Anisotropy of strength and plasticity in lath martensite steel
Introduction
As a result of increasing crash safety and fuel economy performance requirements, there is a demand for high-strength steels with higher capabilities in the automobile industry [1], [2]. Among these high-strength steels, dual-phase steel and transformation-induced plasticity steel have attracted much attention because of their excellent balance between strength and ductility [3], [4]. In these steels, the distribution and morphology of the martensitic phase and the tendency towards martensitic transformation impact the mechanical properties [5], [6], [7], [8]. Therefore, in high-strength steels, it is important to mechanistically understand the effect of the martensite on the mechanical characteristics.
In low-to-medium carbon steels, the martensite microstructure is composed of a hierarchical structure that includes the prior austenite grains, packets, blocks, and laths, as schematically shown in Fig. 1 [9]. The martensite laths are formed based on the Kurdjumov-Sachs (K-S) orientation relationship from austenite. There are twenty-four martensite variants originating from a single austenite grain. The packet is characterised by an aggregation of blocks with a common {111} habit plane from the prior austenite grain. Thus, the packet can contain six martensite variants. Recent progress in microstructural characterisation techniques such as electron back-scatter diffraction (EBSD) analysis, transmission electron microscopy, and three-dimensional tomography has helped clarify the details of the martensite microstructure [9], [10], [11], [12]. An EBSD study by Stormvinter et al. [10] revealed that the carbon content has a strong impact on the variant pairing tendency of martensite. With a low carbon content, variant pairing occurs within a close-packed group that consists of six variants with a common habit plane, and its tendency decreases with increasing carbon content [10]. It was found [9] that the packet was composed of three parallel blocks with different crystallographic orientations, and each block consisted of a pair of variants with a specific K-S orientation relationship, i.e. sub-block (Fig. 1). In addition, a three-dimensional atom probe study by Morito et al. revealed [11] that the austenite films were retained between the martensite laths and contained high carbon when compared to the martensite laths. Although the lath martensite has a complicated microstructure, the microconstituents have defined crystallographic orientation relationships, as previously mentioned. Therefore, it is necessary to analyse the deformation behaviour from a crystallographic perspective to elucidate the strengthening of the lath martensite.
Recent advances in nano/micro-mechanical testing techniques such as nanoindentation [13], [14], microbending [15], microcompression [16], and microtensile testing [17], [18] have made it possible to evaluate the mechanical characteristics of the microconstituents in martensite structures. Microtensile tests of single packet structures revealed a habit-plane-orientation-dependent yielding, suggesting that a block boundary effectively acts as a barrier to the motion of dislocations [17]. A compression study using micropillars with block and packet boundaries revealed significant strain hardening due to the geometric constraint by both the boundaries [16]. A microbending study indicated that a block boundary restricts the dislocation motion, while a sub-block boundary does not significantly impact the strengthening [15]. In contrast, a uniaxial microtensile study by Du et al. [18] revealed that both block and sub-block boundaries act as barriers to the dislocation motion, while the contribution of the sub-block to the strengthening is slightly lower than that of the block boundary. Therefore, the role of each boundary in the plasticity of the lath martensite structure is controversial.
Simulations by Maresca et al. [19], [20], [21] have recently indicated that localised shearing along the lath habit plane dominates the plasticity of the lath martensite structure. Microtensile tests of single block structures were performed in the present study with special focus on the crystallographic orientation. A crystal plasticity analysis was employed to identify the slip activity in the microtensile specimens.
Section snippets
Material and experimental methods
The material used in the present study was a low-carbon, low-alloy steel that was composed of 0.13C, 0.25Si, 0.92Mn, 0.02Ni, 0.83Cr, 0.18Cu, and 0.32Mo (in mass%), with the remaining steel being composed of Fe. Samples with dimensions of ~10×10×1 mm3 were heated to a temperature of 1473 K, and held at this temperature for 120 s. This was followed by water-quenching, which was performed to obtain a fully lath martensite microstructure. After heat treatment, the samples were polished with emery
Experimental results
Fig. 3 shows the stress-strain curves obtained using microtensile testing of the single block specimens. The SB1 specimen, in which the habit plane was highly shear stressed, exhibited the lowest yield and flow stresses. The yield stress, which was defined as the stress at a plastic strain of 0.2%, was equal to 809 MPa for the SB1 specimen. This value was lower than the 935 MPa and 1148 MPa yield stress values for the SB2 and SB3 specimens, respectively, which were unfavourably oriented for the
Crystal plasticity finite element analysis method
In this study, a crystal plasticity finite element method (CPFEM) was used to identify the slip activity in the microtensile experiments. The constitutive model used in the method was a rate-dependent large strain crystal plasticity model proposed by Peirce et al. [24]. The slip rate of the slip system i is calculated bywhere and m are the reference slip rate and strain rate sensitivity parameter, respectively. In Eq. (1), and are the resolved shear
Conclusions
Microtensile testing combined with crystal plasticity finite element analysis was used to elucidate the anisotropic plastic deformation behaviour in single block structures of lath martensite. The contributions of the block and sub-block boundaries to strengthening were also analysed. The conclusions are summarised as follows:
- (1)
As in single packet structures, the yielding in single block structures without block boundaries depended on the habit plane orientation. The resolved shear stress at
Acknowledgement
The present work was supported by a Grant-in-Aid for Scientific Research (A) 15H02302 from the Japan Society for the Promotion of Science (JSPS). TM and YM gratefully acknowledge support from the ‘Program for Advancing Strategic International Networks to Accelerate the Circulation of Talented Researchers’ R2608.
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2022, Materials Science and Engineering: ACitation Excerpt :It was confirmed that the habit plane is inclined at 45° with respect to the LD for the I-type specimens and parallel to the LD for the P-type specimens. The details of habit plane determination based on the K–S orientation relationship are described elsewhere [19]. The relationship between tensile strength and strain-to-failure for the SP structure of lath martensite with various carbon contents is shown in Fig. 6a. Steels generally possess higher strength and lower elongation as the carbon content increases at the macroscale, including several strengthening mechanisms.