G. Spanos

Three-dimensional analysis of proeutectoid cementite precipitates

Abstract Serial sectioning and computer-aided visualization of three-dimensional reconstructions were used to reveal the three-dimensional morphology, connectivity and distribution of proeutectoid cementite precipitates in a model Fe–1.34% C–13.0% Mn alloy. Only dendritic grain boundary precipitates and Widmanstatten precipitate morphologies were observed in the present alloy. Unlike earlier morphological studies of similar alloys, intragranular precipitates were not observed. Scanning and transmission electron microscopy of deeply etched specimens corroborated and augmented these results. Three-dimensional analysis has also made possible the first quantitative three-dimensional measurements of cementite precipitates and yielded a more precise classification of cementite precipitates than was possible with classical two-dimensional techniques.

Three-dimensional analysis of microstructures

When using traditional metallographic techniques, materials scientists often make assumptions about the shape, distribution, and connectivity of three-dimensional (3D) features that lie buried within the material. Recent work has shown that these assumptions can be incorrect. An approach to 3D analysis of microstructures is demonstrated here in three different alloy steels. Proeutectoid cementite, ferrite, and an entire pearlite colony were characterized using computer-aided visualization of 3D reconstructions from serial section images. The present article describes various experimental techniques, as well as recent results and advances in computer-aided reconstruction and visualization.

Microstructural changes in HSLA-100 steel thermally cycled to simulate the heat-affected zone during welding

The microstructural changes that occur in a commercial HSLA-100 steel thermally cycled to simulate weld heat affected zone (HAZ) behavior were systematically investigated primarily by transmission electron microscopy (TEM). Eight different weld thermal cycles, with peak temperatures representative of four HAZ regions (the tempered region, the intercritical region, the fine-grained austenitized region, and the coarse-grained austenitized region) and cooling rates characteristic of high heat input (cooling rate (CR) = 5 °C/s) and low heat input (CR = 60 °C/s) welding were simulated in a heating/quenching dilatometer. The as-received base plate consisted of heavily tempered lath martensite, acicular ferrite, and retained austenite matrix phases with precipitates of copper, niobiumcarbonitride, and cementite. The microstructural changes in both the matrix and precipitate phases due to thermal cycling were examined by TEM and correlated with the results of (1) conventional optical microscopy, (2) prior austenite grain size measurements, (3) microhardness testing, and (4) dilatometric analysis. Many of the thermal cycles resulted in dramatic changes in both the microstructures and the properties due to the synergistic interaction between the simulated position in the HAZ and the heat input. Some of these microstructures deviate substantially from those predicted from published continuous cooling transformation (CCT) curves. The final microstructure was predominantly dependent upon peak temperature(i.e., position within the HAZ), although the cooling rate(i.e., heat input) strongly affected the microstructures of the simulated intercritical and finegrained austenitized regions.

Determining phase volume fraction in steels by electron backscattered diffraction

Abstract The relative sharpness of electron backscattered diffraction (EBSD) patterns is used to quantify the volume fraction of ferrite in low-carbon steels. EBSD pattern quality (PQ) maps are shown to improve imaging of martensite and ferrite over optical and secondary electron imaging. Ferrite volume fraction in isothermally reacted Fe–0.12C–3.28Ni are quantified using PQ maps by manual point counting and image thresholding.

Application of orientation imaging microscopy to study phase transformations in steels

Abstract In this investigation, the methodology, advantages, and limitations of using Orientation Imaging Microscopy (OIM) to study microstructures in multiphase steels are examined. In particular, proeutectoid ferrite precipitation has been investigated in a hypoeutectoid steel (Fe–0.12%C–3%Ni), while the proeutectoid cementite transformation has been studied in a hypereutectoid steel (Fe–1.3%C–13%Mn). By utilizing misorientation maps and inverse pole figure maps to determine the orientation differences between individual crystals, the internal morphology of the solid-state precipitates has been determined. What appear by other types of imaging to be monolithic single crystals are shown to be aggregates of finer crystals with misorientations between them. The relative quality of the Electron Backscatter Diffraction (EBSD) patterns has been correlated to specific phases and to interphase and grain boundaries using pattern quality maps (also referred to as image quality maps). These techniques have been adapted for quantitative identification of different microconstituents, including volume fraction measurements. By making comparisons among optical micrographs, conventional SEM micrographs, pattern quality maps, inverse pole figure maps, and misorientation maps, the advantages and limitations of different types of OIM mapping for studying different aspects of phase transformations in steels are elucidated.

Three dimensional morphology of cementite precipitates

Conventional light and electron microscopy techniques usually allow for characterization of microstructures in only two dimensions. This is a serious impediment to understanding the three dimensional morphology of microstructural features. This paper presents the results of a deep etching study of cementite precipitates in a model alloy composed of Fe-1.34 wt.% C-13.1 wt.% Mn. The high carbon level enables a large volume fraction of cementite precipitates to be produced while the high manganese and carbon levels together suppress the martensite start temperature to below room temperature. During deep etching of this alloy, the austenitic matrix is preferentially removed and the morphology of the cementite precipitates is revealed. This also enables complimentary crystallographic and interfacial structure studies which are underway. Although previous investigations used deep etching as a supplementary technique for characterizing cementite plates, the current work places emphasis on determining the three dimensional morphology of both grain boundary and intragranular precipitates previously categorized under the two dimensional Dube morphological classification system.

Correlation between the crystallography and morphology of proeutectoid Widmanstatten cementite precipitates

Abstract Electron backscatter diffraction was used in conjunction with deep etching to examine the relationship between the crystallography and three-dimensional morphology of cementite precipitates in an Fe–1.34 wt% C–13.1 wt% Mn steel. Orientation relationships (ORs) between more than 200 proeutectoid Widmanstatten cementite precipitates and the surrounding austenite were determined to be on or very near either the well-known Pitsch OR or the Farooque–Edmonds OR. Scanning electron microscopy of the same specimens after deep etching was used to determine the three-dimensional morphology of each of the precipitates for which an OR was determined. These precipitates could be identified as either monolithic plates or conglomerates of laths. Results show that monolithic plates consistently exhibit the Pitsch OR and conglomerates of laths have the Farooque–Edmonds OR, indicating that the precipitate morphologies are dictated by their orientation relationships.

The formation mechanism(s), morphology, and crystallography of ferrite sideplates

The formation mechanism(s), morphology, and crystallography of secondary ferrite sideplates were investigated with transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron backscatter pattern (EBSP) analysis, and optical microscopy in a high-purity Fe—0.12 wt pct C—3.3 wt pct Ni alloy isothermally transformed at temperatures of 550 °C, 600 °C, 650 °C, and 675 °C. The results indicate that two different mechanisms contribute to the formation of these sideplates at austenite grain boundaries. On the first mechanism, primary sideplates form initially, followed by rapid lateral impingement along their bases, resulting in a region along the grain boundary which very early in the growth process resembles an allotriomorphic film. On the second mechanism, sympathetic nucleation of ferrite sideplates occurs atop pre-existing ferrite allotriomorphs, resulting in ferrite:ferrite grain boundaries and significant crystallographic misorientations between the sideplates and the allotriomorphs with which they are associated. These results indicate that “secondary sideplates” and the allotriomorphs from which they evolve arenot composed of monolithic single crystals formed by a morphological instability mechanism but are instead composed of multiple crystals formed by individual nucleation events. Previous investigations in Ti-Cr alloys and a high chromium stainless steel suggest that the findings presented here may be applicable to a number of other alloy systems as well.

Three-dimensional observations of proeutectoid cementite precipitates at short isothermal transformation times

Abstract Three-dimensional (3D) analysis techniques were used to examine a model Fe–1.34% C–13.0% Mn alloy to reveal connectivities of proeutectoid cementite precipitates whose growth was arrested at an early stage. The present results indicate that grain boundary cementite precipitates nucleate at austenite grain boundary edges and corners, then grow and spread on the grain boundary faces. No cementite precipitates were found to connect solely to austenite twin boundaries, which appear to act as barriers to precipitate growth rather than as potential nucleation sites. Cementite precipitates were all connected to austenite grain boundaries or cementite grain boundary precipitates, confirming 3D observations made earlier on a specimen transformed at a longer isothermal transformation time. Widmanstatten lath precipitates appear to emanate only from grain boundary cementite precipitates. While edges of several Widmanstatten plate precipitates were observed to intersect with areas of ‘clean’ austenite grain boundary, they may or may not nucleate directly on austenite grain boundaries.

Three-dimensional visualization of microstructures

This case study describes a technique for the three-dimensional analysis of the internal microscopic structure (microstructure) of materials. This technique consists of incrementally polishing through a thin layer (approximately 0.2 /spl mu/m) of material, chemically etching the polished surface, applying reference marks, and performing optical or scanning electron microscopy on selected areas. The series of images are then processed employing AVS and other visualization software to obtain a 3D reconstruction of the material. We describe how we applied this technique to an alloy steel to study the morphology, connectivity, and distribution of cementite precipitates formed during thermal processing. The results showed microstructural features not previously identified with traditional 2D techniques.