Micromechanical Behavior Of Ferrite-cementite Structures

Plain carbon steel (0.17%-0.97%C) with various ferrite-cementite structures were formed by heat treatments or thermo-mechanical control processes (TMCP). The statistical relationship between mechanical properties and microstructural parameters, the evolution of dislocation substructure and the load partitioning between ferrite and cementite during tensile testing were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and in-situ high-energy X-ray (In-situ HEXRD). On this basis, the mechanism of the microstructure to affect the mechanical behavior of plain carbon steel with ultrafine or fine-grained ferrite matrix (α)+cementite particle (θ) structures was proposed, and the yield strength model and the stress-strain model during uniform plastic deformation for the steel were also established with physical metallurgy concepts. The results were as following:The yield strength of plain carbon steel with an ultrafine or fine (α+θ) structure had a relationship not only with the ferrite grain size but also with the size, volume fraction and location of cementite particles, particularly for medium or high carbon steel. The specific role of cementite particles varied with their location during yielding, in addition to the generation of geometrically necessary dislocations (GNDs) to enhance the yield strength for plain carbon steel with an ultrafine or fine (α+θ) structure. The role of grain boundaries in the yield strength was weakened by the intergranular cementite particles in comparison with single-phase ferrite with the same grain size, and the movable dislocations were blocked by the intragranular cementite particles, leading to an increase in the frictional stress.The work-hardening process of plain carbon steel with an ultrafine or fine (α+θ) structure during uniform plastic deformation can be divided into Stage Ⅰ, transition stage and Stage Ⅱ. In the Stage Ⅰ, the GNDs density around cementite particles was increased and the dislocation cell structures (DCSs) were formed initiatively by the dislocations connecting with cementite particles. In the transition stage, the DCSs became dense gradually. In the Stage Ⅱ, the densification of DCSs was completed and the dislocation walls connecting with cementite particles had no significant change. The increase in the rate of dislocation storage led to the increase in the work-hardening rate during the Stage Ⅰ and the refinement of DCSs caused the increase in the work-hardening rate during the Stage Ⅱ for plain carbon steel with an ultrafine or fine (α+θ) structure. Moreover, the increase in the particle parameter fld, where f and d are volume fraction and mean diameter of the cementite particles, respectively, is beneficial to storing dislocation quickly and forming refined DCSs. However, the refined ferrite grains is disadvantageous to the formation of DCSs.The load transfer between the ferrite and cementite particles began at the macroscopic yielding for plain carbon steel with an ultrafine or fine (α+θ) structure, mainly resulting from the accumulation of GNDs in the vicinity of the ferrite/cementite interfaces. The stress for ferrite (σα) or the stress for cementite particles (σθ) changed markedly during the Luders strain stage, in which the σα decreased rapidly due to the generation of movable dislocations from dislocation sources and the σθ increased rapidly resulting from the accumulation of GNDs, respectivery. The increase in fld led to the increase in both of the σα and the σθ during work-hardening, which was attributed to the accumulation of high-density dislocations in the ferrite matrix, especially around the ferrite/cementite interfaces. The σα and the σθ increased continuously but their increasing rates decreased gradually during work-hardening, accompanied by the increase in the dislocations density and the decrease in the rate of dislocation accumulation.The established stress-strain model demonstrated that the GNDs density increased with the increase in the fld, and the statistically stored dislocations (SSDs) density improved with the increase in the average grain size of the ferrite matrix for plain carbon steel with the ultrafine or fine (α+θ) structure. The decrease in the geometric slip distance (λ) resulted in a linear increase in the rate of dislocation accumulation caused by grain boundaries and hard particles (k) but a non-linear increase in the rate of dynamic recovery (k2). The k2of fine (α+θ) steel is lower than that of ultrafine (α+θ) steel at a same λ, resulting from the strong ability to form low-energy dislocation structure (LEDS, e.g., dislocation walls and DCSs) for fine (α+θ) steel. The decrease in effective geometric slip distance (k2λ) and the decrease in k2can led to the increase in the ultimate strength and the uniform elongation for plain carbon steel with the ultrafine or fine (α+θ) structure, respectively.