Electron backscattering diffraction study of acicular ferrite,bainite, and martensite steel microstructures

Abstract

This study deals with acicular ferrite, bainite, and martensite microstructures observed in three low alloy steels. Electron backscattering diffraction (EBSD) was used to assess crystallographic features of these microstructures. In each area studied by EBSD mapping, ‘crystallographic packets’ defined as clusters of points sharing the same crystallographic orientation were compared with ‘morphological packets’ observed in the corresponding light micrograph. Microtexture studies suggested that acicular ferrite and upper bainite grow with Nishiyama– Wassermann relationships with the parent austenite phase, whereas lower bainite and martensite consist of highly intricate packets having Kurdjumov–Sachs relationships with the parent phase. In all cases three highly misoriented texture components were found within each former austenite grain. Electron backscattering diffraction also gave information about the cleavage and intergranular reverse temper embrittlement fracture mechanisms of these steels. In conclusion, it is shown that EBSD is a powerful tool for studying phase transformation and fracture mechanisms in steels on a microscopic scale.     

Transient Liquid Phase Bonding of Ni—base Single Crystal Superalloy

The Ni-base single crystal superalloy was bonded by the transient liquid phase(TLP) bonding,using a Ni-base flexible metal cloth as an insert alloy.TLP bonding of superalloy was carried out at 1473-1523K for 0.5-24h in vacuum.The [001] orientation of each test specimen was aligned perpendicular to the joint interface.The bonded region was observed by optical microscopy, and the microstructural and compositional analyses across the bonded interlayer were performed by using a scanning electron microscopy(SEM) .The electron back scattering diffraction(EBSD) method was applied to determine the crystallographic orientation.The resultsindicated that the chemical homogeneity across the bonded region can be achieved,and γˊphase both in the bonded interlayer and in the superalloy substrate is almost identical,while the bonded interlayer had almost matched the crystallographic orientation of the bonded substrates.     

Microstructure and crystallography of 20MnCr5 steel tempered at different conditions

The microstructure and crystallography of 20MnCr5 steel tempered at 180 °C for 2 h were examined by electron back scattered diffraction (EBSD) in a field-emission scanning electron microscope. The crystallographic features of martensite in a prior austenite grain area were studied by pole figures (PFs). Compared to the theoretical PFs calculated with Kurdjumow–Sachs (K–S) and Nishiyama–Wassermann (N–W) orientation relationships (ORs), it is shown that both of the ORs exist in the investigated specimens. Misorientations distribution and the result of transmission electron microscope analysis indicating that [011]/54.7° lath boundary existed in the microstructure also prove the coexistence of K–S and N–W ORs. When the steels were further tempered at 350 °C/20 min or 500 °C/20 min, the length of boundaries in equal areas decreased and the crystallographic features still fulfill the coexistence of K–S and N–W ORs.     

Preferred orientations of primary Al4Sr dendrites in a rapidly solidified Al–Sr alloy

Preferred crystallographic orientations of primary Al₄Sr dendrites in a rapidly solidified Al–23 Sr (wt.%) alloy have been investigated using transmission electron microscopy (TEM). The Al₄Sr dendrites with 90° branches are dominant in the Al–23 Sr alloy melt-spun at 500 rpm and the dendrite orientation is the 110 direction. Wheel speed has a significant effect on the morphology and preferred orientation of the Al₄Sr dendrites in the melt-spun Al–23 Sr alloy.     

Grain boundary and fracture surface analysis of Fe–C–P steel

Abstract

The present paper investigates the distribution of grain boundary types and fracture surface crystallography in an Fe–C–P alloy. It is shown that electron backscatter diffraction (EBSD) is an effective technique with which to conduct these investigations. The proportions of both Σ1 and particularly Σ3 (in coincidence site lattice notation)present after various heat treatments were higher than would have been expected for random generation. There was limited evidence that both higher annealing temperatures and longer annealing times promoted generation of Σ3 type boundaries. The standard EBSD technique was modified and extended to encompass both the novel ‘matched fracture’ specimen approach and direct mapping from fracture surfaces to provide crystallographic information. A correlation was noted between higher aging temperatures and proportions of cleavage fracture. Furthermore, there was a strong correlation between cleavage fracture surfaces exhibiting river markings and an {001} surface orientation.     

Determination of crystallographic orientation of dwell-fatigue fracture facets in Ti-6242 alloy

A technique to determine the crystallographic orientation of the fracture facets has been described. The spatial orientation of the facet plane is determined in a scanning electron microscope (SEM) using a quantitative tilt fractography technique. The crystallographic orientation of the grain, across which a particular fracture facet had been produced, is determined using the electron backscattered diffraction (EBSD) technique in an SEM. These two pieces of information were combined to obtain the crystallographic orientation of the fracture facet normal. This technique was used for the characterization of dwell-fatigue fracture facets at the crack-initiation site in Ti-6242 alloy. Our results indicate that these facets are not exactly aligned with the basal plane, but are inclined at ∼10° to it.     

SEM/EBSD based in situ studies of deformation induced phase transformations in supermartensitic stainless steels

Abstract

Recent year's equipment design has enabled the combination of in situ deformation tests with near real time electron backscatter diffraction (EBSD) mapping of the microstructure evolution in the scanning electron microscope (SEM). The present work involves studies of deformation induced phase transformations in supermartensitic steel containing ～40 vol.-% retained austenite at room temperature. The martensite formation was initiated already at low strains, and increased gradually with increasing plastic strains up to ～10%. It was observed that the martensite formed homogeneously within the microstructure, independent of the crystallographic orientations of the retained austenite. But no new martensite variants, besides those already present in the as received condition, did form during deformation. At the same time, the mutual distribution of these variants remained approximately constant throughout the deformation process.     

EBSD analysis of fatigue crack initiation behavior in coarse-grained AZ31 magnesium alloy

Plane bending fatigue tests had been conducted to investigate fatigue crack initiation mechanism in coarse-grained magnesium alloy, AZ31, with hexagonal close-packed (hcp) crystallographic structure. The initial crystallographic structure was analyzed by an electron backscatter diffraction (EBSD) method. Subsequently, a fatigue test was periodically terminated and time-series EBSD analyses were performed. Basal slip and primary twin operated predominantly. In a twin band, secondary twin operated, and resulted in the fatigue crack initiation. The crack initiation was strongly affected by Schmid factors in the grains and twin bands.     

Transmission electron microscope observation of Si deposited on W(110) surface

The (110) surface of a thin W crystal deposited with Si has been investigated by using a transmission electron microscope (TEM). It was found that an ordered Si–W alloy layer with a regular periodic arrangement of antiphase boundaries (APBs) is formed, as a result of intermixing of deposited Si with the W substrate at room temperature (RT). A crystal structure model of this W–Si superstructure is proposed in this study. After 800°C annealing of Si deposited at RT, grains of tetragonal WSi₂ are formed on the W–Si ordered alloy layer. Grains of WSi₂ are also formed when Si is deposited on a W substrate maintained at 900°C. These WSi₂ grains have epitaxial orientation relationships with the W(110) substrate as follows: (110)W//(01 ) WSi₂ and [001]W//[100]WSi₂.     

Crystallographic Analysis of Martensite in 0.2C-2.0Mn-1.5Si-0.6Cr Steel using EBSD

The crystallography of martensite formed in 0.2C-2.0Mn-1.5Si-0.6Cr steel was studied using the electron backscattered diffraction(EBSD) technique.The results showed that the observed orientation relationship(OR) was closer to that of Nishiyama-Wassermann(N-W) than Kurdjumov-Sachs.The martensite consisted of parallel laths forming morphological packets.Typically,there were three different lath orientations in a morphological packet consisting of three specific N-W OR variants sharing the same {111} austenite plane.A packet of martensite laths with a common {111} austenite plane was termed a crystallographic packet.Generally,the crystallographic packet size corresponded to the morphological packet size,but occasionally the morphological packet was found to consist of two or more crystallographic packets.Therefore,the crystallographic packet size appeared to be finer than the morphological packet size.The relative orientation between the variants in crystallographic packets was found to be near 60 /<110>,which explains the strong peak observed near 60 in the grain boundary misorientation distribution.Martensite also contained a high fraction of boundaries with a misorientation in the range 2.5-8.Typically these boundaries were found to be located inside the martensite laths forming sub-laths.     

A Crystallography Study of Bainitic Transformation in a Cu-Zn-Al Alloy

The crystallographic nature of initialisothermal phase transformation in Cu-26Zn-4Al(wt-％) was investigated. The kinetic transforma-tion curve, morphology, crystal structure,substructure, orientation relationship and twinrelationship of bainite plates have been studiedby means of optical and transmission electronmicroscopy. The experimental results showedthat the characteristics of initial 8→bainitetransformation are not exactly consistent withthat found in martensite transformation, forexample, orientation relationship between matrixand bainite does not exist in martensitetransformation.     

Analysis of local crystallographic orientation in an annealed Ti60 billet

A heterogeneous microstructure in terms of local orientation distribution is often found in near-α titanium alloys. The presence of large regions with similar crystallographic orientation, called ‘macrozones',could drastically decrease the fatigue performance of titanium alloys. The present work reports on the crystallographic orientation of a near-α titanium alloy, Ti60, billet after annealing in an α+β phase field.The texture was found to be weak, and no obvious macrozone was found in our measured zone where the variant selection is suppressed. Meanwhile, in-depth electron backscattered diffraction(EBSD) analysis was applied to evaluate the final microstructure, and the mechanisms by which they formed were analyzed.     

New observations on formation of thermally induced martensite in Fe–30%Ni–1%Pd alloy

Kinetical, morphological, crystallographical and thermal characteristics of thermally induced martensite in an Fe–30%Ni–1%Pd alloy has been studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and X-ray diffraction method. Kinetics of transformation was found to be as athermal. SEM and TEM observations and X-ray method revealed α′ (bcc) martensite formation in the austenite phase of alloy by thermal effect. The crystallographic orientation relationship between austenite and α′ (bcc) martensite was found to be having Kurdjumov–Sachs (K–S) type relationship. In addition, the lattice parameters of austenite and martensite phases were calculated from X-ray diffraction patterns.     

AA6111合金在热轧过程中保持时间对晶体织构的影响

使用EBSD研究了AA6111合金在两次热轧过程中不同保持时间对晶体织构演变的影响.结果显示:变形织构主要由α和β纤维组成.从主要织构组分的取向强度看出,随着保持时间的增加,Brass和Copper理想织构组分显著增加,S织构没有明显变化.     

基于晶体取向q值计算再结晶分数的改进方法

以大变形高纯铝在300℃的退火试样为例,提出了一种基于EBSD数据中q值计算再结晶分数的改进方法,该方法通过分离不同取向的q值数据,对于不同的取向数据分别采用Tarasiuk提出的q值积分算法计算再结晶分数,即与完全再结晶状态及变形态的q值数据进行对比并积分计算,再按各取向的再结晶分量权重求和得出总体的再结晶分数.结果...     

Texture evolution analysis of warm-rolled Fe–28Mn–6Si–5Cr shape memory alloy

Since texture control tends to be a promising way to improve the shape memory effect (SME) of polycrystalline Fe–Mn–Si shape memory alloys, rolling texture evolution of an Fe–28Mn–6Si–5Cr shape memory alloy was systematically investigated with orientation distribution functions (ODFs) and electron backscattering diffraction (EBSD) analysis. At the rolling temperature of 873 K, Copper-type texture components, including D, S, Goss, as well as a weak Brass, obviously develop before 44% rolling reduction. With increased rolling reduction to 57%, D orientation abruptly disappears, which indicates a texture transition has occurred. S orientation and α fiber texture except the Goss orientation undergo a decrease accompanying the intensification of γ fiber texture. In the whole deformation processes, Goss orientation is the dominant texture component while no pronounced Brass component is observed. The dominant Goss component can be attributed to the preferred Goss orientation both in shear bands and in matrix. When the rolling temperature is decreased to 573 K, even at the early deformation stage, 42% rolling reduction, both D and Brass orientations are not observed. EBSD analysis confirms that the texture evolution is promoted to the early deformation stages at lower rolling temperature.     

EBSD study of micromechanisms involved in high deformation ability of DP steels

Notched and un-notched tensile specimens of fine grained commercial DP780 steel were deformed in uniaxial tension until fracture. Micro-texture analysis was performed by using an FE-SEM equipped with an EBSD detector and the data were analyzed to quantify orientation gradients within the microstructure of the deformed specimens in terms of Image Quality, Inverse Pole Figure and Taylor Factor map. High deformation ability of DP steels was found to be mostly due to such mechanisms as grain rotation, void creation and evolution, substructure formation within the ferrite grains and the highly plastic stretching of martensite during the deformation process. The true strain of martensite was measured up to 64% and 74% for the un-notched and notched specimens, respectively.     

Electron backscattered diffraction analyses combined with environmental scanning electron microscopy: potential applications for non-conducting,uncoated mineralogical samples

Abstract

Operational techniques have been developed which combine electron backscattered diffraction (EBSD) analysis with orientation contrast and secondary electron imaging in environmental scanning electron microscopy (ESEM). This is specifically directed towards the analysis of uncoated, non-conducting materials such as ceramics and mineral surfaces, where surface charge effects have a severe detrimental effect on EBSD microstructural analysis. Initial experimentation to establish optimum chamber conditions was based on single Ge crystals, using both H₂O vapour and N₂ imaging gases. Reliable EBSPs with correct crystallographic solutions were collected up to pressures of ～4 torr (1 torr ≈1.33 mbar) under H₂O vapour conditions and ～2 torr in N₂, well above the minimal values for gaseous secondary electron detector imaging and surface charge neutralisation. Using these initial pressure and gas type guidelines, polycrystalline structures (with grain mosaics of 50–100 µm) in etched steel were analysed by automated, long duration crystal orientation mapping (COM). Optimum chamber conditions were established at ～1 torr pressure in H₂O vapour environments. At higher gas pressures, increased electron scattering generated more unsolved EBSPs, requiring advanced filtering to reduce map noise. For tests with non-conductors, a suite of single crystal and polycrystalline minerals (garnet, calcite, and olivine) were analysed. Optimum EBSPs were obtained under H₂O vapour conditions at pressures of 0.6–1.8 torr, and beam conditions were sufficiently stable for the collection of manual COMs. This new method of combining EBSD and ESEM will greatly improve the potential for microstructural analysis of sensitive, non-conducting ceramic surfaces.     

Quantitative microstructural characterisation of Fe–30Ni alloy after martensitic transformations by means of stereological and magnetic methods

In the Fe–30Ni alloy investigated a martensitic transformation can occur both during quenching or plastic deformation. Martensite formed during plastic deformation, depending on the thermo-mechanical treatment applied, exhibits a different morphology from that achieved during quenching and forms the so-called composite-like structure. The morphology and volume fraction of martensite depends both on strain and temperature. In the present studies Fe–30Ni alloy was deformed by monotonic rolling in one path and perpendicular rolling in the temperature range M_D–M_S. The aim of the investigations was a determination of martensite volume fraction depending on the strain and temperature. To examine the influence of strain, the alloy was deformed by rolling in one path or perpendicular rolling at a temperature of − 30 °C, in the strain range of 10–30%. The dependence of temperature was investigated by rolling with 30% strain in a temperature range from − 30 °C to − 80 °C. The variants of thermo-mechanical treatment performed enabled us to achieve different martensite morphologies and volume fractions. Microstructural analysis was performed by means of light microscopy and transmission electron microscopy. The results of quantitative microstructural analysis of martensite and retained austenite volume fractions formed in different thermo-mechanical treatments were compared with those obtained by magnetic measurements. The fraction of deformation-induced martensite determined varied from 2% to 86%. The partial volume fractions V_V^MF of martensite formed in different deformation directions were also determined. It was found that the influence of the temperature on the martensite volume fraction is more pronounced than the influence of strain.     

Microstructural influence on small fatigue cracks in a ferritic–martensitic steel

Microstructure effects on fatigue crack initiation and propagation in ferritic–martensitic dual phase steel were investigated. Slip bands were formed in ferrite grains after several thousand cycles with ensuing crack initiation due to dislocation pile-up. Subsurface observations using a focused ion beam (FIB) and crystallographic analyses using electron backscatter diffraction (EBSD) measurements showed that crack initiation occurred as a result of the activation of a slip system having a high Schmid factor. Surface crack nucleation occurred quite frequently at ferrite/martensite and ferrite/ferrite boundaries, with crack propagation in the ferrite grains. This initiation mode can be attributed to the mismatch stresses at ferrite/martensite phase boundaries and at high angle grain boundaries.