Materials Science Forum Vols. 495-497

Abstract: Regions with sharp local textures, called macrozones, have been characterised in a bimodal IMI834 billet, containing 30% of primary αp grains surrounded by secondary αs colonies. It is shown that the αs colonies have been inherited according to a strong variant selection during the β→αs phase transformation. In each observed macrozone, the favoured variants have in average their c-axes in the same macroscopic direction as the αp grains. A detailed analysis of neighbouring αp grains and αs variants clearly shows that the variants favoured at β/αp boundaries are those able to share their c-axes with a neighbouring αp grain. The sharpness of such a variant selection mechanism is strongly related to the local orientation distribution of neighbouring αp/β grains at high temperature. This explains the differences in variant selection sharpness observed from one macrozone to the other.

Abstract: The transformation texture associated with martensite formation in the titanium alloy Ti- 6Al-4V has been investigated. Samples were heated into the fully b phase and quenched to form a microstructure of very fine a' martensite with no evidence of diffusional transformation at the prior b grain boundaries. EBSD texture measurements on the martensite showed that within each prior b grain, although typically all 12 variants of a’ were formed, the fractions of variants was far from uniform. The a’ texture was markedly different from values calculated using equal variant probability, also indicating that significant variant selection was occurring during martensitic transformation. This effect was modelled on the basis of elastic interaction between martensite events.

Abstract: Electron backscattered diffraction (EBSD) has been used to determine microtexture in a zirconium alloy tube subject to interrupted pilgering and therefore exhibiting varying amounts of deformation as a function of axial position along the transition between the initial and final size. Texture and hardness measurements have been made as a function of the distance through the tube wall thickness and along the tube length. Texture results have been compared with co-located hardness measurements. The results show a systematic variation in the deformation texture with changes in Q (the ratio of the reduction in thickness to reduction in diameter). This is consistent with previous observations of the effect of Q on texture evolution in zirconium alloys. It is demonstrated that the texture measurements can be correlated well with the anisotropy in strength determined from hardness measurements.

Abstract: Titanium alloys are widely used in various industrial, domestic, and medical applications such as turbine blades, bicycle frames, knee implants, etc. The two-phase titanium alloy Ti-6Al-4V (wt. percent) is considered to be a workhorse alloy for many applications in these diverse fields. Despite the large body of work on this alloy, the question of the transformation mechanism from the hcp a to the bcc b phase, occurring on heating to temperatures above the a/b transus at ~980°C, is still unresolved. Due to experimental difficulties, it has not yet been clearly determined whether the increase in b volume fraction occurs by fresh nucleation of b crystals within a phase grains or the growth of preexisting b grains. Since the Burgers orientation relationship holds only if the b grains are nucleated within the a grains, the outcome of this question greatly affects texture-modeling efforts for this system. The Burgers orientation relationship predicts that the {0001} crystal direction in a grain of the a phase becomes a {110} crystal direction in a grain of the b phase after the transformation. In this work we present experimental results from in-situ texture measurements performed on the HIPPO neutron diffractometer at LANSCE. Using the combination of time-offlight neutrons and full-pattern Rietveld analysis allowed us to determine the orientation distribution functions of both phases at room temperature, 800°C, 1020°C and again at room temperature. We found strong indications that the b phase indeed grows from grains preexisting at room temperature. Upon re-transformation from b to a we found that the Burgers relationship is followed.

Abstract: Irradiation of metal materials with low-energy ions is accompanied by the long-range effect, consisting in distinct texture and structure changes at the depth, exceeding, at least, by 104 times the thickness of the layer of ion retardation. In order to ascertain mechanisms of this effect, a layer-by-layer X-ray study was carried out as applied to ion-plasma treated cladding tubes of Zr- 1%Nb alloy for nuclear reactors. Impacts of pulse treatments with helium plasma by two different regimes and the ion energy of ~ 1 keV were compared. It was found, that only by minimal surface melting the ion-plasma treatment causes unpredictable bulk texture changes, consisting in arising of the axial texture component. The melted surface layer suppresses shock waves, associated with braking of ions and supposedly responsible for stimulation of dislocation processes at long distances from the surface.

Abstract: The effect of hot working strain on texture evolution in the near-α titanium alloy Ti 834 has been investigated using hot axisymmetric compression testing and EBSD. Testing was undertaken sub β transus with 22% α, which upon cooling produced a bimodal microstructure of primary α in a matrix of transformed β. Two distinct deformation induced texture components were identified: i) transverse [(0001)||compression direction] and ii) off-basal [(0001) »^ compression direction]. Orientation image mapping was then used to identify the microstructure associated with each texture component. The transverse texture is associated with the transformed β and the offbasal component is the deformation texture of the primary α.

Abstract: Owing to their lower modulus, great corrosion resistance and superior biocompatibility, titanium alloys are increasingly used as artificial joint replacements. However bone bonding capability of these materials needs to be improved. Many studies are currently conducted to improve the osseo-integration of titanium based implants. In the present study, the role of crystallographic texture of titanium alloy Ti-6Al-4V on bone bonding capability was investigated in vitro systems. X-ray diffraction analysis was used to determine preferred orientation in each substrate. These substrates were seeded with preosteoblast cells to examine cell attachment and proliferation. Attachment of cells was assessed by counting the number of adhered cells within 30-240 min. The proliferation rate of cells was measured between the 3rd-11th days of incubation. The results suggest that the substrate with (100) orientation shows better osteoblastic cell adhesion and proliferation rate than the (110).

Abstract: The mechanisms governing the very first stage of static recrystallization in two hexagonal alloys (commercially pure titanium and low alloyed zirconium) are investigated in this paper. Initially fully recrystallized and equiaxed materials were cold-rolled to 80% thickness reduction and subsequently recrystallized at 500°C for short times. High resolution EBSD maps were acquired in a FEG-SEM before and after annealing in order to see where and how the new grains appear. Nonoriented nucleation mechanisms are involved in both materials, and there is a strong correlation between the local deformation substructures and the recrystallization kinetics. Recrystallization is extremely fast in the areas where the deformation cells are small and highly misoriented, i.e. in the areas which underwent severe grain fragmentation. Twinning plays an important role for that purpose in the studied titanium sheet.

Abstract: Some ancient metallic art craft, utensils, silverware and weapons are externally undistinguishable from modern ones. Not only the general aspect and shape but also some uses have not changed through the ages. Moreover, when just some small pieces can be recovered from archaeological sites, the samples can not easily be ascribed to any known use and consequently identified. It is clear that mechanical processing has changed along history but frequently only a "microscopic" inspection can distinguish among different techniques. Some bronze samples have been collected from the Quarto d’Altino (Veneto) archaeological area in Italy (paleovenetian culture) and some model samples have been prepared by a modern artisan. The sample textures have been measured by X-ray Diffraction techniques. (111), (200) and (220) pole figures were used to calculate Orientation Distribution Functions and further recalculate pole figures and inverse pole figures. The results were compared with modern forging technology results. Textures are able to discern between hammering ancient techniques for sheet production and modern industrial rolling procedures. However, as it is demonstrated in the present work, forgery becomes difficult to detect if the goldsmith, properly warned, proceeds to erase the texture history with some hammering post-processing. The results of this contribution can offer to the archaeologists the opportunity to take into consideration the texture techniques in order to discuss the origin (culture) of the pieces and the characteristic mechanical process developed by the ancient artisan. Texture can also help the experts when discussing the originality of a certain piece keeping however in mind the cautions indicated in this publication.