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Internal nitridation kinetics were determined for a UNS N08810/800H alloy using a general model of the form $$x^{n}=kt$$ . Nitridation behavior was studied at service-relevant temperatures 800 °C to 1000 °C in a 95 pct $$\hbox {N}_2/5\,{\text{pct}}$$ $$\hbox {H}_2$$ atmosphere for times 50 to 750 hours. Optical and scanning electron microscopy were used for microstructural characterization and measurement of nitride penetration. AlN, $$\hbox {Cr}_2\hbox {N}$$ , and CrN were formed, and the experimentally observed precipitation sequence was consistent with equilibrium calculations for this alloy using Thermo-Calc. A combination of diffusivity data determined using DICTRA and experimentally verified equilibrium calculations showed that Wagner’s analysis for internal oxidation kinetics was valid for AlN penetration. Parabolic kinetics closely approximated measured AlN penetration. This suggests that extension of AlN penetration models to other temperatures and Fe-Ni-Cr-Al alloy systems is reasonable. $$\hbox {Cr}_2\hbox {N}$$ penetration did not conform to Wagner’s analysis. Deviation from parabolic behavior was evident, and general model penetration predictions for $$\hbox {Cr}_2\hbox {N}$$ were experimentally validated. Using the experimentally determined models, time–temperature–precipitation diagrams for AlN and $$\hbox {Cr}_2\hbox {N}$$ penetration were constructed.

During operation of a shipping container handler, a twistlock pin supporting one side of two stacked shipping containers fractured, causing the shipping containers to fall. An investigation revealed that the alloy steel pin had a ferritic/bainitic microstructure with an average hardness of 29.5 HRC and a typical Charpy V-notch impact energy of 17 J at room temperature. Examination of the fracture surface revealed a cleavage fracture that initiated from a quench crack. It was determined that the primary cause of the failure was improper heat treatment.

Low-cycle fatigue (LCF) failures can be expected when rebars are subjected to a small number of cycles at large stress/strain amplitudes. Major seismic events are often preceded or followed by other events of smaller/larger magnitude. During each event, the residual fatigue life of the steel reinforcement may reduce, eventually leading to failure of the bars due to the cumulative damage. Strain ageing may reduce even more the residual fatigue life of steel. This paper presents the results of an experimental testing campaign conducted on samples fabricated from New Zealand Grade 300E steel. A benchmark LCF life was obtained for 12-mm steel reinforcing bars subjected to axial-strain-controlled completely reversed cyclic tests at amplitudes ranging between 0.78 and 2.75%. The reduction in fatigue life attributed to strain ageing was obtained by comparing the benchmark fatigue life with that of steel reinforcing bars precycled up to 33% and 66% of the benchmark life and artificially strain aged for an equivalent period of 1 year at 15 °C. The Coffin-Manson and Koh-Stephens models were employed to evaluate the experimental results. The study demonstrates that, depending on the strain amplitude, strain ageing can cause a reduction in total and residual fatigue life ranging from 20 to 70%. Strain ageing should not be ignored during the assessment of earthquake-damaged steel reinforcing bars.

Grain size variations are common in thermomechanically processed alloys with non-uniform cold work. A method to produce samples with grain size gradients was developed using Incoloy 800H. Two tensile samples with non-uniform gages were designed with finite element analysis and manufactured. Measured strain profiles were consistent with designs, and maximum von Mises strains of $$({18.6 \pm 6.1})$$ and $$({13.6 \pm 4.6})$$ pct were obtained. After annealing, an area $${40 \, {\hbox {mm}} \times 5}\, {\hbox {mm}}$$ was mapped by electron backscatter diffraction. Totals of 2849 and 2569 grains were identified after merging twins. Both samples had duplex grain structures as defined in ASTM E1181. Grains were binned into 4 mm strips to evaluate the spatial grain size distribution. Grain size gradients of 0.0081 and 0.0112 $${\hbox {mm}^2/\hbox {mm}}$$ were obtained. Simulated grain growth of the linear gradient microstructure was consistent with accelerated growth predictions. This new method of making samples will enable laboratory studies of gradient grain size effects in realistic industrial alloy microstructures. Further, samples could be used for parallel, single specimen experiments on phenomena that depend on grain size such as complexion transitions and fatigue. Most importantly, high-throughput parallel testing of microstructures enabled by our method could accelerate materials discovery and qualification in fields such as high-entropy and nuclear alloys.

As part of a steering linkage mechanism in a jet boat, two D-ring shackle pins were safety wired together in order to prevent either of the shackle pins from falling out. Relative movement of the D-ring shackles from maneuvering the boat caused fluctuating stresses in the stainless steel safety wire, resulting in a fatigue fracture of the wire. Without the constraint of the safety wire, a pin from one of the D-ring shackles became dislodged, resulting in the failure of the steering mechanism, loss of control and crash of the boat. As well as a demonstration of the key features of the components that led to the determination of a fatigue fracture, the significant flaws in the installation and configuration of the steering linkage are discussed.

A total of fifty-eight Incoloy 800H samples were creep tested between temperatures of 1023 K and 1293 K, stresses of 14.1 to 105 MPa, and average grain sizes of 87.7 to 315 µm. Combined with data obtained by the National Institute for Materials Science (NIMS), a deformation mechanism map (DMM) for Incoloy 800H was produced. Optimization of the fit of the constitutive creep equations to the experimental data was performed using a global search iterative numerical optimization tool called a genetic algorithm (GA). It was found that the data were well represented by both high-temperature and low-temperature power-law creep mechanisms, but the extent of the influence of diffusion-based creep mechanisms, most specifically Coble creep, will require further investigation. A training and test method was performed to validate the solution and to test the extrapolability of the dataset. It was determined that the extrapolability of the data in all directions of the DMM was generally low.

A matrix of nine experiments was conducted on aluminum alloy 2139 samples made by an electron beam directed energy deposition additive manufacturing process to characterize the effect of magnesium vaporization loss on microstructure and properties. The nine-sample matrix included three different magnesium levels deposited onto material at three different temperatures. The goal was to quantify the variability in the deposited material as a function of allowable magnesium content and the local temperature of the substrate or previous layer. The results show that temperature variations in the substrate up to 100 K have no significant effect on the amount of magnesium loss; however, relatively large variations in Mg content, up to a ±0.25 wt%, can significantly affect the microstructure and mechanical properties. This is due to a change in how the precipitate phase forms, which influences the mechanism by which the alloy is strengthened.

An aircraft fuel line was found to have fractured and caused an in-flight fire. A brief history of the general problem of cracking in brazed stainless steel fuel injection lines places this failure in context. The fracture mechanism was likely to be fatigue due to vibrations. Other factors contributed to the failure, such as an inappropriate braze repair and incorrect bend radius and location dimensions. The effectiveness of current airworthiness directives and manufacturer service bulletins are discussed. Alternative designs are suggested to prevent recurrence.

The 2010 and 2011 Christchurch seismic events have highlighted the limitations of the current knowledge in assessing the residual capacity of earthquake-damaged reinforced concrete (RC) buildings. An important challenge during the assessment phase was determining the residual ductility and the remaining low-cycle fatigue life of damaged rebars. Low-cycle fatigue is a possible failure mechanism of steel reinforcing bars when subjected to large-amplitude cyclic loads, such as due to earthquakes. While a single seismic event may not cause rebar failure, the low-cycle fatigue life will be reduced due to plastic strain. Also, New Zealand (NZ)-manufactured Grade 300E is prone to strain ageing. This phenomenon causes a change in mechanical properties, such as increase in yield and ultimate tensile strength, return of a discontinuous yield point, reduction in ductility and rise in the ductile/brittle transition temperature, and must be considered in damage assessment. This paper discusses the effects of strain ageing on the monotonic and cyclic steel mechanical properties. Low-cycle fatigue tests were conducted on Grade 300E steel rebars. Reinforcing bar samples were subjected to constant and fully-reversed strain amplitude cycles. Strain amplitudes ranged from 0.5% to 3%. The strain-fatigue life curve for the un-aged steel was determined. The strain ageing effects on the fatigue life of Grade 300E were then investigated. Specimens were cyclically tested up to the 33% and 66% fatigue life previously determined and “artificially” aged at 100°C. Finally, they were cyclically tested until failure. The experimental data were analyzed and low-cycle fatigue models were calibrated using the Coffin-Manson empirical relationship. Fatigue lives of the un-aged and aged samples were then compared. Preliminary observations suggested that strain-ageing triggers a premature crack initiation which propagates until failure.

Carburization is the major degradation mechanism of the HP alloy stainless steel tubes used for ethylene pyrolysis. Due to the changes in the microstructure and magnetic properties of the heat-resistant HP grade (Fe-Cr-Ni) alloy tubes over their service life, the level of carburization can be detected non-destructively using eddy current methods. In the current project, the microstructure of a carburized ex-service Nb-modified HP alloy tube has been characterized via image analysis. The phase fractions and phase distributions have been measured in order to determine the microstructural features that contribute to the eddy current non-destructive testing response, to assist in improving the accuracy of remaining life estimates. Issues in differentiating the large number of phases were encountered when using electron backscattered images for image analysis, and thus, energy-dispersive x-ray spectroscopy maps have been used, in combination with the software programs ilastik, for automated image segmentation, and FIJI, for image processing. The chromium depletion of the austenite matrix of the tube, and thus the magnetic response, was observed to be predominantly influenced by the volume fraction of M7C3 and M23C6 chromium carbides, with potential contribution from the chromium-containing η-carbide intermetallic. Carbonitrides of the type Cr2(C,N) were seen to have negligible impact. A comparison between microstructures and magnetic response has determined that eddy current non-destructive testing can detect and measure carburization.

Modern seismic codes are based on capacity design and the hierarchy of strength philosophy that allows inelastic response in case of severe earthquakes. Thus, in most traditional systems, e...

A hybrid material has been developed to improve creep performance in pressurized metallic pipes subjected to high-temperatures. Model materials were selected for an investigation of reinforcement design parameters in architectured materials. Brass pipes (65 wt.% Cu/35 wt.% Zn) with austenitic stainless steel reinforcement were pressurized and creep rupture tested at 673 K. Compared to unreinforced pipes of equal dimensions, a 47-times reduction in the effective strain rate was observed with a 50° reinforcement angle. A ‘neutral angle’ of 54.7 ± 1.5° was determined experimentally, where tangential (hoop) and longitudinal stresses on the pipe can be balanced and strains minimized. For initial angles below the neutral angle, creep strain was shown to facilitate a shift in orientation towards the neutral angle. For an initial angle of 42°, this shift towards the neutral angle resulted in instantaneous creep rate dropping from 170% of the mean creep rate to 60% of the mean creep rate over 820 h, when the final angle was measured to be 50°. A high-temperature prototype (tungsten braid oriented at 53° over a 253MA stainless steel pipe) was shown to give a creep life extension in excess of 300-times at 1313 K. Keywords: Hybrid, Creep, Architectured reinforcement, Neutral angle

Additive manufacturing (AM) is an emerging technology capable of producing near net shape structures in a variety of materials directly from a computer model. Standard metallic alloys that were developed for cast or wrought processing have largely been adopted for AM feedstock. In many applications, these legacy alloys are quite acceptable. In the aluminum alloy family, however, there is a significant performance gap between the casting alloys currently being used in AM processes and the high strength/toughness capability available in certain wrought alloys. The precipitation hardenable alloys, most often used in high performance structures, present challenges for processing by AM. The near net shape nature of AM processes does not allow for mechanical work prior to the heat treatment that is often necessary to develop a uniform distribution of precipitates and give peak mechanical performance. This paper examines the aluminum (Al) alloy 2139, a composition that is strengthened by homogeneous precipitation of Ω (Al2Cu) plates and thus ideally suited for near net shape processes like AM. Transmission electron microscopy, microhardness, and tensile testing determined that, with proper processing conditions, Al 2139 can be additively manufactured and subsequently heat treated to strength levels comparable to those of peak aged wrought Al 2139.

A highly carburized HP40-Mod alloy ethylene pyrolysis tube was characterized by means of scanning electron microscopy with backscatter electron imaging, electron back-scattered diffraction, energy dispersive spectroscopy, and etching using the NACE International Standard Test Method for evaluation of carburization of ethylene pyrolysis tubes. The response of the tube to eddy current non-destructive testing was measured using the carburization crawler under development at Quest Integrity NZL Ltd. The matrix was significantly depleted of chromium, as low as 4 wt% Cr at the inner wall. M23C6 carbides transformed to M7C3 at the inner wall region and NbC carbides partially transformed to the chromium-rich η-carbide at the outer wall region, both of which likely contributed to the chromium depletion of the matrix. The present results indicate that the NACE etchant attacks the austenitic matrix where chromium content is below approximately 12wt%. A comparison to other ex-service tubes indicates that matrix chromium content and the location of the M23C6/M7C3 transformation front are useful microstructural characteristics for interpreting eddy current NDT response.

The microstructures of two as-cast heats of HP alloy stainless steels modified with niobium and titanium were examined with particular attention paid to the interdendritic niobium-titanium-rich carbides formed during solidification of these alloys. Generally, these precipitates obtain a blocky morphology in the as-cast condition. However, the (NbTi)C precipitates may obtain a nodular morphology. To provide further insight to the origin of the two different morphologies obtained by the (NbTi)C precipitates in the HP-NbTi alloy, the microstructure and crystallography of each have been studied in detail using scanning electron microscopy, transmission electron microscopy, various electron diffraction methods (EBSD, SAD, and CBED), and energy-dispersive X-ray spectroscopy.

Aluminium carbide (Al4C3) has been identified in ex service and laboratory test specimens of cast irons exposed to liquid aluminium. In cast iron/aluminium couples, the formation of Al4C3 and κ-Fe3AlC was confirmed for the first time. The growth kinetics of Al4C3 were parabolic with an activation energy 145±23 kJ mol−1 (1023–1223 K), which is consistent with the reported activation energy for growth in graphite/liquid aluminium couples. The aim of this work is to shed light upon the origin and growth of Al4C3 in cast iron/liquid aluminium couples in the temperature range of 1023–1223 K.

Modern seismic codes are based on capacity design and hierarchy of strength principles that allow the formation of plastic hinges in sacrificial elements within a structure. An accurate estimation of the mechanical properties of the steel reinforcement is required to achieve a ductile failure mechanism. After being subjected to plastic deformation, many types of low-carbon steel experience a time- and temperature-dependent phenomenon known as strain-ageing. This phenomenon causes changes in the mechanical properties such as increase in yield and reduction in ductility. Therefore, in the aftermath of an earthquake event, the hierarchy of strength between beams and columns in a subassembly and/or between flexure and shear within the same structural element of a damaged structural system could potentially be compromised. In this paper, the extent of strain-ageing in New Zealand Grade 300E and Grade 500E reinforcement is investigated through experimental testing in which reinforcing bar samples were pre-strained up to selected strain limits and then subjected to natural (at 15 °C) and accelerated ageing (at 100 °C), at different time intervals, up to one year. The changes in mechanical properties were monitored and quantified. These changes must be accounted for during post-earthquake assessments and repairing design and are recommended to be incorporated in future code provisions.

The microstructures of two as-cast heats of niobium-modified HP stainless steels were characterized. Particular attention was paid to the interdendritic niobium-rich carbides formed during solidification of these alloys. At low magnifications, these precipitates are grouped in colonies of similar lamellae. Higher magnifications revealed that the lamellae actually obtain two distinct morphologies. The type I morphology exhibits broad planar interfaces with a smooth platelike shape. Type II lamellae have undulating interfaces and an overall reticulated shape. To provide further insight into the origin of these two different morphologies, the microstructure and crystallography of each have been studied in detail using high resolution scanning electron microscopy, transmission electron microscopy, various electron diffraction methods (electron backscatter diffraction (EBSD), selected area diffraction (SAD), and convergent beam electron diffraction (CBED)), and energy dispersive X-ray spectroscopy.

A series of grain boundary engineering (GBE) procedures was performed on samples of 800H, a common iron-nickel base alloy. The effects of varying the GBE parameters (deformation percentage and annealing temperature/time) were examined using electron backscatter diffraction orientation mapping. GBE samples exhibited up to a 70 pct increase in Σ3 n boundary length fraction compared to the as-received condition. The incorporation of these Σ3 n boundaries was also measured using a simplified model of two-dimensional grain boundary transport and its electrical analogy. The results showed that GBE resulted in increased resistance to grain boundary diffusion compared to the as-received condition. The evolution of a GBE microstructure was also studied on a per-cycle basis, and the results are shown to be consistent with previous studies.

This article describes the root cause failure analysis of a mechanical vapor recompressor (MVR) impeller used in a dairy processing plant. An impeller blade was thrown during commissioning after approximately 150 h of service. Visual examination of the blade’s fracture surface indicated that the crack growth mechanism was fatigue and this was confirmed by scanning electron microscope (SEM) examinations. A detailed investigation was commissioned to determine the source of the oscillatory stresses that must have been present to cause fatigue. A vibration analysis indicated large torsional oscillatory stresses, leading to fatigue, were induced by excitation of the 1st and 2nd torsional vibration modes of the MVR impeller and motor system. The excitation was induced by the control system for the motor variable speed drive (VSD).

Samples of annealed and cold rolled 70%Cu-30%Zn brass and 800H alloy were characterized using electron backscatter diffraction (EBSD) pattern analysis to determine average grain diameter, with the ability to differentiate between random high angle grain boundaries and annealing twin boundaries. Hardness measurements of both materials and minimum creep rates for the brass alloy were measured and compared to average grain diameter measurements, which had been calculated with twin boundaries both included and excluded (in accord with ASTM E 112). The present results indicate that annealing twin boundaries contribute to mechanical properties and therefore should be considered in structure-property correlations.

A comprehensive, critical, and up to date review is presented for the proeutectoid cementite transformation in steels. It is believed that many of the new findings, features, and concepts presented here for this classic phase transformation in steels serve as a model which may be more broadly applicable to test against many other phase transformations systems as well. There were a number of early investigations of cementite morphology, and this review considers those early results in light of many newer studies that provide critical new insight into cementite morphologies in both two and three dimensions. A number of different orientation relationships (ORs) between proeutectoid cementite and the austenite matrix from which it forms have been reported in the literature, in some cases leading to confusion, and they are critically evaluated here, as are the habit plane, growth direction, and interfacial structure of various morphologies of proeutectoid cementite. Quantitative experimental a...

To provide new insight into microstructural development during creep, sequential EBSD (electron backscatter diffraction) orientation maps were obtained from fixed surface areas at intervals in creep tests of a high-pressure die-cast Mg–8.5Al–1Ca–0.3Sr alloy. Samples were tested at 200 °C at various stress levels to achieve power-law creep behavior. In all specimens, steady-state creep was marked by the formation of tension twins. At intermediate stress levels, specimen failure was initiated by the formation of defects at boundaries with high crystallographic misorientation. However, at high stress levels, the failure mechanism was assisted by the crack formation at interdendritic boundaries with low crystallographic misorientation.

To provide new insight into microstructural development during creep, sequential electron backscatter diffraction (EBSD) orientation maps were obtained from fixed surface areas during interrupted creep tests of extruded pure magnesium. Samples were tested under two distinctly different conditions, i.e., low-temperature power-law creep and high-temperature power-law creep. Three aspects of microstructural evolution during creep were observed. In both specimens, transient creep was marked by the elimination of tension twins formed during the extrusion process. As steady-state creep rates were obtained, both specimens exhibited an ever-decreasing contribution by basal slip accompanied by an ever-increasing contribution by prismatic slip. In the low-temperature specimen, prismatic slip was manifested by the formation of a network of new fine subgrains along original grain boundaries, resulting in a bimodal grain size distribution. However, the high-temperature specimen developed linear prismatic slip lines while maintaining its original grain size.

A three dimensional view of creep voids in a hydrogen reformer tube is presented. By separating and reconstructing various microstructural features present, the proper representation of creep voids in 3D can be fully studied. Useful measurements of parameters such as void volume, void-to-void distance, and grain boundary angles were obtained. The data presented here represent the initial collection of creep void information for use in various creep void nucleation and growth models. Additional data are currently being collected for material subjected to different service conditions.

A magnesium–8 wt% lithium alloy was prepared to give a microstructure of hcp (α) and bcc (β) phases, with the α phase appearing as plates in the β matrix. A near Burgers OR was observed between the two phases, i.e., [ 0 0 0 1 ] α / / [ 0 1 ¯ 1 ] β , ( 1 ¯ 1 0 0 ) α / / ( 2 ¯ 1 1 ) β . An additional ∼0.5° rotation of the bcc lattice around [ 0 0 0 1 ] α / / [ 0 1 ¯ 1 ] β axis, as well as the α/β interface orientation, were both predicted by a moire plane approach and observed experimentally.

This paper describes the three-dimensional (3-D) microstructure of a pearlite colony in a carburized electrolytic iron. The original data was obtained by serial sectioning methods around 1960, and is revisited with modern 3-D visualizations. After a brief historical introduction, the structure of the pearlite colony is discussed, using surface renderings and blue-red stereoscopic images.

The techniques of serial sectioning and three-dimensional reconstruction were used to characterize creep voids occurring in an ex-service hydrogen reformer tube. The influence of variations in temperature, stress and microstructure types on the characteristics of creep voids are considered in the study. The results revealed that the highest void density per grain boundary area occurred at the outer wall of the tube where the temperature is highest and the microstructure consists of large columnar grains, compared to the inner wall of the tube consisting of smaller equiaxed grains.

Al-Fe-Si intermetallic particles in both unmodified and highly modified sand-cast eutectic Al-11.7 pct Si alloys were characterized using scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy, and convergent beam and selected area electron diffraction. The only two Al-Fe-Si intermetallics observed in this particular alloy are (1) “Chinese-script” morphology, consistent with a description of body-centered cubic α-Al19Fe4MnSi2 and (2) plate-shaped, consistent with tetragonal δ-Al3FeSi2. The authors are unaware of any other characterization of δ-Al3FeSi2 using convergent beam electron diffraction (CBED) and selected area diffraction (SAD) techniques.

We report on the growth of unusual star shaped Bi islands and ordered arrays of nanorods on highly ordered pyrolitic graphite (HOPG) substrates. The rods and stars grow with {0 1 2}Bi||{0 0 1}HOPG and are preferentially aligned with the high symmetry directions of the substrate. As more Bi is deposited, the film undergoes a transition to the {0 0 1} orientation. These features are a result of a complex interplay between kinetics, thermodynamics, and the crystallography of the substrate and island material.

An ex-service hydrogen reformer tube composed of Fe–0.45% C–25% Cr–35% Ni–1.0% Si–1.5% Nb–1.0% Mn (approximate wt.%) was examined via serial sectioning and 3D reconstruction to characterize creep voids in terms of their size, location, and surrounding precipitates. In addition, electron backscatter diffraction (EBSD) technique was used to investigate any crystallographic influence on the location of these voids. Observations in 3D show that void distribution is not uniform in terms of size, shape, and location. Most of the voids are oblong and occur at grain edges and corners. In addition, all observed voids were in contact with the Cr-rich M 23 C 6 precipitates. EBSD results showed that 80% of the M 23 C 6 precipitates surrounding these voids have an irrational crystallographic orientation relationship (OR) with the austenite matrix, contrary to the expectation that grain boundary precipitates nearly always obtain rational OR with respect to one adjacent grain.

We report on the growth of ultra-thin bismuth films on the basal plane of highly ordered pyrolitic graphite (HOPG) substrates. Scanning electron microscopy and atomic force microscopy have been used to characterize the morphology, and the crystallographic orientation was obtained using electron back scatter diffraction. Low coverage films are comprised of islands with a striped surface morphology, and show the orientation relationship { 0 1 1 ¯ 2 } Bi ∥ { 0 0 0 1 } HOPG with preferred in-plane orientations 〈 1 1 2 ¯ 0 〉 Bi ∥ 〈 1 0 1 ¯ 0 〉 HOPG . With increasing film thickness, we identify an unusual orientation transformation to the commonly found Bi{0 0 0 1} (trigonal) orientation.

Iron-rich intermetallic phases are well known to be strongly influential on mechanical properties in Al–Si alloys. Two important phases have been previously designated as the relatively benign α-phase (with a ‘Chinese Script’ morphology) and the detrimental β-phase (plate or needle-shaped). While the common morphologies and effects of these phases are generally agreed upon, there is some confusion in the literature regarding the chemistry and/or crystallography of these phases. Using a combination of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), electron backscatter diffraction pattern analysis (EBSD), transmission electron microscopy (TEM) and convergent beam electron diffraction (CBED), the present work showed that intermetallic phases in cast eutectic Al–Si alloy intermetallics are often misidentified in the current metallurgical literature. The α-phase particles were consistent with cubic Al19Fe4MnSi2 rather than hexagonal. The phase often identified a β-phase is actually consistent with the tetragonal Al3FeSi2 (more correctly designated as δ-phase) rather than monoclinic or orthorhombic.

Microstructural characterization of Mg-12Li-xCa alloys (x=0−15 wt.%) was performed via a combination of several analytical techniques. The addition of Ca to a Mg-12Li alloy resulted in as-cast microstructure with a primary dendrites of β-(Mg, Li), a solid solution of Mg in bcc Li, and a lamellar interdendritic eutectic of the β-phase and CaMg2. The amount of the eutectic in cast Mg-12Li-xCa alloys corresponded to the Ca content. Cold-rolling the alloys re-distributed the micro-constituents. Surface oxidation was shown to preferentially occur on the β-phase.

The three-dimensional (3-D) shapes and distributions of grain-boundary-nucleated proeutectoid ferrite precipitates have been obtained by computer-aided 3-D reconstruction of serial sections of an Fe-0.12 wt pct C-3.28 wt pct Ni alloy. Isothermal transformation for short times at 650 °C was used to produce a low volume fraction of ferrite, which appeared as both Widmanstatten shapes and more-equiaxed grain-boundary precipitates. While the two-dimensional (2-D) cross sections of these precipitates appeared to fit into the previously accepted categories of precipitate morphologies, 3-D reconstructions revealed important aspects of connectivity and shape that were not observed earlier. A partially revised morphological classification based on these 3-D observations is provided here. Significant differences between the 3-D morphology of proeutectoid ferrite and that of proeutectoid cementite are also discussed, indicating (among other things) that the 3-D morphological classifications of ferrite are generally more complex than those of proeutectoid cementite.

The crystallography and three-dimensional (3-D) morphology of Widmanstatten proeutectoid ferrite precipitates are examined in an Fe-0.12 wt pct C-3.28 wt pct Ni steel isothermally reacted at 650 °C, 600 °C, and 550 °C. This article integrates new orientation mapping (OM) results with the findings of a companion article to this one on the 3-D morphology of proeutectoid ferrit[1] and an earlier transmission electron microscopy (TEM) study which is reanalyzed here in light of the new OM and 3-D results. All of these studies were performed for the same alloy and heat treatments. The 3-D morphologies and distributions of proeutectoid ferrite precipitates are now known to often be quite different from those deduced by conventional two-dimensional (2-D) microscopy techniques. The present crystallographic studies indicate that “primary” ferrite (nucleated directly on prior austenite grain boundaries) forms monolithic single crystals and can be approximated as elongated triangular pyramids. “Secondary” ferrite morphologies can be described as laths and plates branching into the austenite from a thick and/or broad allotriomorphic ferrite base. These secondary Widmanstatten branches are composed of many misoriented crystals with ferrite: ferrite boundaries between them and appear to approach a common orientation as they extend into the austenite grain. Implications of the current findings on existing growth and crystallography models are discussed, and a preliminary hypothesis or mechanism of ferrite formation has been proposed to account for the present observations.

Intermetallic phases in sand cast eutectic Al–Si alloys were characterized using a combination of SEM, EDS and EBSD pattern analysis. Chinese script α-phase particles were consistent with cubic Al19(Fe,Mn)5Si2. Plate-shaped β-phase particles were consistent with tetragonal Al3(Fe,Mn)Si2.

Hardness, optical-microscopy and X-ray diffraction studies on the strengthening phase in β-phase magnesium–lithium alloys containing different content of aluminum were carried out to give some new characteristics of the strengthening phase affecting lattice distortion and α-Mg precipitation in the β-matrix. In the presence of the strengthening-phase precipitates, the matrix lattice undergoes substantial strain characterized by peak broadening. The peak width in the β-matrix phase pattern can provide an indication of lattice strain caused by the strengthening-phase precipitates. The origin of α-Mg precipitation resulting from the decomposition of the strengthening phase into stable AlLi compound is also explained in the present work.

The crystallography and morphology of proeutectoid grain boundary cementite precipitates were studied in an isothermally transformed Fe-1.3%C-12%Mn steel. Grain boundary cementite precipitates develop with complex variations of a morphology that can be described as fern-like dendrites growing preferentially within and along austenite grain boundaries. The dendritic morphology was made most readily apparent by deep etching specimens of the alloy whose growth had been arrested after relatively short times during the solid state austenite→cementite transformation. Trace analysis revealed that these precipitates usually have no crystallographically preferred primary or secondary dendrite arm growth directions. Electron Backscattered Diffraction pattern analysis showed that most of the grain boundary cementite precipitates approximated one of the known cementite–austenite crystallographic orientation relationships (OR) with at least one of its adjacent austenite grains. Occasionally, the grain boundary cementite exhibited an OR that achieved ‘best fit’ directions between the cementite and austenite in both adjacent austenite grains.

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.

A novel transmission electron microscopy (TEM) sample preparation technique was applied during a study of proeutectoid cementite precipitates in a Fe–1.34 wt.% C–13.0 wt.% Mn alloy. In isothermally transformed specimens, the material contains interconnected networks of grain boundary and Widmanstatten cementite precipitates. By immersing conventionally prepared (dimpled and electropolished) TEM specimens in an etching solution, the three-dimensional morphology and internal structure of entire electron transparent cementite precipitate networks were revealed to provide insights that were not previously possible. This paper describes the experimental technique and illustrates the benefits of this technique by comparing present results with observations of thin foil specimens of the same alloy.

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.

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.

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.