The massive transformation in titanium aluminides: Initial stages of nucleation and growth

Rapid solidification by twin-anvil splat quenching captures the initial nucleation and growth of the α → γ _m massive transformation in titanium aluminides. Splat quenching Ti₅₂Al₄₈ and Ti₅₀Al₄₈Cr₂ from the liquid at slightly below the melting point produces an equiaxed α solidification structure. Solid-state cooling rates that approach 10⁶ K/s arrest the α → γ _m massive transformation with 1- to 5-μm-sized γ _m nuclei, especially in the Ti₅₀Al₄₈Cr₂ alloy. Classical massive-transformation heterogenous nucleation occurs at α:α grain boundaries with an orientation relationship of [111] _γ //[0001] _α and . The γ _m nucleus then grows into the adjacent α grain without the orientation relationship by forming an incoherent α:γ interface with {111} _γ facets. Orthogonal variants of the tetragonal c-axis in the γ _m product suggest that the massive transformation initially produces an fcc structure which subsequently orders into the L1₀ phase. Nucleation of γ _m is not only observed at α:α grain boundaries and triple points, but also within the α grains. The intragranular γ _m nucleation, which is believed to be heterogeneous, occurs with the same orientation relationship as for the intergranular nuclei. However, the intragrain nuclei do not form {111} _γ facets and retain a curved α:γ _m interface. Although analysis of the {111} _γ faceted growth using weak-beam dark-field (WBDF) imaging shows no evidence for any type of misfit-compensating dislocations, lattice imaging of the {111} _γ facets with high resolution transmission electron microscopy (HRTEM) reveals that the planar interface exhibits a slight curvature, produced by atomic steps of (111) planes. These experimental data have been used to estimate a ratio of ledge spacing (λ) to ledge height (h) for the {111} _γ facets as λ/h=41, which is similar to calculated values for a ledge growth mechanism of massive transformations in Cu-Zn and Ag-Cd alloys. This article is based on a presentation made at the symposium entitled “The Mechanisms of the Massive Transformation,” a part of the Fall 2000 TMS Meeting held October 16–19, 2000, in St. Louis, Missouri, under the auspices of the ASM Phase Transformations Committee.     

Early stages of recrystallization in nickel

This paper reports on an investigation into the early stages of recrystallization in nickel from the standpoint of the misorientation between newly formed recrystallized grains and the surrounding deformed microstructure. For the case of recrystallization “nuclei”(i.e., <l-μm diameter), half the sampled interfaces were close to low-gS coincident site lattice (CSL) misorientations, particularly gS = 3. Established new grains(i.e., >l-μm diameter) tended to be either not low-gS CSLs or less close to a low-gS CSL than for the “nucleus” case. The interpretation of these data is that the very earliest stage of recrystallization is facilitated at a boundary which is more mobile than average, often a characteristic of a low-gS CSL. After nucleation of recrystallization has been established, the important criterion for the recrystallization interface is a large capacity for dislocation absorption, which is associated with general boundaries rather than CSLs. The number of boundaries bordering the new grains and their geometrical characteristics were consistent with a model which relates sides per grain to grain boundary energy.     

Initial stages of bismuth oxide formation

Data on electron diffraction study of bismuth films during heating are presented for various partial oxygen pressures in the gas phase. The formation of bismuth oxide phases is completed in several seconds.     

Initial stages of recrystallization in aluminum of commercial purity

In commercial aluminum with a purity of 99.4 pct, the formation and growth of recrystallization nuclei were studied by techniques such asin-situ annealing in a high voltage electron microscope, transmission electron microscopy and light microscopy. Sample parameters were the initial grain size (370 and 19 microns) and the degree of deformation (50 and 90 pct reduction in thickness by cold-rolling). It was found that the initial grain boundaries and high angle boundaries within the original grains are preferential sites for recrystallization nuclei, and that the effect of such sites is enhanced by the FeAl₃ particles present in the commercial aluminum as impurities. The nucleation temperatures determined by high voltage electron microscopy and transmission electron microscopy decrease markedly when the initial grain size is decreased both after 50 and 90 pct cold rolling; a less pronounced temperature decrease is obtained by increasing the degree of deformation. The size of the recrystallization nuclei, the recrystallization temperature and the recrystallized grain size are reported for the four sample states, and finally the structural and kinetic observations are discussed.     

Morphological evolution during the early stages of aluminide coating growth on a single-crystal nickel superalloy surface

The (100) surface of a single-crystal Ni alloy was aluminized as a function of time to study the development of the resulting coating microstructure. A chemical vapor deposition (CVD) reactor, which was specially configured for short-term aluminizing experiments, was used to prepare coating specimens at 1150 °C. After 5 minutes, γ′-Ni₃Al particles ∼100 nm in size randomly nucleated on the alloy surface. Within 20 minutes, a coating layer consisting of preferentially oriented, columnar β-NiAl grains was formed with the segregation of refractory elements (i.e., Ta and W) from the alloy to the coating grain boundaries. The lateral growth of the columnar grains was observed to be relatively rapid for up to 45 minutes, but slowed considerably between 45 and 180 minutes. While the columnar nature of the coating did not change significantly after 20 minutes, the surface features continually evolved, with the appearance of a small amount of the γ′ phase, which coincided with the segregation of the refractory elements to the coating surface.     

Adhesion molecules in the nickel allergic reaction

Nickel is the major cause of allergic contact dermatitis, and to increase our understanding of this immune reaction we studied changes in the expression of adhesion molecules on mononuclear cells during nickel stimulation in vivo and in vitro. Nickel-induced lymphocyte cultures were used in vitro, the cells being examined with monoclonal antibodies (Mabs) and by flow cytometry. Mononuclear cells from skin biopsies of in vivo cutaneous nickel reactions were studied with Mabs and immunohistochemistry. The expression of adhesion molecules in vitro was differential: the number of cells carrying CD11c, CD29, CDw49b, CDw49d, CDw49e, CDw49f, CD54, CD56 and ELAM-1 being significantly overrepresented among the nickel-induced lymphoblasts whereas the number of blasts carrying CD44 was underrepresented and those of CD11a, CD18, CD58 and LAM-1 remained unchanged. CD4+ cells gained adhesion molecules during nickel-induced blast transformation whereas CD8+ cells lost most of their adhesion molecules. The in vivo results were in agreement with the in vitro ones except that CDw49b, CDw49f, CD56 and ELAM-1 could not be detected in a 96-hour nickel reaction in vivo. In conclusion, the nickel allergic reaction favors the expression of certain adhesion molecules, and this expression is induced on CD4+ cells while CD8+ cells tend to lose such molecules. The changes were more sensitively detected with the in vitro method.     

Phase relations of a silicide/silica reaction couple at 2273 K

The phase relations of a zirconium silicide/silica reaction couple have been investigated at 2273 K in air. After annealing times from 4 to 49 hours, the reaction couples, created by encapsulating zirconium disilicide in a quartz ampule, developed an interdiffusion zone and an inner core consisting of solidified zirconium silicide. The interdiffusion zone consisted of a silica layer, zirconia precipitates, and prior liquid silicide globules dispersed in prior liquid silica. Zirconia precipitates formed from the oxidation of the silicide melt between the protective silica layer and the two liquid regions of silicide and silica, as a result of oxygen diffusion. At 2273 K, the inner core consisted of liquid zirconium silicide (ZrSi)_L, although the microstructure analysis indicated formation of proeutectic ZrSi₂ and a eutectic microstructure of ZrSi₂ and Si upon solidification. A constant oxygen potential developed within the interdiffusion zone and protected the silicide from oxidation even after 49 hours at temperature.     

Initial stages of recrystallization in aluminium containing both large and small particles

The recrystallization behaviour has been studied in a dispersion strengthened aluminiumalumina alloy containing a bimodal distribution of fine alumina particles (<0.1 /μm) and coarse FeAl₃ particles (0.2–4 μm). The formation and growth of recrystallization nuclei were studied by in situ annealing in a high voltage electron microscope, conventional transmission electron microscopy and light microscopy. The parameters studied were the initial grain size and the degree of deformation (50 and 90% reduction in thickness by cold-rolling). It was found that recrystallization nuclei formed preferentially at the initial grain boundaries and at deformation bands; the effect of such sites was enhanced by the FeAl₃ particles which, however, in isolation were not strong nucleation sites. The recrystallization kinetics in the aluminium-alumina alloy were retarded or accelerated due to the presence of alumina or FeAl₃ particles respectively. However, the FeAl₃ particles did not lead to a significant grain refinement ; the recrystallized grain size of the aluminium-alumina alloy was large. Finally, the structural and kinetic observations are discussed and related to results from a previous study of the initial stages of recrystallization of aluminium of commercial purity.     

Microstructure and mechanical behavior of reaction hot-pressed titanium silicide and titanium silicide-based alloys and composites

Titanium silicide (Ti₅Si₃) and its composites show promise for applications at temperatures higher than 1000 °C. Dense Ti₅Si₃ was processed by reaction hot pressing of a TiH₂/Si powder mixture, which involved decomposition of TiH₂ into Ti and H₂ at around 800 °C, a chemical reaction between Ti and liquid Si at 1500 °C forming Ti₅Si₃ in situ, and densification under pressure. The use of fine TiH₂ particles led to the formation of a relatively fine-grained microstructure with fewer microcracks and higher hardness and fracture toughness values than those expected for a coarse-grained Ti₅Si₃. The addition of 8 wt pct Al as an alloying element led to the formation of Al_0.67Si_0.08Ti_0.25 and Al₂O₃ in situ and a solid solution of Al in Ti₅Si₃. Both alloying with Al and the addition of TiC as a reinforcement phase improved the room-temperature fracture toughness. Fracture toughness measurements were performed by three-point bend testing of single-edge notch bend (SENB) specimens, as well as by indentation techniques using different models, and the data have been compared. The role of different operating toughening mechanisms such as crack deflection, bridging, branching, and energy dissipation through microcracracking have been examined. The investigation has also shown that Ti₅Si₃ maintains a high yield strength value up to 1200 °C.     

Interdiffusion in thin conductor films — chromium/gold,nickel/gold and chromium silicide/gold

The diffusion of chromium, nickel and chromium silicide into gold has been studied using thin film techniques. Electrical measurements of film diffusion couples were made at several temperatures and these data are related to the interdiffusion of the metals. It was found that the activation energy for resistance changes in the films is somewhat greater than one-half of that for bulk diffusion indicating the predominance of a grain boundary diffusion mechanism; however, a drastically lowered temperature coefficient of resistance also indicates some interdiffusion by lattice diffusion. Under the conditions of these experiments, the chromium, nickel, and chromium silicide diffuse from the substrate through the gold to the outer surface of the gold where they are oxidized.     

Initial stages of the formation of lead oxide phases

Metal lead films are studied by electron diffraction during heating depending on temperature T and oxygen partial pressure p _O2 in the gas phase. It is found that the formation of oxide phases begins from the α-PbO₂ modification. As p _O2 increases or T increases, intermediate oxides nPbO₂ · mPbO and Pb₃O₄ form and then transform into the β-PbO modification.     

1—（5—硝基—2—吡啶偶氮）—2，7—萘二酚与镍的显色反应及其分析应用

研究了1-（5-硝基-2-吡啶偶氮）-2,7-萘二酚与镍的显色反应。在Tween-80存在下,试剂与镍于pH5.5～6.5的缓冲溶液介质中形成稳定的蓝色络合物,表观摩尔吸光系数ε650＝1.02×105,镍的浓度在0～4μg／10mL范围内服从比尔定律。所拟方法用于钢样及矿样中微量镍的测定,结果满意。