The microstructure and the strength of the lath martensite in Fe–0.2C and Fe–0.2C–2Mn alloys were analyzed as a function of the prior austenite grain size. The size of martensite packets formed ...within individual austenite grains was controlled by the austenite grain size but not affected by the Mn addition. However, the further subdivision of packets into blocks differed significantly in the two alloys, and at a given austenite grain size a smaller block size was observed in the Mn containing alloy. The yield strength of the two alloys was related to the packet size and the block size, respectively, and the results suggested that the block size is the key structural parameter when analyzing the strength–structure relationship of lath martensite in low carbon 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.
Local distribution of orientation relationship between austenite and martensite is quantitatively examined using lath martensite in an ultra-low-carbon steel. Orientation relationship is ...systematically deviated between near Greninger–Troiano and Nishiyama–Wasserman orientation relationships around variant boundaries of the smallest misorientation pair, which is observed as the most frequent variant pair in the steel. Misorientation between this variant pair is decreased at some boundaries of the pair. Formation mechanism of the pair is discussed using the observed boundary characters.
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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.
Background
Measurement of local stress amplitude at the stress concentration point is a key component of mechanical design from the viewpoint of preventing metal fatigue. Copper electroplating ...method, which is a stress measurement method utilizing grains grown in a copper foil by cyclic loading, is suitable for such microscopic stress measurement. Reports have shown that the maximum shear stress and principal stress, which are important components in the evaluation of fatigue strength, can be measured by examining the density and crystallographic features of grown grains.
Objective
Electron backscatter diffraction (EBSD) analysis of grown grains can be utilized to understand the crystallographic features of the grains, but EBSD equipment requires technical skill to operate, which makes it inconvenient. In this paper, we explore the feasibility of stress measurement by analyzing the crystallographic features of grown grains using the X-ray diffraction (XRD) method, which is more versatile than the EBSD method.
Methods
Cyclic loading tests were conducted using smooth specimens with copper foil adhered under various biaxial stress conditions of bending and torsion. The surfaces of the grains-grown copper foil were then analyzed by the XRD method.
Results
The peak of diffraction intensity tended to shift from the (220) plane to the (111) plane as the biaxial stress ratio increased. We quantified this tendency using the Lotgering factor and developed an empirical formula for determining the relationship between the Lotgering factor and the biaxial stress ratio.
Conclusions
Our proposed empirical formula enables principal stress measurement within a biaxial stress ratio
C
ranging from –0.45 to 0, and the measurement accuracy is comparable to that of the conventional EBSD method.
Crystallography of upper bainite in Fe–Ni–C alloys Furuhara, T.; Kawata, H.; Morito, S. ...
Materials science & engineering. A, Structural materials : properties, microstructure and processing,
09/2006, Letnik:
431, Številka:
1
Journal Article
Recenzirano
The upper bainite structures were investigated in Fe–9Ni–(0.15–0.5)C (mass%) alloys transformed at temperatures between 723 and 623
K. A bainite packet similar to lath martensite is formed and is ...partitioned into blocks containing lath-shaped bainitic ferrite with the same parallel close-packed plane relationship of the K–S relationship. Its feature is categorized into three types: (A) a packet contains laths of two K–S variants with a small misorientation (sub-blocks), (B) a packet is divided by blocks largely misoriented and each block contains sub-blocks and (C) a packet is divided by blocks containing a single variant of laths. A packet of type (A) is formed at 723
K in each alloy whereas the packet type becomes (B) or (C) at lower temperatures. As a result, bainite blocks are refined with decreases in transformation temperature and carbon content. In contrast, blocks and packets of lath martensite are refined with increasing carbon content.
Lath martensite structure has overwhelming industrial significance in most heat-treatable commercial steels. High dislocation density is one of major factors for high strength of lath martensite. It ...is known that the dislocation density in the lath martensite is an order of 1014 or 1015 m-2. However, the quantitative studies on the dislocation density in martensite are very few. One of the difficulties in the measurement of dislocation density by transmission electron microscopy is a determination of thin foil thickness. Kehoe and Kelly determined the foil thickness from extinction thickness fringes in an inclined boundary and measured the dislocation density within the laths in Fe-C alloys with carbon content from 0.01 to 0.1 mass%. They showed that the dislocation density increases with carbon content. However, the effect of carbon content more than 0.1% on the dislocation density is not clear yet. In the present paper, the dislocation densities within the lath martensite in Fe-C (0 to 0.8 mass% C) and Fe-Ni (0 to 23 mass% Ni) alloys were studied by TEM through precise determination of foil thickness using the convergent-beam electron diffraction (CBED) method. Compositions: 0.0026, 0.18, 0.38, 0.61, and 0.78 mass% C, and 11, 15, 23, and 31 mass% Ni.
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.
Ultrafine Structure and High Strength in Cold-Rolled Martensite Huang, X.; Morito, S.; Hansen, N. ...
Metallurgical and materials transactions. A, Physical metallurgy and materials science,
10/2012, Letnik:
43, Številka:
10
Journal Article, Conference Proceeding
Recenzirano
Odprti dostop
Structural refinement by cold rolling (10 to 80 pct reductions) of interstitial free (IF) steel containing Mn and B has been investigated from samples with different initial structures: (a) lath ...martensite, (b) coarse ferrite (grain size 150
μ
m), and (c) fine ferrite (22
μ
m). Unalloyed IF steel with a coarse grain size (120
μ
m) has also included based on a previous study. Deformation microstructures and structural parameters have been analyzed by transmission electron microscopy and electron backscatter diffraction, and mechanical properties have been characterized by hardness and tensile testing. At low to medium strains, lath martensite transforms into a cell block structure composed of cell block boundaries and cell boundaries with only a negligible change in strength. At medium to large strains, cell block structures in all samples refine with increasing strain and the hardening rate is constant (stage IV). A strong effect of the initial structure is observed on both the structural refinement and the strength increase. This effect is largest in lath martensite and smallest in unalloyed ferrite. No saturation in structural refinement and strength is observed. The discussion covers the transformation of lath martensite into a cell block structure at low to medium strains where the driving force is suggested to be a decrease in the dislocation line energy. Medium to large strain-hardening mechanisms are discussed together with structure-strength relationships assuming additive stress contributions from dislocations, boundaries, and elements in solid solution. Good agreement is found between flow stress predictions and stress values observed experimentally both in the initial undeformed martensite and in deformed samples.
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