Effect of Solute Carbon on the Evaluation of Dislocation Density in as Quenched Martensite by X-ray Diffraction

Abstract

X-ray diffraction analysis is one of powerful tools on the dislocation analysis and this method can be applied reasonably for many metals with isotropic crystal structure such as bcc and fcc. In this study, modified Williamson-Hall analysis was applied for martensitic steels containing 0.006 -0.26 mass% carbon and proved that the value of dislocation density increases with increasing the carbon content. However, martensitic steels containing solute carbon have bct structure characterized by different lattice constants on a-axis and c-axis. With increasing solute carbon, a-axis shrinks but c-axis is elongated. This leads to the peak séparation in an X-ray diffraction peak and causes an increase of the full-width at half-maximum (FWHM) in the diffraction peak. This suggests that the value of dislocation density is over estimated due to the effect of peak separation in as quenched martensitic steels with solute carbon. It was found that the increment of apparent dislocation density Δρ' is expressed by the following equation as a function of the amount of solute carbon (mass%C), independent of the values of true dislocation density and the screw component of dislocation. Δρ[m −2 ] =1.68×10 17 (mass%C) 2 As a result, it is concluded that the true dislocation density is constant at 4.5×10 15 m 2 in martensitic steels which have solute carbon more than 0.14 mass% at least.

Quantitative Analysis of Hardening Due to Carbon in Solid Solution in Martensitic Steels

“…It is generally known that the dislocation density (N dis ) in lath martensite increases with increasing C total . 2,3) Figure 5 shows N dis in the as-quenched specimens estimated by the mWH/WA method (solid circle) against previously reported values. 2,11) N dis increased with an increase in C total ; however, the difference between the 0.3C and 0.6C steels was as small as approximately 0.3 × 10 15 m − 2 .…”

“…2,11) N dis increased with an increase in C total ; however, the difference between the 0.3C and 0.6C steels was as small as approximately 0.3 × 10 15 m − 2 . Takaki et al 2) reported that N dis becomes almost constant over 0.14 mass% of C total in lath martensite, which is in accordance with the results of this study.…”

“…It is generally known that the hardness of as-quenched martensitic carbon (C) steels increases with an increase in the C content; however, the strengthening mechanism in martensitic C steels has not been fully clarified because C affects multiple strengthening factors, which are yet to be separately evaluated. In fact, it has been reported that increasing C increases the dislocation density 1,2) and refines the block size 3) in martensitic C steels; therefore, the dislocation strengthening and grain refinement strengthening may increase with an increase in the C content, in addition to the solid solution strengthening owing to supersaturated C in solid solution (solute C).…”