Microstructure Of Lath Martensite Research Articles

Analysis on surface morphology formation process of Al-Si coated 22MnB5 steel during laser welding

A special morphology is formed on the upper surface of an Al-Si coated steel laser welded joint. Due to the uneven mixing of the coating element and the base metal (BM), the brittle phase is formed at the fusion line on the upper surface of the welded joint, resulting in its mechanical properties not meeting the requirements. In this study, the surface morphology formation process of Al-Si-coated steel laser-welded joints is analyzed. The results show that a wavy structure is formed at the edge of the weld under the action of the Karman vortex street. On the one hand, a multiphase microstructure of δ-ferrite, lath martensite (LM) and Fe-Al intermetallic compounds (Fe-Al IMCs) is formed at the convexity of the wavy structure. The brittle phase of the fusion line at the upper surface of the welded joint is δ-ferrite. On the other hand, since the melting point of the Al-Si coating at the concavity (between the adjacent convexity) is much lower than the temperature of the surrounding liquid metal, the Al-Si coating at this position is melted into the molten pool under the action of heat conduction and flows to the weld center under the action of gravity, forming river-like Fe-Al IMCs on the surface of the weld. By removing the coating layer or reducing the welding speed, the formation of a Karman vortex street can be effectively avoided, and finally, δ-ferrite precipitation at the weld surface fusion line can be inhibited.

Influence of Mo Content on the Precipitation Behavior of 13Ni Maraging Ultra-High Strength Steels

This study offers valuable insights into the precipitation behavior of 13Ni maraging steels, emphasizing the role of molybdenum content in their microstructure, strengthening, and precipitate evolution. Precipitate morphology and crystallography were examined using a combination of high-resolution transmission electron microscopy and selected area electron diffraction. Strengthening mechanisms were assessed through Vickers hardness measurements. All the examined samples exhibited a lath martensite microstructure and displayed an increasing hardness over the aging time. The molybdenum content not only influenced the presence of retained austenite in the initial microstructure but also affected the type of precipitates formed during the early aging stages. Initially, Ni3Mo precipitates were formed, succeeded by the formation of more stable Fe2(Mo,Ti) Laves precipitates. The ultra-high strength of 13Ni maraging steels arises from the combination of the precipitate type and size distribution. The base composition of 13Ni maraging steels achieved a peak hardness of 798 HV1 through the precipitation of Laves Fe2(Mo,Ti) phases ranging from 3 to 14 nm in diameter.

Improving the ductility of ultrahigh strength lath martensite through heterogeneous carbon distribution

This investigation aims to improve the ductility of ultrahigh strength (>2 GPa) press hardening steels (PHS) by exploiting the heterogeneous carbon distribution in lath martensite. The proposed strategy is demonstrated in a medium carbon PHS with a lath martensite microstructure produced by quenching austenite. It is found that by enhancing auto-tempering, the solute carbon content in the coarse laths formed earlier during martensitic transformation can be reduced, leading to the softening and ductilizing of these laths. The softened coarse laths of small volume fraction provide significant improvement in the post-uniform elongation (PUE) without compromising the ultrahigh strength of lath martensite. The remarkable improvement in the PUE of ultrastrong lath martensite achieved by exploiting heterogeneous carbon distribution is expected to be instructive for the development of ultrahigh strength PHS with good ductility and fracture toughness.

Low-temperature bonding between F82H and Cr assisted by metastable Cr interlayers applied by pulsed laser deposition

The joining of F82H and pure Cr metal has been demonstrated at temperatures as low as 798 K, assisted by the prior application of a thin metastable Cr interlayer to both bonding surfaces using pulsed laser deposition. Bonded F82H/Cr interfaces were characterised by SEM-EDS and TEM analysis, with respect to diffusion behaviour, chemical composition and microstructure. At higher bonding temperatures, an interface layer formed on the Cr side of the joint, which was rich in M23C6, though its structure and thickness was different depending on whether Cr interlayers were applied. As the bonding temperature was decreased stepwise, the interface layer thickness reduced to a minimum of ∼60 nm and appeared to adopt a film-like structure. Although the chemical composition of the thinner interface layers could not be determined, they were most likely M23C6 in a film-like form, which subsequently formed particles, and then columns, as the bonding temperature and interface layer thickness increased. Hardness around the interface increased as compared with the bulk F82H and Cr, though the peak hardness value decreased as the interface thickness decreased. The hardness of bulk F82H and Cr was similar before and after bonding at all temperatures, suggesting little impact on mechanical properties. Diffusion of Cr into F82H and Fe into Cr was not detected at any of the conditions tested, and the original tempered lath martensite microstructure of F82H appeared to have been retained.

Evolution of the F82H/Cr interface after solid-state diffusion bonding below Ac1 temperature: examination of microstructures and hardness

F82H steel and pure Cr metal were joined by solid-state diffusion bonding under high vacuum at various temperatures below Ac1 in the range 964 to 1072 K for 240 min. Bonding was successful at all temperatures and the diffusion behaviour, chemical composition and microstructure at the F82H/Cr interface and interdiffusion zone were characterised using SEM-EDS and TEM analysis. A hard, M23C6-rich interface layer formed on the Cr side of the joint due to diffusion of C from the F82H steel. The thickness of the M23C6-rich interface layer reduced with reducing temperature, from ∼1.1 µm when bonded at 1072 K, to ∼0.2 µm at 964 K. Cr diffusion into F82H was only observed at the highest temperature of 1072 K, with minor Cr enrichment detected at a depth of <1 µm into the F82H. The slow diffusion behaviour below Ac1 meant a lath martensite microstructure appears to have been retained throughout the F82H after bonding below the typical tempering temperature, and hardness testing of the F82H suggests no post-bonding heat treatment would be required. Simple models for the interface evolution of the F82H/Cr joint both above and below Ac1 are proposed, which can act as reference points for the further development of Cr-coated F82H and its likely behaviour in a range of fusion power plant operational scenarios.

Influence of Martensite/Bainite Dual Phase-Content on the Mechanical Properties of EA4T High-Speed Axle Steel.

In this work, we have investigated the effect of martensite/bainite dual phase content on the mechanical properties of EA4T high-speed axle steel. For evaluation and control of the strength, ductility, and toughness of steel, the microstructure of lath martensite (LM) and granular bainite (GB) was clarified through an optical microscope (OM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). Besides, the tensile fracture morphology was studied by scanning electron microscopy (SEM). For this purpose, this study conducted a quantitative analysis of the LM and GB fractions using the Pro Imaging software-2018 of OM. The remarkable effect of the LM/GB structure on mechanical properties is discussed. The results have shown that by increasing the volume fraction of the GB structure, the LM structure is refined and its microhardness and strength are improved. Meanwhile, the micro strength of LM follows the Hall-Petch relationship with the lath martensite packet size. Subsequently, the mechanical property prediction model of EA4T steel based on the LM/GB content was established by regression analysis of all experiment dates. When the LM fraction in the steel is about 40-70%, a superior combination of strength, ductility, and toughness can be obtained in EA4T steel.

Preliminary investigation of Cr-coated F82H: Evolution of microstructure and mechanical properties at the F82H/Cr interface during solid-state diffusion bonding above Ac1 temperature

Solid-state diffusion bonding was employed to join F82H steel and pure Cr metal under high vacuum at various temperatures above Ac1 in the range 1114 to 1323 K for 60 min, and at 1323 K for 120 min. After successful bonding at all temperatures, the F82H/Cr interface and interdiffusion zone were characterised using SEM-EDS, TEM and EBSD analysis, with respect to diffusion behaviour, chemical composition and microstructure. A thin interface layer formed on the Cr side of the joint, which decreased in thickness with decreasing bonding temperature, and was rich in M23C6 with mixed columnar and particulate nature. The depth of Cr diffusion into F82H decreased with decreasing bonding temperature. At temperatures ≥1231 K a layer of ferrite grains formed in the F82H region adjacent to the interface, due to the combined stabilising effect of enhanced Cr concentration and depleted C. At the lowest bonding temperature of 1114 K, a lath martensite microstructure was retained throughout the F82H due to a significantly lower extent of Cr and C diffusion. The diffusion coefficient of Cr in F82H was determined at all temperatures examined, and the activation energy of Cr diffusion in F82H >Ac1 was approximated as 225 KJ mol−1. Finally, hardness testing of the interface and interdiffusion zone suggests the F82H properties were retained after bonding at the lowest temperature of 1114 K, however at higher temperatures the newly formed ferrite region was softer. Further research is required to understand the impact of the hard M23C6-rich interface layer on mechanical properties of the joint, and whether microstructural change can be minimised, or even eliminated, below the Ac1 temperature.

Strain rate dependence of strengthening mechanisms in ultrahigh strength lath martensite

The strain rate dependent mechanical properties of lath martensite are important for developing ultrahigh strength steels for automobile applications. Here, taken 2 GPa grade press hardening steel as a model material, we explored the strain rate sensitivity of lath martensite and the underlying physics. Uniaxial tensile tests were carried out over a wide range of strain rates from 10−3 to 1450 s−1 to determine the rate dependent mechanical properties. Strain rate exerts minor effect on mechanical properties at strain rates below 102 s−1. In contrast, both yield strength and work-hardening rate substantially increase with strain rate during high-strain-rate deformation. Microstructural evolution was characterized using electron backscatter diffraction and transmission electron microscopy. Particularly, synchrotron X-ray diffraction was used to measure dislocation density. The respective strengthening contributions from the friction stress, dislocations and high-angle block boundaries in lath martensite were evaluated on the basis of the measured microstructural parameters as well as related phenomenological models for predicting their strengthening effects. It is found that the enhanced yield strength during high-strain-rate deformation is due to a larger lattice friction for dislocation slip. The higher work-hardening rate is attributed to the enhanced mechanical heterogeneity within the current lath martensite microstructure at higher strain rates, which leads to larger strain gradient and thus promoted generation of geometrically-necessary dislocations.

Influence of scanning strategies on laser-cladded AISI 420 steel

ABSTRACT In this study, the influence of different laser scanning strategies on substrate deformation, microstructure and properties of laser-cladded thin-walled 420 martensitic stainless steel (MSS) plate have been carefully investigated. The continuous offset-out scanning strategy (COSS) and continuous raster scanning strategy (CRSS) was compared with the subarea offset-out scanning strategies (SOSS) and subarea raster scanning strategy (SRSS) with different sequences. The results show that the minimum and maximum macroscopic deformation was generated using COSS and CRSS, respectively. The temperature distribution with characteristic of quasi-symmetry was generated by COSS, which can largely suppress the substrate deformation, in marked contrast to a long ellipse symmetry by CRSS. All the laser-cladded 420 MSS layers exhibit lath-martensite microstructure, while some cracks are distributed by using SOSS. The optimal comprehensive performance of the laser-cladded 420 MSS can be obtained using COSS, with smallest outermost deformation of 1.57 mm, superior hardness of 54.9 HRC and wear rate of 0.9626 × 10−5 mm3/N·m.

Liquid metal embrittlement susceptibility of zinc-coated martensitic sheet steels

This paper investigates the liquid metal embrittlement (LME) behavior of three martensitic steels (MS-1, MS-2, and MS-3), each of 1500 MPa tensile strength. Their LME response was measured via hot ductility testing at a fixed strain rate over a range of temperatures (700–950 °C). Despite having the same room temperature strength and general lath martensitic microstructure, the three MS steels exhibited different LME responses. When LME occurred, it proceeded in an intergranular manner within a limited temperature range and with the most severe response at 850 °C. The lath martensitic microstructure was investigated in detail to correlate it to the differences in the LME response. It was found that the three MS steels had different microstructural morphologies and prior austenite grain sizes (PAGS). The LME sensitivity appears to correlate with these microstructural parameters. The MS-2 steel exhibited higher LME susceptibility and possessed the smallest PAGS and a high fraction of low-angle grain boundaries. In contrast, MS-1 was immune to LME at all tested temperatures and had a large PAGS and a high fraction of large-angle grain boundaries. To better compare hot ductility results between steels, a “LME index” was formulated based upon the measured LME severity over the tested temperature range, with a large LME index indicating a severe LME response. Among the three MS steels, MS-2 had the highest LME index while MS-1 had the lowest.

Microstructure Characteristics and Wear Performance of a Carburizing Bainitic Ferrite + Martensite Si/Al-Rich Gear Steel

In this paper, a new low-carbon alloy gear steel is designed via Si/Al alloying. The carburizing and austempering, at a temperature slightly higher than the martensitic transformation point (Ms) of the surface and much lower than the Ms of the core, for different times, were carried out on the newly designed gear steel. After heat treatment, a series of different microstructures (superfine bainitic ferrite + retained austenite, superfine bainitic ferrite + martensite + retained austenite, and martensite + retained austenite) were obtained on the surface, whilst the low-carbon lath martensitic microstructure was obtained in the core. The microstructure of the surface was examined using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The phase composition was analyzed using X-ray diffraction (XRD). The hardness and wear resistance of the surface as well as the hardness distribution of carburizing layer of the samples with different microstructures were studied. The results show that the Si/Al-rich gear steel, after carburizing and austempering at 200 °C for 8 h, not only has excellent mechanical properties but also has high wear resistance, which meets the technical requirements of heavy-duty gear steel. The research work in this paper can provide a data reference for the application of carburized steel with mixed microstructures of bainitic ferrite and martensite in the design of heavy-duty gear.

Corrosion Assessment for Aging Treatment of Rolled and Selective Laser Melting 18Ni 300 Maraging Steel

In this study, we investigate the microstructure and the corrosion performance of an 18Ni 300 maraging steel after prepared by selective laser melting (SLM) technique and the effects of aging. The commercial 18Ni 300 maraging steel fabricated by rolling followed by aging acts as a comparison. The SLM specimens showed fine microstructures with homogeneous cellular and columnar dendrites, which resulted in relatively uniform corrosion, while the conventionally rolled specimens displayed a typical lath martensite microstructure. The aging treatment accelerated the corrosion rate possibly due to the galvanic corrosion between intermetallic precipitates Ni3(Ti, Mo) and martensite matrix, as well as the corrosion of Ni and Co which diffuses into the matrix. The formation of unmelted particles made of (Al, Ti)O during the SLM process is deleterious to the corrosion resistance as they lead to localized attack. The long-term immersion makes for the accumulation of thick and corrosion product layer, which prevents further dissolution of the metal substrate by preventing oxygen and ion species from contacting metals.

The influence of parent austenite characteristics on the intervariant boundary network in a lath martensitic steel

The influence of the parent austenite deformation state on the intervariant boundary network (i.e., population, plane orientation, and connectivity) of a lath martensitic microstructure was investigated using conventional EBSD mapping and five-parameter boundary analysis approach along with quantification of boundary connectivity using homology metrics. The lath martensite largely revealed a bimodal misorientation angle distribution, closely matched with the Kurdjumov–Sachs (K–S) orientation relationship. The application of deformation significantly changed the distribution, gradually reducing the intensity of the 60° misorientation angle peak. This was largely ascribed to substructure development within the parent austenite upon deformation, which stimulates particular variant/s having a habit plane (011)α′ closely parallel to the primary (111)γ and/or secondary (1overline{1 }1)γ slip plane. The interaction of these variants eventually promoted specific intervariant boundaries (e.g., 60°/[111], 10.5°/[011], and 49.5°/[110]) at the expense of 60°/[011]. The application of deformation in the parent austenite did not change the intervariant boundary plane character distribution, which mostly exhibited an anisotropic character terminated on {110} planes because of the displacive nature of the martensitic transformation. However, the extent of anisotropy progressively decreased with increasing strain in the austenite prior to transformation. The grain boundary network connectivity was markedly altered due to the local variant selection induced by the deformation. Deformation in the austenite regime generally decreased the connectivity of boundaries having a {110} plane orientation. The intervariant boundaries with the {110} twist character also displayed a similar trend, though the connectivity of {110} tilt boundaries progressively enhanced with increasing strain. The former was closely matched with a decrease in the population of 60°/[110] intervariant boundaries with the strain. The current findings suggest that the intervariant boundary network of lath martensite can be manipulated through changes in the parent austenite deformation state which, in turn, can be used to enhance key final product properties such as toughness.Graphical abstract

Influence of Heat Input on Microstructure and Mechanical Properties of Gas Tungsten Arc Welded HSLA S500MC Steel Joints

High-strength low alloy (HSLA) S500MC steel is widely used for chassis components, structural parts, and pressure vessels. In this study, the effects of heat input during automatic gas tungsten arc welding (GTAW) on microstructure and mechanical properties of thermomechanically controlled processed (TMP) S500MC steel were investigated. A butt joint configuration was used, and welding was performed in autogenous mode. Six different levels of heat input namely 1.764 kJ/mm, 1.995 kJ/mm, 2.035 kJ/mm, 2.132 kJ/mm, 2.229 kJ/mm, and 2.33 kJ/mm were considered. Microstructural investigations revealed a different microstructure than base metal in the fusion zone (FZ) of all welded joints which was most likely due to a lath martensitic microstructure surrounded by retained austenite. With increased heat input, the amount of retained austenite and the size of packets increased. In the heat-affected zone (HAZ), two distinct regions of coarse grain (CG-HAZ) and fine grain (FG-HAZ) were observed. Due to the presence of carbides in the HAZ, mostly a martensitic microstructure with smaller packets, compared to FZ, was formed. By increasing the heat input and through the dissolution of carbides, the dimension of packets increased. Due to microstructural changes and grain growth, in both the FZ and the HAZ, the mechanical properties produced by TMP were lost in these regions. However, failure occurred in the base metal of all samples with a maximum tensile strength of 690 MPa. Thus, tensile strength in the weld zone and HAZ were higher than the base metal even for the highest heat input indicating the formation of a good joint between S500MC plates with GTAW, regardless of heat input.

High-quality remanufacturing of HSLA-100 steel through the underwater laser directed energy deposition in an underwater hyperbaric environment

Underwater laser directed energy deposition (UDED) can be employed to repair and maintain the offshore engineering structures due to its advantages of flexible adjustment of feedstock materials and controllable heat input to the structures. For the first time, preprepared HSLA-100 steel plates were successfully remanufactured by UDED at an ambient pressure of 0.3 MPa (water depth of 30 m). The relationships between the hyperbaric underwater environment, solidification process, microstructures and mechanical properties of the HSLA-100 steel were clarified. The results show that the cooperation of surrounding water, central gases and gas curtain gas with large flow rates increased the cooling rates of the underwater melt pool. A lath martensitic microstructure with high dislocation densities and a number of inclusions was formed in the sample remanufactured by UDED. The in-situ precipitation of Cu-enriched nanoparticles was caused by the unique intrinsic heat treatment involved in the underwater deposition process. The average diameter of the Cu-enriched nanoparticles increased with increasing laser energy density. The microstructure of the sample remanufactured by UDED was harder than that of the sample remanufactured by in-air DED. The Charpy impact toughness and tensile properties of the samples remanufactured by UDED were close to those of the sample remanufactured by in-air DED. This work demonstrates the feasibility of high-quality remanufacturing of HSLA-100 steel via UDED in a hyperbaric underwater environment. The results obtained in this study could provide useful guidance for the application of UDED to offshore engineering structures.

Combining crystal plasticity and electron microscopy to elucidate texture dependent micro-mechanisms of tensile deformation in lath martensitic steel

A modified 9Cr-1Mo steel having lath martensitic microstructure has been subjected to the hot-rolling at three different temperatures followed by a normalization at 1025°C to form different crystallographic textures after thermomechanical processing. The samples hot-rolled at 875°C, 1000°C and 1050°C showed major texture components as Goss (i.e. {110}<001>), Cube (i.e. {001}<01¯0>) and Gamma (i.e. {111}<1¯1¯2>), respectively. Next, these samples have been uniaxial tensile tested at quasi-static strain rate at room temperature, and tensile properties are evaluated. The results indicated almost similar strength levels for Goss and Cube oriented specimens, and significantly reduced strength for Gamma oriented samples. However, the Cube and Goss oriented samples showed different strain hardening rates owing to the occurrence of deformation induced twinning and anti-twinning phenomenon as revealed by the Visco-plastic self-consistent polycrystal plasticity simulations. Simulation results were validated with experimental observations using high-resolution transmission electron microscopy. Anisotropic parameters have also been simulated considering the difference in initial crystallographic orientations. Study of deformation micro-mechanism at different length scale of martensitic units (e.g., prior-austenite grain, martensitic packets, blocks, sub-blocks, and laths) revealed negligible rotations at the prior-austenite grain level, whilst the lattice rotations were found to be significant at martensitic sub-block (i.e. variant) length scale. The investigation indicated that some specific types of martensitic variants generally participated in large lattice rotation during deformation for differently textured samples.

Homogenization timing effect on microstructure and precipitation strengthening of 17–4PH stainless steel fabricated by laser powder bed fusion

Effective post-heat treatment is critical to achieve desired microstructure for high performance of additive manufacturing (AM) components. In this work, the timing influence of homogenization and its tuning mechanism on microstructure-property relationships of 17–4PH steels fabricated by laser powder bed fusion have been studied systematically. A mixed microstructure of lath martensite, δ-ferrite and precipitation of NbC, nano-oxides and fcc ε-Cu were observed in the as-built sample. To explore an optimum post-heat treatment, homogenization was performed at 1150 °C for different times followed by aging at 482 °C for 1 h with the help of thermodynamic simulations. It is identified that homogenization for 1 h followed by aging leads to the best combination of strength and ductility due to the refinement of martensite and precipitation distribution. The optimal tensile properties are comparable to the traditionally fabricated 17–4PH material. The martensite growth and coarsening of NbC and nano-oxides during long-term homogenization impair the strength and toughness significantly. The secondary phases of NbC, nano-oxides and CRPs play critical roles in the strengthening and fracture via their precipitation and distribution by tuning the homogenization time. This study demonstrates that altering the martensite lath and precipitation behavior using rational homogenization heat treatment is an effective pathway to improve the mechanical properties of additive manufactured 17–4PH alloys.

Interfacial plasticity mediated by lath boundaries in reduced-activation ferritic/martensitic steels

• The onset lath boundary sliding sensitively depends on the microscopic structure of the interfacial planes. • The lath boundary sliding is mediated by the motion of dislocation arrays lying on the interfacial plane. • For predominant lath boundary sliding: - The Burgers vector of the interfacial dislocations must lie on the interfacial plane. - The interfacial plane must be parallel to the common slip planes in the BCC structure, i.e., {110}. In the hierarchically-arranged crystallographic structure of reduced-activation ferritic/martensitic (RAFM) steel, the smallest but the most abundant microstructural unit, i.e., the lath, plays an important role in dislocation plasticity. Due to the multi-scale complexity in the lath-martensitic microstructure, miniaturized mechanical characterization at very fine scale is required to understand the deformation mechanism associated with laths. In this study, uniaxial micro-compression tests combined with rigorous crystallographic analysis were performed to figure out the plastic deformation mechanism of lath boundary sliding in RAFM steels. These experimental results were further interpreted via molecular dynamics simulations to discover the underlying dislocation mechanisms. We found that the amount of lath boundary sliding is controlled by the crystallographic orientation of the lath boundary plane, the direction of Burgers vectors of the interfacial dislocations, and the magnitude of resolved shear stress on the lath boundary plane. Also, the effect of normal stress on the lath boundary plane was investigated.

The investigation on Johnson-Cook model and dynamic mechanical behaviors of ultra-high strength steel M54

Ultra-high strength steel M54 has been considered as promising candidates for structural applications in spacecraft and aircraft, because of its excellent static and dynamic characteristics. Numerical simulation is widely accepted as an effective way to help understand the dynamic response to structures under impact loadings. In this study, the Johnson-Cook (J-C) constitutive parameters and failure parameters used in ANSYS were accurately determined by fitting static and dynamic experimental data. Based on the obtained experimental results, the good combination of ultra-high strength and good elongation under quasi-static condition was ascribed to its special chemical composition, the microstructure of ultrafine lath martensite with high dislocation density, ultra-high martensite content, uniform distribution of nano-sized M2C carbides and the existence of reverted austenite films. The dynamic compressive properties at high strain rates were investigated systematically by split Hopkinson pressure bar (SHPB) system. A remarkable strain rate strengthening effect came into evident on flow stress of the steel. The dynamic strengthening mechanisms were considered as fine-grain strengthening, substructure strengthening, and dislocation strengthening. Also, adiabatic shear behaviors of the steel under dynamic compressive tests were investigated. The results demonstrated that the strain rate was a key basis to judge stability of the M54 steel under impact loading. Recrystallization mechanism was responsible for the appearance of fine equiaxed grains within adiabatic shear bands (ASBs). The evolution of the ASBs was simultaneously revealed. The obtained experimental results and established J-C constitutive model could enhance the scientific understanding of dynamic mechanical behaviors of the M54 steel and provide reliable constitutive parameters for better guiding relevant engineering applications.

Relationship between strength and hardness in single packet and single block structures of lath martensite

Microtensile tests were performed to elucidate the mechanical properties using micro-scale specimens extracted from single packet (SP) and single block (SB) structures of lath martensite in steels with low- and medium-carbon contents. In addition, micro-Vickers hardness and nanoindentation tests were conducted to measure the hardness corresponding to single substructural units. A habit-plane-dependent-yielding occurred in the SP structures, and the anisotropic yielding behaviour was also observed in the SB structures excluding the block boundaries, independent of the carbon content. While the low-angle grain boundaries are included in the SB structure, they are difficult to hinder the dislocation motion. Therefore, this indicates that the plastic anisotropy on microstructure of lath martensite is attributed to the dislocation cell structures formed during the early stage of deformation. The tensile strength of the SP and SB structures were correlated with the micro-Vickers hardness and nanohardness, respectively. The average correlation coefficient for the SP structures coincided with the Tabor’s constant, whereas the average coefficient for the SB structures was half lower than that of the Tabor relation.