Materials Science Forum Vol. 753

Abstract: Functions of an observation and an analysis in electron microscope, such as scanning electron microscope (SEM) or transmission electron microscope (TEM) are indispensable to evaluate advanced materials. Therefore a specimen preparation technique, that is a front end of the electron microscopy, has become highly important, thus a choice of it affects a result of the evaluation. The authors was combined a cooling stage in FIB and applied it for evaluation of metals with low melting point. The electron microscopic evaluation of Lead solder, Indium, Tin and Bismuth, metals with low melting point, has been always discussed if the results represent the actual physics. Metals with low melting point are heat sensitive materials, so the comparison of cross-sectioning with room and low temperature, it can be said that low temperature cross-sectioning has less effect and keeps the actual physics of the sample. In this paper, some knowledge from comparisons of cross-sectioning with room and low temperature for metals with low melting point are reported.

Abstract: Changes in the microstructure and crystallographic orientations during in-situ heating of folded Al 0.1%Mg have been followed by SEM and EBSD. The folding process results in both strain and texture gradients across the folded region which in turn can influence the recovery and recrystallisation processes as well as crystallographic texture. This work is an extension of ex-situ heating experiments on folded nickel 200, titanium and ferritic steel [1,2]. The present findings illustrates that during isothermal in-situ heating at 295oC nucleation and growth starts close to the surface where the deformation is highest, new grains form and grow in a region about quarter depth of the sheet thickness. After this grain growth occurs resulting in large grains that meet up at the centre line. These results are consistent with those found in ex-situ heated Ni200 alloy [2], where fine grains were found in the compressed and tensile regions with large grains in the middle of the sheet.

Abstract: Abstract. With the advance of electron diffraction techniques in individual orientation analysis, traditional crystallographic characterization methods could be simplified, thus offering chances to develop some new approaches. In recent years, our group has devoted to working on possible extensions of the SEM and TEM based techniques for crystallographic analyses on a microstructure- and orientation-specific level. Several methods are illustrated in this paper, including the determination of dislocation type and Burgers vector without recourse to the traditional g.b invisibility condition, the identification of twinning mode and complete twinning elements for any crystal symmetry that requires minimum initial data input, and the characterization of specific interface plane or slip plane using only one sample observation plane instead of two perpendicular sample planes. These new extensions of characterization methods have proven to facilitate the related microstructural examinations.

Abstract: Heavily deformed 18-carat yellow gold samples show a recrystallization peak at 700 K during the first heating. The mechanical loss spectrum of polycrystals shows a relaxation peak at about 780 K, which is absent in single crystals made from the same alloy. Stepwise deformation of a single crystal from 2 % to 10 % causes an increase of the high temperature mechanical loss background and the appearance of a high temperature peak. At 8 % deformation the high temperature peak disappears and the peak that is normally observed in polycrystals appears. The increase of the exponential background is interpreted as due to the introduction of new dislocations whereas the high temperature peak is attributed to a relaxation mechanism in the sub grain boundaries. The peak of polycrystalline samples located at intermediate temperatures depends on the grain size: with grain growth, the peak position shifts to higher temperatures. The peak temperature can be related to the mean grain size.

Abstract: It is known that there is lattice rotation around the particles after deformation, in a so-called Particle Deformation Zone (PDZ), which is thought to be important in randomizing the texture after recrystallization though Particle Stimulated Nucleation (PSN). However, the role of the local distribution of stored energy and its link to local lattice rotation is not well understood, making it impossible to successfully predict PSN efficiency. Here, we present a new method for studying the deformation around particles with the use of Digital Image Correlation (DIC) of High-Resolution Scanning Electron Microscopy (HRSEM) image and Electron BackScatter Diffraction (EBSD) map. Combining these two techniques makes it possible, for the first time, to relate the local deformation fields to remnant changes in local lattice orientation. Initial measurements are made on a model Al-Si alloy deformed in plane-strain condition by channel die compression up to 30%. Our analysis shows that the material deforms heterogeneously with high levels of deformation localized along slip bands. EBSD analysis shows that the lattice distortion in these bands is minimal. The HRDIC analysis clearly shows particles interrupt the slip banding. Local lattice rotation measured by EBSD is considerably less than expected from the measured deformation.

Abstract: Laser ultrasonics for metallurgy (LUMet) is an innovative sensor technology for in-situ measurement of microstructure evolution during thermomechanical processing. This unique sensor has been attached to a Gleeble 3500 thermomechanical simulator for dedicated laboratory studies during processing of steel, aluminum, magnesium and titanium samples. Advanced processing software has been developed for the measurement of grain size and texture evolution from laser ultrasonic signals. Results of austenite grain growth measurements in low carbon steels will be described to demonstrate the capabilities of the LUMet technique. Further, applications of the system to measure recrystallization of ferrite and austenite formation during intercritical annealing simulations of dual phase steels will be presented. The ability to rapidly acquire data both during a single test and for multiple conditions over a range of conditions from different samples has important implications on expediting process modelling and alloy design. Although certain limitations exist, the LUMet technique offers a very reliable characterization platform with a number of potential applications in metallurgical process engineering.

Abstract: Integrated model based control systems are an essential part of modern plant operations. The production requirements have evolved from achieving geometric tolerances to becoming a large, flexible, but accurate metallurgical instrument. Within the steel industry, on-line process and microstructure monitoring systems have been realised for many years on hot strip mills. At recent conferences (including ReX&GG IV), Siemens has described the successful implementation of these models in steel plate production; in applications where there is increased complexity in through thickness microstructure and the final products have safety critical applications. Even with advanced on-line systems and tailored production routes there is a relatively comfortable process window in modern steel manufacture. For the plant builder, light structural metals titanium and magnesium, and harder nickel based high temperature alloys represent another level of complexity and process control requirements. This paper examines the potential of microstructure modelling with respect to the process design for manufacturing wrought products in alternative alloy systems. Metallurgical aspects will be considered and their practical implications discussed. The continued expansion of these materials into markets where high strength/weight ratios, corrosion resistance or high temperature properties are desirable (e.g. automotive, offshore, chemical, aerospace) make this worthwhile from an industrial perspective.

Abstract: Effects of local variations in the deformation microstructure on subsequent recrystallization are discussed and illustrated by three examples. The three examples consider local variations on different length scales and are: 1. Effects of local variations in the deformation microstructure on the formation of protrusions on migrating boundaries. 2. Effects of an inhomogeneous spatial distribution of second phase particles on growth. 3. Effects of stored energy and orientation variations on recrystallization kinetics.

Abstract: The texture and microstructure evolution during uniaxial compression of Zircaloy-4 at 500C has been studied. X-ray diffraction was used to measure the bulk texture compressive after strains of 20% and 50%. Early stages of texture evolution involve the strengthening of the fibre through the action of prismatic slip. With further deformation, the basal poles move toward the rolling direction from the normal direction as a result of a non-prismatic slip system. A detailed EBSD study in a small region showed that the grains of the fibre are more stable than those of the fibre. The latter rotates further during deformation and shows a greater spread of grain average misorientation (GAM). This could be due to a greater accommodation of plastic strain in these grains and/or a difference in recovery rates between the two fibres.