The cellular interlamellar and growth-front interphase boundaries in Cu-3 Wt pct Ti

Examination of the cellular colony interlamellar and growth-front interphase boundaries in Cu-3 wt pct Ti reveals an influence of crystallography at both of these interface types. Analysis of the interlamellar boundaries demonstrates that different arrangements of interphase misfit-compensating defects exist and combinations of misfit dislocations (MDs), structural ledges (SLs), or direction steps (DSs) were observed to dominate strain reduction between lamellae, even within the same colony. Detailed analysis also demonstrated that the actual interlamellar orientation relationship (OR) is (111) _α ‖ (010) _β with [-101] _α ‖ [501] _β , which is 0.28 deg in misorientation from the reported OR. The effect of crystallography was also apparent at the cellular growth front, as evidenced by the misfit-compensating structure observed with transmission electron microscopy (TEM) at the grain-boundary segments and the sharp faceting of all β precipitate-growth interfaces. This article is based on a presentation in the symposium “Interfacial Dislocations: Symposium in Honor of J.H. van der Merwe on the 50th Anniversary of His Discovery,” as part of the 2000 TMS Fall Meeting, October 11–12, 2000, in St. Louis, Missouri, sponsored under the auspices of ASM International, Materials Science Critical Technology, Sector, Structures.     

Comparison of orthorhombic and alpha-two titanium aluminides as matrices for continuous SiC-reinforced composites

The attributes of an orthorhombic Ti aluminide alloy, Ti-21Al-22Nb (at. pct), and an alpha-two Ti aluminide alloy, Ti-24Al-11Nb (at. pct), for use as a matrix with continuous SiC (SCS-6) fiber reinforcement have been compared. Foil-fiber-foil processing was used to produce both unreinforced (“neat”) and unidirectional “SCS-6” reinforced panels. Microstructure of the Ti-24A1-11Nb matrix consisted of ordered Ti₃Al (α ₂) + disordered beta(β), while the Ti-21 Al-22Nb matrix contained three phases: α₂, ordered beta (β ₀), and ordered orthorhombic(O). Fiber/ matrix interface reaction zone growth kinetics at 982 °C were examined for each composite system. Although both systems exhibited similar interface reaction products(i.e., mixed Ti carbides, silicides, and Ti-Al carbides), growth kinetics in theα ₂ +β matrix composite were much more rapid than in theO +β ₀ +α ₂ matrix composite. Additionally, interfacial reaction in theα ₂ +β} composite resulted in a relatively large brittle matrix zone, depleted of beta phase, which was not present in theO +β ₀+α ₂ matrix composite. Mechanical property measurements included room and elevated temperature tensile, thermal stability, thermal fatigue, thermo-mechanical fatigue (TMF), and creep. The three-phase orthorhombic-based alloy outperformed the α₂+β alloy in all of these mechanical behavioral areas, on both an absolute and a specific(i.e., density corrected) basis.     

Static and in-situ high-resolution transmission electron microscopy investigations of the atomic structure and dynamics of massive transformation interfaces in a Ti-Al alloy

Static and in-situ high-resolution transmission electron microscopy (HRTEM) and three-dimensional near-coincident-site (NCS) atomic modeling were used to determine the atomic structure, growth mechanisms, and dynamics of massive transformation interfaces in a Ti-Al alloy. Results from these experiments show that massive γ _M grains often have an irrational orientation relationship (OR) and structurally incommensurate (incoherent) interface with respect to the parent α (retained α ₂) phase, although the degree of commensurability varies, depending on the orientation and planarity of a particular α ₂/γ _M interface. In-situ HRTEM observations of several α ₂/γ _M interfaces during growth revealed evidence of both continuous and stepwise mechanisms of interface motion, again, depending on the orientation of the interface plane for a given OR between the two phases. These findings are discussed within the contex of the massive transformation and the motion of interphase boundaries in solids. 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.     

The microstructural details of β-Sn precipitation in a 5Sn-95Pb solder alloy

Solders of Pb-rich compositions, such as 5Sn-95Pb, are commonly used in electronic packaging applications, and this demanding use necessitates that the microstructure-property-processing relationships of the solder be understood fully. In this study, the microstructure of 5Sn-95Pb solder was characterized using a variety of metallographic techniques. The effect of cooling rate on the precipitation of β-Sn from supersaturated α-Pb was determined. On slow cooling, β-Sn precipitates discontinuously with resultant β-Sn lamella alternating with the α-Pb. Upon rapid cooling, the β-Sn precipitates with a nominally homogeneous distribution. These discrete precipitates were found to have a platelike shape with a (111)_Pb habit plane and an orientation relationship of (111)_Pb‖(010)_Sn and [011]_Pb‖ [001]_Sn. Regions of the β-Sn precipitates that curved away from this habit plane formed ledges. Upon heating, the precipitates were found to dissolvevia a ledge mechanism. Prolonged aging of both the fast and slow cooled 5Sn-95Pb resulted in a coarsening of the β-Sn precipitates with a resultant decrease in strength. Furthermore, the strength of the aged alloy was observed to be independent of cooling rate. D.R. FREAR, formerly Graduate Research Assistant, Lawrence Berkeley Laboratory     

The Stress-induced omega phase transformation in Ti-V alloys

The deformation behavior of bcc (β) Ti-V single crystals containing the athermal ω phase was studied using tensile tests, optical and electron microscopy and X-ray and electron diffraction. The stress-strain curves were characterized by a marked yield point and an essentially zero work hardening rate. The deformation structures consisted of wavy slip and stress-induced plates. Both X-ray and electron diffraction showed that the stress-induced plates consist of one variant of the omega phase, with the orientation relation to the parent? phase close to (0001)ω ‖(001)ω; (•1120)ω ‖(110)β. The habit plane was of the type 554. A small amount of ά (hcp) phase was present within the ω plate, with orientation relation to the β phase close to the Burgers relationshipi.e. (0001)_ά, ‖(110)β; [•1120]ά ‖[•111]_β. It is suggested that the ω phase may be an intermediate structure in the stress-induced bcc → hep transformation. R. R. AHRENS, formerly Research Assistant in the Department of Materials Science and Engineering, Cornell University     

Deformation of an alloy with a lamellar microstructure: experimental behavior of individual widmanstatten colonies of an α-β titanium alloy

The deformation behavior of individual Widmanstatten colonies comprised of aligned lamellae of ductile phases has been investigated. Based on the α β Ti alloy, Ti-8Al-lMo-lV, this study shows the existence of a large (>2X) variation in the critical resolved shear stress for yielding of individual colonies. Schmid’s Law is not obeyed except for prism slip parallel to the β lamellae. In addition, colonies with a high yield stress exhibit a high work hardening rate and fine, uniform slip. This behavior appears to be independent of the α-phase slip system (basal, prism, or pyramidal) and of the microstructure (α βvs α α′ (martensite)). The experimental behavior is correlated to several colony orientation parameters including the stress axis, the slip plane, the slip direction and the orientation of the α β interface. The yield stress of a colony is found to increase as the slip direction of the dominant macroscopic slip plane approaches normality to the α β interface. These results indicate that the macroscopic flow behavior of colonies comprised of ductile lamellae depends on the ability of a slip system, once activated in the softer phase, to shear through the harder phase. The data also indicate that the interaction stresses at the phase interfaces are not a principal factor controlling macroscopic yielding in α β Ti alloys. Finally, the alignment of a slip system in the α-phase with a potential slip system in the β-phase lamellae does not appear to affect the yield stress strongly. formerly Graduate Students, Department of Metallurgical Engineering, Michigan Technological University     

Surface and Interface Energies of Complex Crystal Structure Aluminum Magnesium Alloys

Alloying Al with Mg can improve its structural properties but also can lead to the formation of grain-boundary precipitates of β-Mg₂Al₃ that lead to failure by intergranular fracture and corrosion. Simulating the properties of the β phase is difficult because it has a complex structure with more than 1000 atoms per unit cell. We approximate the experimental β structure by the β′ structure, which has about 300 atoms per unit cell, and we compute the fracture behavior of the material from density functional theory calculations of relevant surface and interface energies. We report also on experimental measurements of the orientation and fracture properties of the α-Al(Mg)–β-Mg₂Al₃ interface and compare them with the atomistic simulations. We have computed the surface energy of face-centered cubic α-Al with up to 10 at. pct Mg, as well as the decohesion energy of β′-Mg₂Al₃ and the interfacial decohesion energy between β′-Mg₂Al₃ and pure α-Al with geometry similar to that observed experimentally. We find that the β′-Mg₂Al₃ decohesion energy is nearly isotropic and is lower than the pure Al surface energy and the α-Al–β′-Mg₂Al₃ interface decohesion energy. This result is consistent with the experimental observations of fracture within the β phase rather than at the α-Al(Mg)–β-Mg₂Al₃ interface or within the α-Al(Mg) phase.     

Beta decomposition processes in Hf-Rich Hf-Nb alloys

The decomposition of the bcc β-phase by both athermal and isothermal processes has been investigated in Hf-rich Hf-Nb alloys. An all β-phase structure is retained in chillcast alloys containing 30 to 50 at \ pct Nb (Cb), although electron diffraction streaking effects and the behavior of the temperature coefficient of electrical resistivity indicate the presence of a bcc lattice instability similar to that reported in solute lean Ti and Zr alloys. Aging a Hf_0.65Nb_0.35 alloy at 400 and 600°C resulted in the direct precipitation of a fine dispersion of a-phase needles; this morphology differs from the discs of transition α (α_t) which Carpenteret al observed in Nb-rich Nb_0.68Hf_0.32. During continued aging, the needles grow selectively to form colonies or groups of needles in which both the individual needles and the groups of needles have major axes aligned along 〈110〉_β type directions. The initial a-phase particles exhibit the Burgers orientation relationship with the parent matrix; continued aging changes the electron diffraction patterns in a way that is similar to that observed in aged Ti-Mo and Ti-Mo-Al alloys where they were attributed to the α- phase having a different crystallographic relationship to the β-phase (Type 2 a-phase). The observed changes in the electron diffraction patterns of aged Hf_0.65Nb_0.35 cannot be described as resulting from strained Burgers α-phase.     

Electron microscopy study of the early stage of the martensitic transformation in a U(Ga) alloy

Transmission electron microscopy was used to study the early stages of development of the isothermal martensitic α′ platelets within the metastableβ matrix in a U-1.6 at. pct Ga alloy. Analysis of the electron diffraction patterns allowed determination of the orientation relationship (0Ī1)_β ∥ 〈001〉_α′ and [100]_β′ oriented at 12 deg with respect to [010]_α. The habit plane, as determine by trace analysis, is the (411)_β plane.     

Microstructure Evolution during Friction Stir Welding of Mill-Annealed Ti-6Al-4V

In this study, mill-annealed Ti-6Al-4V plates were successfully friction stir welded over a wide range of processing parameters using a tungsten-1 pct La₂O₃ tool. Two K-type thermocouples embedded in the tool indicated that approximately 25 pct of the heat generated during welding was transferred out of the workpiece and into the tool. The thermocouple data, combined with observations of the microstructure, indicated that the stir zone of all welds exceeded the β transus. The microstructure and texture of two representative welds made just above and high above the β transus were investigated with scanning electron microscopy and electron backscatter diffraction (EBSD). The β phase orientations were reconstructed with a fully automated technique from the as-collected α phase data through knowledge of the Burgers orientation relationship. The results suggest that the fine β grains in the stir zone are formed from the base material ahead of the advancing tool by dissolution of secondary and primary α phase, and there is no further recrystallization. These grains subsequently deform by slip and rotate toward the orientations that are most stable with respect to the shear deformation induced by the tool. In the highest temperature weld, diffusion tool wear in the form of periodically spaced bands provided an internal marker of the tool/workpiece interface during welding. The flow patterns evident within the tungsten-enriched bands suggest that flow is considerably more chaotic on the advancing side than in the central stir zone.     

Microstructural characterisation of near-α titanium alloy Ti-6Al-4Sn-4Zr-0.70Nb-0.50Mo-0.40Si

Microstructural stability in the near-α titanium alloy (alloy 834) containing Ti-6Al-4Sn-4Zr-0.70Nb-0.50Mo-0.40Si (in weight percent), in the β and(α + β) solution-treated and quenched conditions, has been investigated. The β transus for this alloy is approximately 1333 K. Solution treatment in the β phase field at 1353 K followed by quenching in water at room temperature resulted in the formation of α′ martensite platelets with high dislocation density and stacking faults. Thin films of β are found to be sandwiched between interface phases, which, in turn, are sandwiched at the interplatelet boundaries of lath martensite. The interface phase is a subject of much controversy in the literature. Solution treatment at 1303 K in the(α + β) phase field followed by quenching in water at room temperature resulted in the near-equiaxed primary α and transformed β. Both the β and(α + β) solution-treated specimens were aged in the temperature range of 873 to 973 K. While aging the —treated specimen at 973 K,(α + β)-treated specimen, even at a lower temperature of 873 K for 24 hours, caused precipitation of suicides predominantly at the interplatelet boundaries of martensite laths. Electron diffraction analysis confirmed them to be hexagonal suicide S₂ witha = 0.70₂ nm andc = 0.36₈ nm. The above difference in the precipitation could be attributed to the partitioning of a higher amount of β- stabilizing elements as well as silicide-forming elements to the transformed β in the(α + β) solution-treated condition. However, ordering of theα′ phase was observed under all of the aging conditions studied. The ordered domains were due to the longer aging times, which cause local increases in the level of theα-stabilizing elements. Formerly Research Associate, Department of Metallurgical Engineering, Baranas Hindu University.     

Crystallographic Texture and Volume Fraction of <Emphasis Type="Italic">α</Emphasis> and <Emphasis Type="Italic">β</Emphasis> Phases in Zr-2.5Nb Pressure Tube Material During Heating and Cooling

The phase transformations in an as-received Zr-2.5Nb pressure tube material were characterized in detail by neutron diffraction. The texture and volume fraction of α and β phases were measured on heating at eight different temperatures 373 K to 1323 K (100 °C to 1050 °C) traversing across the α/(α + β) and (α + β)/β solvus lines, and also upon cooling at 1173 K and 823 K (900 °C and 550 °C). The results indicate that the α-phase texture is quite stable, with little change in the {0002} and { 11[`2]0 } \left\{ {11\bar{2}0} \right\} pole figures during heating to 1123 K (850 °C). The β-phase volume fraction increased while a slight change in texture was observed until heating reached 973 K (700 °C). On further heating to 1173 K (900 °C), there appears a previously unobserved α-phase texture component due to coarsening of the prior primary α grains; meanwhile the transformed β-phase texture evolved markedly. At 1323 K (1050 °C), the α phase disappeared with only 100 pct β phase remaining but with a different texture than that observed at lower temperatures. On cooling from the full β-phase regime, a different cooldown transformed α-phase texture was observed, with no resemblance of the original texture observed at 373 K (100 °C). The transformed α-phase texture shows that the {0002} plane normals are within the radial-longitudinal plane of the pressure tube following the Burgers orientation relationship of (110)<sub>bcc</sub>//(0002)<sub>hcp</sub> and [[`1]11]_\textbcc //[11[`2]0]_\texthcp [\bar{1}11]_{\text{bcc}} //[11\bar{2}0]_{\text{hcp}} with a memory of the precursor texture of the primary α grains observed on heating at 1173 K (900 °C).     

Orientation and structure of planar facets on the massive phase γ m in a near-TiAl alloy

The orientation and structure of planar facets on the γ _m massive phase formed in a Ti-46.5 at. pct Al alloy have been characterized using a combination of transmission electron microscopy (TEM) and electron diffraction. The planar γ _m /α ₂ interfaces are irrational with respect to both the α ₂ matrix phase and the γ _m phase, and there is neither evidence of a rational orientation relationship across such facets, nor resolution of a linear defect structure within the interface planes in conventional electron diffraction patterns and amplitude-contrast images, respectively. However, when imaged parallel to a particular direction in the interface, these irrationally oriented interfaces are invariably parallel to the Moiré plane, which is defined by the intersection between two sets of closely-packed planes in the γ _m and α ₂ phases. The relationship is such that there is an effective continuity of these lattice planes across the interface and a one-dimensional coherency within the planar interface. This is interpreted to imply that such interface facets adopt a configuration of reduced energy and that they are not random and fully incoherent, as often described. It is suggested that these planar facets may migrate in a glissile manner normal to the interface plane by nucleation and rapid lateral movement within the interface plane of interfacial defects that have the form of Moiré ledges, which are defined by the spacing of the Moiré pattern that may be formed by overlap of the relevant crystal planes across the interface. 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.     

Reaction diffusion and phase equilibria in the V-N system

The formation of phase bands in in situ diffusion couples of the V-N system was studied by the reaction of vanadium sheet with pure nitrogen within the temperature range 1100 °C to 1700 °C and the nitrogen pressure range 2 to 24 bar. Under these conditions, phase bands of β-V₂N and δ-VN_1−x develop. The morphology of the β-V₂N/α-V(N) interface depends on the saturation state of the α-V(N) core. If the nitrogen content in α-V(N) is high, the interface has a jagged appearance, whereas at low nitrogen contents of the α-V(N) phase, the interface is planar. Electron probe microanalysis (EPMA) was used to measure the diffusion profiles within the couples. The homogeneity regions of the nitride phases were established and the phase diagram accordingly corrected. From the growth rates of the phase bands, the mean composition-independent nitrogen diffusivities in β-V₂N and δ-VN_1−x were derived. These diffusivities follow an Arrhenius equation with activation energies of 2.92 (β-V₂N) and 2.93 eV (δ-VN_1−x ). By using δ-VN_1−x as a starting material and a low nitrogen pressure during annealing, it could be shown that the direction of nitrogen diffusion can be reversed, i.e., β-V₂N is formed on the surface of the couple as a result of out-diffusion of nitrogen.     

Reaction diffusion and phase equilibria in the V-N system

The formation of phase bands in in situ diffusion couples of the V-N system was studied by the reaction of vanadium sheet with pure nitrogen within the temperature range 1100 °C to 1700 °C and the nitrogen pressure range 2 to 24 bar. Under these conditions, phase bands of β-V₂N and δ-VN_1−x develop. The morphology of the β-V₂N/α-V(N) interface depends on the saturation state of the α-V(N) core. If the nitrogen content in α-V(N) is high, the interface has a jagged appearance, whereas at low nitrogen contents of the α-V(N) phase, the interface is planar. Electron probe microanalysis (EPMA) was used to measure the diffusion profiles within the couples. The homogeneity regions of the nitride phases were established and the phase diagram accordingly corrected. From the growth rates of the phase bands, the mean composition-independent nitrogen diffusivities in β-V₂N and β-VN_1−x were derived. These diffusivities follow an Arrhenius equation with activation energies of 2.92 (β-V₂N) and 2.93 eV (δ-VN_1−x ). By using δ-VN_1−x as a starting material and a low nitrogen pressure during annealing, it could be shown that the direction of nitrogen diffusion can be reversed, i.e., β-V₂N is formed on the surface of the couple as a result of out-diffusion of nitrogen.     

Role of cold work and SiC reinforcements on the β′/β precipitation in Al-10 pct Mg alloy

Elevated temperature (above 100 °C) precipitation behaviors were studied in A1-10 wt pct Mg alloy and the same alloy reinforced with SiC particles through electrical resistivity, hardness, differential scanning calorimetry (DSC), and microscopy. Two distinct hardness peaks/resistivity drops, as associated with two precipitation events, were identified: (1) α (solid solution) → β′ (metastable hex precipitate) → β (Al₃Mg₂, stable complex cubic precipitate); and (2) α → β. Equilibrium β precipitates, transformed from metastable β′, were observed to possess a wide variet of orientation relationships with the matrix and were often observed to be twinned. A more restricted orientation relationship (only three variants) between β and matrix was observed in direct decomposition of α to β, and β precipitates, within these orientation relationships, were never observed to be twinned. In a predominantly binary Al-Mg system, direct precipitation of β was observed to dominate. However, the presence of trace amounts of boron nitride and/or boron (or a large supply of matrix dislocations) either from cold work, or (as in case of composites) from the thermal mismatch between the SiC and Al matrix, produced both precipitation events with event 1 dominant.     

Correlation between Microstructures and Oxidation Resistance in Zr-Nb-Ti Alloys

Oxidation behavior of Zr-10Nb-10Ti and Zr-10Nb-20Ti (compositions are in atomic percent) alloys has been investigated in air between 300 °C and 700 °C. Higher Ti content in the alloy enhances the oxidation resistance. The calculated isotherms by PANDAT[1,2] show that 20Ti enters a three-phase (αZr-hcp, βNb-bcc, and βZr-bcc) region at 500 °C, while 10Ti alloy continues to be a two-phase (αZr and βNb) alloy until 550 °C and then enters the three-phase (αZr, βNb, and βZr) region. Both alloys have a single-phase βZr solid solution at 700 °C, which is detrimental for the oxidation resistance. The βNb phase greatly contributes to the oxidation resistance in these two alloys. The common oxidation products have been identified as TiO₂, ZrO₂, and Nb₂O₅. Both alloys suffer from pest oxidation at temperatures between 500 °C and 550 °C, respectively (20Ti and 10Ti), up to 700 °C. X-ray diffraction (XRD) indicates strong peaks for monoclinic structure of ZrO₂ at temperatures above 600 °C.     

Microstructural investigation on the precipitation of α″ nitrides and α″ → γ′ nitride transformation in ion-nitrided pure iron

The precipitated characteristics of α″-Fe₁₆N₂ nitrides in the diffusion layer of ion-nitrided pure iron were investigated with transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). Three sets of α″ nitrides, whose habit planes are (100)_α,(010)_α,(001)_α, respectively, do not precipitate simultaneously from the diffusion layer, which is different from the normal homogeneous precipitation in Fe-N alloys. Unlike the typical disc-shaped morphology reported widely, the α″ nitrides in the diffusion layer appear as ribbonlike slices. They grow on {001}_α matrix planes with a parallel orientation relationship, and the direction of their length is parallel to the <110>_α direction. The interface between the α″ nitride and α matrix and a 7 deg [111]/(112) low-angle-tilt grain boundary in the α″ nitride were examined with HREM. The distributions of dislocations at the interface and the grain boundary were investigated. During microstructural examination, it was observed that a γ′-Fe₄N nitride could grow on an α″ nitride directly. The orientation relationship during the α″ → γ′ nitride transformation was determined as to be (001)_γ//(110)_α,[110]_γ//[111]_α.     

Effect of microstructure variations on the formation of deformation-induced martensite and associated tensile properties in a β metastable Ti alloy

This article focuses on the effect of the microstructure on the activity of different deformation mechanisms and the resulting mechanical behavior of a metastable β Ti alloy (β-Cez). Various types of microstructures were produced, with given volume fractions of β phase (100 or 90 pct). These microstructures differed in the size of their β grains as well as in the distribution, shape, and size of the primary α particles. A statistical approach was also developed to characterize small variations in chemistry of the β phase between the various microstructures. It is shown that, even for similar volume fractions of β phase, changes in the microstructure strongly affect the mechanical response of the alloy. The mechanical response is controlled by the interplay between the two deformation modes operating in this alloy: formation of α″ deformation-induced martensite and activation of slip. The easier formation of stress-induced martensite leads to lower apparent yield stresses and a better work-hardening response. On the contrary, very limited work hardening is obtained when slip is activated solely. The differences in the ability of the martensitic transformation to occur can be understood by considering the effect on M _s and T _o of both the chemistry of the β phase and of constraining effects due to grain sizes and dislocations.