[en] The present study has examined for α/β-Ti alloys the relationship between the morphology and crystallography of Widmanstaetten plates of α-Ti in colonies within a prior grain of β-Ti. Thus, optical metallography, scanning electron microscopy and transmission electron microscopy have been used to characterize the morphological features of the microstructure, whereas orientation-imaging microscopy (OM) and transmission electron microscopy (TEM) have been employed to reveal crystallographic information. It has been discovered that within a prior β-Ti grain, although the growth direction of the Widmanstaetten plates in given colonies may differ by large angles from α-plates in other colonies, they may exhibit very close crystallographic relationships. For example, inclined α-plates may share common basal planes and be related by a rotation of ∼10.5 deg. about the c-axis of the crystals. This phenomenon has been interpreted on the basis of variant selection of the Burgers orientation relationship commonly adopted between the α and β phases in these alloys. Similar relationships have been observed in α-colonies growing either side of a given prior β grain boundary. These latter observations have been used to draw conclusions concerning the precipitation of α on prior β grain boundaries

[en] A previously unidentified, nanometer scale ordered precipitate has been characterized in the metastable β titanium alloy Ti-5Al-5Mo-5V-3Cr (Ti-5553) using conventional transmission electron microscopy (TEM) and high-angle annular dark field (HAADF) high-resolution scanning transmission electron microscopy (HRSTEM). The phase, termed O″, is metastable, the ordering transformation occurring over an intermediate temperature range (e.g., 375 °C), and this ordered face centered orthorhombic precipitate may be observed co-existing with isothermal ω and α phases in the alloy. It is also noted that this transient phase may provide the diffracted intensities observed from another phase, identified tentatively in the literature as orthorhombic martensite, α″ [1].

[en] In the good old days, there was a very significant effort involved in developing new materials using rapid solidification processing. The processing methods included the production of powder and melt-spun ribbon, as well as laser surface melting and the original bold attempts to perform Layerglazing^TM, now known as laser engineered net shaping (LENS^TM). One of the problems associated with the early work in this field involved the need to compact the particulate, usually through the application of heat and pressure, which caused coarsening of the refined microstructures, so minimizing the advantages associated with the rapid heat extraction. The first part of this presentation will present some of the salient features of this early approach to rapid solidification processing, making use of examples from the work of the author's research group.More recently, there has been a resurgence in this type of processing, including a revisit of LENS^TM-type processing and also more recent techniques, not involving melting, such as Kinetic Metallization (KM). Results of work using these processing methods will be presented. Additionally, use of other non-equilibrium processing procedures which permits optimum processing of Ti alloys will be described. (author)

[en] Robert (Bob) Sinclair and Nestor Zaluzec have been working for many years at the leading edge of developments in electron microscopy techniques and applications. Their distinguished careers and some of their notable scientific achievements are briefly highlighted. - Highlights: • Biographical sketch of Bob Sinclair • Biographical Sketch of Nestor Zaluzec • Scientific career highlights

[en] In the present study, the relationship between the crystallographic orientations and growth directions of grain boundary-allotriomorphic-α (GB α) and secondary Widmanstaetten α laths growing from the GB α at grain boundaries separating β grains with specific misorientations has been examined. These relationships have been determined using a variety of characterization techniques, including scanning electron microscopy, orientation imaging microscopy, transmission electron microscopy (TEM) and a dual-beam focused ion beam instrument to provide site-selected TEM foils. Two very interesting cases, one in which the two adjacent β grains are rotated mutually by approximately 10.5^o about a common <1 1 0> direction and the other in which the two β grains are in a twin relationship, i.e. a 60^o rotation about a common <1 1 1> direction, have been studied. It was discovered that the α laths growing into two adjacent β grains from the common grain boundary may have the same orientation in both grains, while they may have either large (∼88.8^o) or small (28.8^o) angular differences in growth directions in the two adjacent β grains, depending on the relative misorientation of the β grains. The growth directions of the α laths growing from such boundaries are explained on the basis of the Burgers orientation relationship between the Widmanstaetten α and the β phases and the interfacial structure proposed previously by various workers

[en] Highlights: • Image contrast with the T3 detector is sensitive to SE1 yield. • SE1 signal is maximized when low accelerating voltages are used in the SEM. • Second phases in a metallic matrix may be differentiated by their SE coefficient. • Insulating oxides exhibit high SE yield. Conductive carbonitrides exhibit low SE yield. • Observed phases in the selected alloys agree with CALPHAD predictions. Novel imaging strategies in the scanning electron microscope aimed at significantly improved image contrast of second phases in metal alloys are described. These include the use of low accelerating voltages, small working distances, and a novel detection system. Contrast is assessed as a function of voltage and optimized imaging conditions which result in much improved image quality are presented. These strategies are applied to two precipitation hardened Ni-base alloys, a cast single crystal and a hot isostatically pressed sample.

[en] Three-dimensional microscopy has become an increasingly popular materials characterization technique. This has resulted in a standardized processing scheme for most datasets. Such a scheme has motivated the development of a robust software package capable of performing each stage of post-acquisition processing and analysis. This software has been termed Materials Image Processing and Automated Reconstruction (MIPAR™). Developed in MATLAB™, but deployable as a standalone cross-platform executable, MIPAR™ leverages the power of MATLAB’s matrix processing algorithms and offers a comprehensive graphical software solution to the multitude of 3D characterization problems. MIPAR™ consists of five modules, three of which (Image Processor, Batch Processor, and 3D Toolbox) are required for full 3D characterization. Each module is dedicated to different stages of 3D data processing: alignment, pre-processing, segmentation, visualization, and quantification.

With regard to pre-processing, i.e., the raw-intensity-enhancement steps that aid subsequent segmentation, MIPAR’s Image Processor module includes a host of contrast enhancement and noise reduction filters, one of which offers a unique solution to ion-milling-artifact reduction. In the area of segmentation, a methodology has been developed for the optimization of segmentation algorithm parameters, and graphically integrated into the Image Processor. Additionally, a 3D data structure and complementary user interface has been developed which permits the binary segmentation of complex, multi-phase microstructures. This structure has also permitted the integration of 3D EBSD data processing and visualization tools, along with support of additional algorithms for the fusion of multi-modal datasets. Finally, in the important field of quantification, MIPAR™ offers several direct 3D quantification tools across the global, feature-by-feature, and localized classes.

[en] Here, we explore mechanisms of ω-assisted α nucleation for microstructure refinement in a metastable β titanium alloy, Ti-20V (wt%). In particular, we analyze quantitatively the individual and combined effects of concentration and stress fields associated with a growing isothermal ω precipitate and the potency of the ω/β interface on the activation energy barrier for α nucleation as a function of the ω precipitate size and coherency state. We find that the complex interplay among these three factors determines the most potent ω particle size for inducing α nucleation and the most favorable nucleation site.

[en] A number of laser and electron beam-based fabrication processes are under consideration for aerospace components, where the ability to obtain a consistent and desirable microstructure and resulting mechanical properties is of critical concern. To this end, this work employs a combination of analytical and numerical modeling approaches to investigate the effects of process variables and size-scale on solidification microstructure (grain size and morphology) in beam-based fabrication of bulky 3D structures. Thermal process maps are developed for predicting solidification microstructure in any material system, and results are plotted on solidification maps to investigate trends in grain size and morphology in Ti-6Al-4V. The results of this work suggest that changes in process variables (beam power and velocity) can result in a grading of the microstructure throughout the depth of the deposit, with a transition from columnar to mixed or equiaxed microstructure at higher powers.

[en] The microstructure of a Ti-Fe compositional gradient sample was investigated using transmission electron microscopy (TEM) and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). Different combinations of phases were observed to form depending on the local concentration of Fe. Interestingly, a nano-scale incommensurate modulated domain was detected in the alloy containing approximately 13.8 to 33.9 wt% Fe using selected area diffraction (SAD). The observed disordered structure with no fixed long range periodicity in the nano-scale incommensurate modulated domain in aberration-corrected HAADF-STEM imaging was found not related to the shuffle mechanism of the ω phase.