Crystallography Of Lath Martensite Research Articles

Substructure and crystallography of lath martensite in as-quenched interstitial-free steel and low-carbon steel

In this study, the substructures and crystallographic features of as-quenched lath martensite in interstitial-free (IF) steel and low-carbon steel (0.2C steel) were investigated using focused ion beam-scanning electron microscopy serial sectioning and advanced transmission electron microscopy. A three-dimensional analysis revealed that the morphology of the so-called lath, the smallest structural unit, exhibited a wedge-shaped plate instead of the ideal lath shape. The habit plane orientations were distributed from (5 7 5)A to (1 1 1)A in the IF steel, whereas they were near to that predicted based on the phenomenological theory of martensite crystallography in the 0.2C steel. In the IF steel, well-developed dislocation cells, tangled dislocations, and curved dislocations could be observed. Conversely, straight dislocations and cementite formation were observed in the 0.2C steel. Moreover, the orientation changed significantly inside a given lath in both steels. The lath boundary comprised regularly spaced steps with (0 1 1)M atomistic terrace planes. Interfacial dislocations corresponding to the discontinuity of the (0 1 1)M lattice planes could be observed at steps, and these dislocations were introduced to compensate for the misorientation between adjacent laths. Additionally, carbon segregations were observed at the lath and block boundaries in the 0.2C steel. Our observation results on the dislocation structures, cementite formation, and segregation of carbon atoms on the lath and block boundaries indicated that the substructure changed significantly during quenching. Based on our experimental results, we discuss the origin of the substructures and crystallographic features of lath martensite.

The microstructure and crystallography of lath martensite with Greninger-Troiano orientation relationship in a Fe-12.8Ni-1.5Si-0.22%C steel

The excellent matching of strength and toughness with respect to lath martensite is closely related to its hierarchical configuration and crystallographic features. Recently, a few literatures reported that the Greninger-Troiano (G-T) relationship, instead of the generally accepted Kurdjumov-Sachs (K S) relation, was determined in some lath martensite steels. However, the underlying reason is still waiting to be explored. In the present study, systematic characterizations were performed in water-quenched Fe-12Ni-1.5Si-0.2C steel firstly, then a modified version of crystallographic model developed by Morris et al. was proposed to clarify the microstructure and the crystallography with G-T relationship. As the main crystallographic features, the orientation relationship between prior austenite and martensitic variants is close to G-T rather than K S relation. Further characterization shows that the block interface is parallel to the annealing twin boundary of prior austenite, indicating the {111} γ habit plane of blocks. Naturally, three crystallographically distinct blocks with {111} γ interfaces can be configured into a packet with self-accommodation manner to minimize macro strain. In fact, such a hierarchical configuration with G-T relationship can be understood clearly by the modified version of crystallographic model. Firstly, regarding the bivariant block as the basic element which undertakes the invariant plane shear, the total shear strain within each packet region can thus be effectively cancelled out on the common habit planes of {111} γ due to the self-accommodation effect, leading to an essentially shear strain-free pattern; Secondly, a higher hierarchy is accompanied with a significant decrease of strain and only a homogenous dilation of 0.02 remains in a prior austenite grain, which is closely related to the unique hierarchy; Thirdly, the G-T relationship is natural preferred for the present steel, which satisfies two conditions in this crystallographic model: (i) a minimum of angular deviation between the {111} γ and the habit plane of bivariant blocks; (ii) three slip systems of dislocations with 〈111〉 α’ Burgers vectors involved during the martensitic transformation, which is also universally observed in the TEM examinations. Besides that, compared with the case of K S relationship, a potential influence of G-T relationship on mechanical properties may embody in inducing a lower ductile-brittle transition temperature and better toughness of lath martensite steels. • G-T relation was determined by a newly-developed scheme. • Microstructure and crystallography were clarified by a modified model. • A higher hierarchy is accompanied with a significant decrease of strain.

Morphology and crystallography of lath martensite in weld metal of high strength steel by double sided laser welding

The morphology and crystallography of the lath martensite in the weld metal of high strength steel by double-sided hybrid fiber laser-arc welding were investigated. The lath martensite was found to have Kurdjumov-Sachs orientation relationship. The amount of the lath martensite in the block was affected by the cooling direction of the weld pool. All the four packets were distributed randomly within a columnar grain. The blocks were approximately in square shape. All the 24 variants appeared within the columnar grain and some regions included certain combinations of two particular variants. The lath martensite in the weld metal had preferred orientation which was lath martensite (1 1 1) ‖ substrate surface.

Effect of Solution Carbon and Nitrogen on the Microstructural Size and Crystallography of Lath Martensite in Fe–N and Fe–C Alloys

Microstructures of lath martensite that contains bcc or bct martensite crystals in Fe–C alloys are known to depend on the carbon content of the alloys. The effect of nitrogen content on microstructure, however, has not yet been elucidated. This study elucidates the effects of carbon and nitrogen content on microstructures via local crystallographic analysis. We found that the packet sizes are similar when the nitrogen content in the alloys are the same as the carbon content, with the packet size decreasing with increasing carbon and nitrogen content. The block and sub-block thicknesses in low nitrogen lath martensite are smaller than those in low carbon lath martensite, whereas those in medium and high carbon and nitrogen lath martensites are similar. Martensite lath thickness and dislocation density in the Fe–N alloy laths are lower than those in the Fe–C alloys laths, and the distribution of misorientation angles between adjacent the blocks and sub-blocks in Fe–N alloys is similar to that in Fe–C alloys.

Investigation on martensitic steel using a quenching-partitioning-tempering process via electron backscatter diffraction analysis

A new quenching-partitioning-tempering (Q-P-T) process was designed based on the recently developed quenching and partitioning (Q & P) treatment by Speer et al. The orientation relationships (ORs) of martensite (α) in Q-P-T steel tempered under different conditions were investigated using a scanning electron microscope with an electron backscatter diffraction unit. The morphology and crystallography of lath martensite within a packet were analyzed. Martensites exhibiting an initial austenite (γ) grain in the orientation image map of different samples were chosen and their {111}α pole figures were drawn. The pole figures were fitted according to the ideal Kurdjumov-Sachs (K-S) and Nishiyama-Wasserman (N-W) ORs, which were calculated using Matlab and with a γ crystal-oriented (001)γ[100]γ as the reference frame. The martensite OR in the Q-P-T steel follows both the K-S and N-W ORs; however, the K-S OR is predominant. This characteristic does not change in the samples further tempered at different temperatures.

Effect of Cooling Rate on Morphology and Crystallography of Lath Martensite in Fe-Ni Alloys

The development of blocks and subblocks in the lath martensite was observed with SEM/EBSD and TEM/Kikuchi diffraction patterns analyses. Morphology of lath martensite formed below Ms temperature was observed using step quenching method in 18 mass% Ni maraging steel. The development of lath martensite structure can be described as follow; lath groups with single K-S variant start to form at the austenite grain boundary. Next, other sub-blocks appear adjacent to the lath groups formed first and these lath groups form macroscopic blocks observed in optical microscopy. The morphologies of the lath martensite in Fe -23 mass% Ni alloy cooled at different cooling rates after austenitization were also observed. The packet size and block thickness decreases with increasing cooling rate, although the sub-block thickness do not change. A packet locally contains small packets with different relationship of close packed planes, and the amount of small packets increases with increasing the cooling rate.

The morphology and crystallography of lath martensite in alloy steels

The morphology and crystallography of lath martensite in two Mn-containing interstitial free steels and a maraging steel were examined in detail by a combination of transmission electron microscopy, electron backscatter diffraction in a scanning electron microscope and optical microscopy. Indirect determination of the orientation relationship between the lath martensite and the austenite was made by analysis of misorientation distributions, and a similar orientation relationship was found for the three alloys in accordance with previous observations in low-carbon steels. Furthermore, the formation of six variants in a given packet and the preferential arrangement into blocks of low-misorientation variant pairs demonstrate a universality of morphology and crystallography of lath martensite. The presence of six variants in a packet can be accounted for by the minimization of the total shape strain introduced during the transformation. The lath boundaries developed within the volume of a certain variant show alternating misorientations and a mixed tilt and twist character characterizing these structures as low-energy dislocation structures.

Crystallography of lath martensite and lower bainite in alloy steels

The crystallographic features of lath martensite and lower bainite were investigated using precise transmission electron microscopy proposed by the authors. It was found that the martensite laths related to each other by a relative rotation of about 60° around the normal to their common close-packed planes {1 1 1} A{1 1 0} M, thus resulting in an approximate pseudo-{1 1 2} M twin relationship between two adjacent laths, that all parallel martensite laths formed in a single packet kept a unique G–T orientation relationship (OR) with the austenite matrix, and that the habit plane of the lath martensite was close to {3 3 5} A. A {3 3 5} f martensite was thus revealed. Furthermore, crystallographic features similar those of lath martensite were also observed in lower bainite formed in the same steel, thus confirming a crystallographic similarity between the lath martensite and lower bainite.

Phase Transformation Crystallography of Lath Martensite

AbstractOur current understanding of martensitic transformations has been based on the Phenomenological Theory of Martensite Crystallography developed in the 1950s. Recently, a Topological Model of martensitic transformations has been presented wherein the habit plane is a semi-coherent structure, and the transformation mechanism is shown explicitly to be diffusionless. This approach is used here to model phase transformation crystallography of lath martensite in ferrous alloys. A range of network geometries is predicted corresponding to orientation relationships varying from Nishiyama-Wasserman to Kurdjumov-Sachs. Experimental observations from the literature of the dislocation and disconnection arrays, habit plane and orientation relationship are in good agreement with the model.

Effect of Austenite Grain Size on the Morphology and Crystallography of Lath Martensite in Low Carbon Steels

The crystallography, microstructure and mechanical property of as-quenched martensite of Fe-0.2C-Mn(-V) alloys of which the prior austenite grain sizes are 370-2 μm were studied. The prior austenite grain, whose size is larger than 28 μm, is divided by several packets. Those packets are subdivided by blocks containing sub-blocks, each of which corresponds to the Kurdjumov-Sachs variant. When the prior austenite grain size is about 2 μm, one packet tends to grow predominantly. Each packet is divided by blocks containing sub-blocks.

Morphology, substructure and crystallography of lath martensite in Fe-C alloys

Morphology, crystallography and dislocation density of lath martensite were examined in Fe-C alloys with various carbon contents (0.0026∼0.78 wt%C). It was found that, in low carbon alloys (≤ 0.18%C), a martensite block observed in optical microscopic scale consists of two groups of laths misoriented each other by about 10 degrees on analysis of microtexture. This result is largely different from the current view on the crystallography of martensite block, in which laths in a block is of the same variant. In contrast, a block contains laths of a single variant in high carbon alloys (≥0.61%C). As carbon content increases, block and packet sizes are refined whereas size of lath does not change significantly. Dislocation density largely increases with carbon content.

The morphology and crystallography of lath martensite in Fe-C alloys

The morphology and crystallography of lath martensite in Fe-C alloys containing various carbon contents from 0.0026 to 0.61% were studied by analyzing electron back scattered diffraction patterns in scanning electron microscopy and Kikuchi diffraction patterns in transmission electron microscopy. As carbon content increases, the sizes of both packet and block decrease. In low carbon steels (0.0026–0.38%C), a block which is observed as having different contrasts under optical microscopy contains two groups (sub-blocks) of laths which are of two K-S variants with a misorientation of about 10 degrees. On the other hand, in the high carbon alloy (0.61%C), a block consists of laths of a single K-S variant.

Crystallography of Lath Martensite in Steels

The double lattice invariant shear version of the phenomenological theory, has been used to explain all the known crystallographic features of lath martensite. When the lattice invariant shear systems are taken to be rational slip systems in martensite with slip directions along [111] and [111], the habit plane solutions converge to an (hhl) hab plane that coincides with, or is close to, (557). Up to 35 different combinations of these rational lattice invariant shear systems lead to (hhl) habit planes that lie 9° to 14° from (111). The theory correctly predicts the habit plane, the orientation relationship, (including the observed spread in the angle between the close packed directions [110] and [111]), the shape strain, the internal substructure of dense dislocations with a/[111] dislocations dominant, and the presence of an array of a/[111] dislocations approximately 3.5 nm apart in the austenite/martensite interface. The theoretical explanation of lath martensite also indicates that approximately twin-related laths can form with the same habit plane, and that this arrangement of laths in a packet will provide a significant degree of mutual accomodation of the shape strain.

LATH MARTENSITES IN LOW CARBON STEELS

The morphology and crystallography of lath martensite in low and medium carbon steels have been studied by transmission electron microscopy and diffraction. The steels have microduplex structures of dislocated lath martensite (a M direction. The overall microstructure and orientations result from minimization of the total strain and shape deformation. Considerable accommodation occurs by deformation of laths (sometimes twinned) and austenite (sometimes tripped to twin martensite). In the meantime, microchemical analyses have shown considerable carbon segregation to the martensite-austenite interface. 4 figures.