Three-dimensional observations of morphology of low-angle boundaries in ultra-low carbon lath martensite

Abstract

The lath martensite structure contains hierarchical substructures, such as blocks, packets and prior austenite grains. Generally, high-angle grain boundaries in the lath martensite structure, i.e. block boundaries, are correlated to mechanical properties. On the other hand, low-angle grain boundaries play an important role in morphological development. However, it is difficult to understand their nature because of the difficulty associated with the characterization of the complex morphologies by two-dimensional techniques. This study aims to identify the morphologies of low-angle boundaries in ultra-low carbon lath martensite. A serial-sectioning method and electron backscatter diffraction analysis are utilized to reconstruct three-dimensional objects and analyse their grain boundaries. A packet comprizes two low-angle grain boundaries - sub-block and fine packet boundaries. Sub-blocks exhibit porous morphology, with two large sub-blocks predominantly occupying a block. Several fine packets with different habit planes from the surrounding regions are observed. Fine packets are present in blocks, which frequently share a close-packed direction with the neighbouring fine packets. In addition, fine packets are in contact with the sub-block boundaries.

Three-dimensional Characterisation of Microstructures in Low-carbon Lath Martensite

“…The micrographs show that the specimens consist of fully lath martensite structures. Using the line intercept method, the sizes of the prior austenite grain in the ULC and LC structures were measured to be 107 μm 13) and 100 μm, respectively. In ULC lath martensite, a block, or a region with similar grey values, has a plate-like shape.…”

“…In ULC lath martensite, we already reported that there are type II blocks, which are enclosed in the type I blocks. 13) However, the ULC type II blocks are not the same as the LC type II blocks since most of the ULC type II blocks belong to different CP groups than the CP group of adjacent type I blocks. 11,13) In the LC specimen, some type II blocks belonging to different CP groups are enclosed in the coarse packet as shown as the white regions in Fig.…”

“…These fine regions correspond to 'fine' packets in ULC lath martensite. 11,13) Hereafter, type II blocks having different CP groups from the surrounding coarse packet are called fine packets. In Fig.…”

“…The specimens were austenitised at 1 273 K for 0.6 or 3.6 ks before being quenched in water to obtain lath martensite. The ULC steel was studied before 13) and re-examined to compare with the microstructure of the LC steel. In this study, we focused on the coarse packets and their blocks in the ULC and LC steels because of the following concept.…”

“…The plate-like blocks contain sub-blocks with a complex shape. 12,13) In high-carbon lath martensite, sponge-like blocks with partially plate-like units appear in coarse packets. 11) As described above, the 3D microstructures of ultra-low and high carbon lath martensite are significantly different.…”