Calculation of crystallographic texture due to displacive transformations

Bhadeshia, H. K. D. H.; Abreu, Hamilton Ferreira Gomes de; Kundu, Saurabh

doi:10.3139/146.101645

Cited by 28 publications

(24 citation statements)

References 35 publications

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“…There are important unresolved difficulties in the calculation of crystallographic texture due to displacive transformation under the influence of stress [1,2]. It is often assumed that it is sufficient to plot calculated crystallographic orientations on stereographic projections or similar devices, and then compare against experimental data which contain not only the location of poles but also the associated diffraction intensity.…”

Section: Motivationmentioning

confidence: 99%

Multiple, Simultaneous, Martensitic Transformations: Implications on Transformation Texture Intensities

Bhadeshia

2013

MSF

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Abstract. A theory is developed for martensite variants that have different start temperatures but exist in the same steel. The method enables the volume fractions of each kind of martensite to be followed as a function of the steel temperature. The problem is relevant to the calculation of detail in transformation texture when phase changes occur under the influence of external stress. It should allow for the first time, the estimation of both the location of crystallographic poles on a stereographic projection, and the diffraction intensity associated with that location. It is found that the increment of transformation as a function of undercooling is identical for all variants, once simultaneous transformation begins. Any variance in the absolute fractions is due to the differences in the martensite-start temperature.

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Section: Motivationmentioning

confidence: 99%

Multiple, Simultaneous, Martensitic Transformations: Implications on Transformation Texture Intensities

Bhadeshia

2013

MSF

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“…32,[34][35][36] The theory predicts that the crystallographic indices representing the orientation relationship and habit plane are in general irrational; that means that the indices cannot be expressed by the division of integers. This necessarily means that there will be 24 variants of the transformation product per austenite grain.…”

Section: )mentioning

confidence: 99%

Problems in the Calculation of Transformation Texture in Steels

Bhadeshia

2010

ISIJ Int.

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The solid-state decomposition of austenite can lead to a non-random distribution of product crystals. Methods of the quantitative characterisation of this texture are extremely advanced, and there is a deep understanding of the relationship between the texture and macroscopic properties. There remain, however, important barriers to the complete calculation of texture, some of which have not been documented in the published literature. It is the purpose of this review to focus on the difficulties in order to set the scene for further progress. The advent of electron back scatter diffraction has led to an explosion of papers on microscopic aspects of crystal orientations; some of the issues relating to this technique are also described.KEY WORDS: transformation texture; crystallography; steel; martensite; bainite. Consider first the spatial resolution. Using a step size of 2-4 nm might suggest a resolution of that order, but Fig. 1 illustrates the difficulties. The boundaries in this image appear convoluted whereas they are not in reality. The grain marked with an arrow is suggested to have a size of about 8 nm 24) but it is questionable whether this is a real grain or an artefact given the size of the surrounding black regions which are crystallographically unresolved pixels. The grain boundaries themselves appear ragged, with a roughness which is comparable to the suggested 8 nm grain. Such roughness is not likely to be real since and thin film deposition does not normally lead to a bimodal distribution in grain size. 1517Consider now the problem of angular resolution when dealing with steels. The influence of imperfections in causing a spread in orientation well beyond the stated angular resolution of the EBSD instrument is illustrated in Fig. 2(a). Instrumental resolution should be of the order of 0.5°b ut the illustrated spread in the orientation of the single grain of austenite is closer to 10°. There will then be a corresponding spread in the orientations of bainite, as illustrated in Fig. 2(b), even if there is a strictly fixed orientation relationship between the parent (g) and product (a) phases. Products which form by displacive transformations will contain excess dislocations leading to a spread in diffracted intensity. Furthermore, the contribution to the spread from different crystals of bainite or martensite will not be identical given the nonuniform distribution of dislocations, plastic accommodation and the fact that the stress state of the austenite changes as more plates of martensite form. 25) There are many publications in the literature which suggest that there is a spread in the g/a orientation relationship rather than one established by the crystallography of the phases. It is then reasonable to question whether there is a true spread in orientation relationship or one which is caused by a neglect of the fact that the austenite is not a perfect crystal. The measurements implying spread have in some cases been deduced from observations on meteorites, which cool incredibly slowly as they evo...

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“…The overall transformation elastic strain tensor can be calculated and compare to the applied stress, resulting in a calculation of which variants are favoured by the applied stress state. [20][21][22][23] Bhadeshia 10 showed that the volume change due to the transformation in bainite is much smaller than the shear component. Babu and Bhadeshia 24 studied the effect of applied stress on the formation of acicular ferrite.…”

Section: Introductionmentioning

confidence: 99%

Effect of variant transformations in fusion zones of gas metal arc welds

Béreš

Abreu

Santos

et al. 2015

Science and Technology of Welding and Joining

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The macro-and microtextures of gas metal arc (GMA / GMAW) welds fabricated using both conventional ferritic and low transformation (LTT) filler metals were examined. Both welds were largely composed of acicular ferrite. The weld textures were found to be distinct, which is observed to be due to variant selection during the austenite-to-ferrite transformation.In situ EBSD performed on thermal cycling of the LTT fusion zone into the austenite phase field confirmed that these transformations satisfy the expected crystallographic relationships.Implications for welding consumable design and finite element process modelling are drawn.Keywords: Texture analysis; Displacive transformation; Variant selection; Gas Metal Arc Weld. IntroductionThe thermal strains that arise during welding give rise to residual stresses and distortion in welded joints that are problematic, both in initial fabrication of a satisfactory structure and its subsequent strength, fatigue and stress-corrosion cracking performance.1,2 However, in the majority of welds a filler metal is added and therefore the possibility exists to select a filler material with the aim of improving the performance of the welded joint. Over recent years, here has been much interest in using filler metals during the welding of steels that exploit the austenite-to-ferrite () transformation strains to absorb some of the thermal strains from welding, mitigating the residual stress and distortion. [3][4][5][6][7][8][9] It has been shown that weld fabrication using, for example, filler metal wires with a lowered  transformation temperature (so-called low transformation temperature or LTT wires) can result in lowered residual stresses and distortion. 8,[10][11][12] Text Click here to download Manuscript: Beres_Text. docx 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 2 Where the transformation is entirely reconstructive, one might initially believe that the transformation strain will simply be due to the volume change between the two unit cells.However, for a fully displacive transformation, one must consider if the applied stress state can bias which orientations of the product phase form from the parent -in the case of the  transformation 24 crystallographic variants of the bcc/bct ferritic (bainite or martensite) phase can form from the austenite.However, even for fully reconstructive transformations some 'memory' is commonly observed in the product phase textures. For example, in Zr and Ti it is now commonly accepted that the simultaneous satisfaction of the orientation relationship (OR) at the parent phase grain boundary by the product phase can result in a memory effect. [13][14][15] Such an effect has also been observed in the macroscopic textures observed using in situ synchrotron diffraction in a interstitial-free (IF) steel. 16For displacive transf...

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Calculation of crystallographic texture due to displacive transformations

Cited by 28 publications

References 35 publications

Multiple, Simultaneous, Martensitic Transformations: Implications on Transformation Texture Intensities

Multiple, Simultaneous, Martensitic Transformations: Implications on Transformation Texture Intensities

Problems in the Calculation of Transformation Texture in Steels

Effect of variant transformations in fusion zones of gas metal arc welds

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