of the Portevin-Le Chatelier mechanism in medium-Mn steels showing the TRIP effect is a complicated issue because of their microstructure consisting of several phases, as well as the TRIP effect exhibited by these steels. The exact characteristics of the factors affecting the PLC effect in AHSS is very important, both from a research point of view and their industrial implementation. This overview concerns the PLC phenomenon in Advanced High Strength Steels (AHSSs), with particular emphasis on advanced medium-Mn TRIP steels. The Nature of PLC Effect in SteelsThe plastic instability phenomenon occurring during the deformation of metallic materials shows two most common forms of propagative bands: Lüders and Portevin-Le Chatelier bands. The Lüders bands refer to the regions of localized strain. They form immediately after the onset of plastic deformation from the yield point drop, followed to a dominant stage of the stress plateau stage. Lüders bands are commonly caused by static strain aging (SSA) [13]. Static strain aging is characterized by an increase in strength properties associated with a decrease in plasticity. The PLC bands are represented by characteristic serrations on stress-strain curves. PLC bands are usually related to the dynamic strain aging (DSA) effect. The occurrence of the PLC bands is much more erratic, and can be observed in various forms (serration types) in comparison to the Lüders bands.There are several theories which explain the PLC effect in metallic materials. However, none has been so far clearly confirmed. The first interpretation of this phenomenon was proposed by Cottrell [1]. From his point of view, the PLC effect is related to the interactions between solute atoms, such as C or N, and mobile dislocations. The presence of serrations on a tensile curve is associated with the rapid release of dislocations from the atmospheres of dissolved atoms, which block their movement. This model is based on the assumption that atmospheres are formed around dislocations due to volume diffusion. In the presence of substitution atoms, diffusion is facilitated by vacancies resulting from plastic deformation. The interstitial gaps located in the vicinity of the dislocation are enl...

In Situ Determination of Phase Stress States in an Unstable Medium Manganese Duplex Steel Studied by High-Energy X-ray Diffraction

“…This suggests that the physical source of the serrations is austenite. This result is consistent with the fact that PLC mechanisms are commonly observed in carbon-rich austenite at room temperature, as in FeMnC TWIP (Twinning-Induce Plasticity) steels [50][51][52]. On the contrary, PLC is reported in ferritic phases only at high temperatures (typically 300-400 • C) [53,54].…”

A Review of the Serrated-Flow Phenomenon and Its Role in the Deformation Behavior of High-Entropy Alloys

“…Once the stress becomes sufficiently large, the dislocation can break free and then migrate until it is pinned again. If the pinning and unpinning processes are repeated cyclically, it leads to a DSA effect [190]. Figure 8 presents an illustration of the pinning-unpinning cycle [152].…”

Plastic Instability in Medium-Carbon Tempered Martensite Steel

“…The plastic instability occurring in the former period is referred to as the Lüders phenomenon, and that in the latter period is the Portevin-Le Châtelier (PLC) phenomenon. Some materials have only one of the two types of plastic instabilities [ 4 , 5 ], and some have both [ 6 ]. The type of plastic instability can be identified from the typical characteristic on the tensile curve: a yield plateau for the Lüders deformation [ 4 , 5 ] or a jerky flow (a series of serrations) for the PLC effect [ 7 , 8 , 9 , 10 , 11 ].…”