Evolution of boride morphologies in TiAl-B alloys

The solidification of γ-TiAl alloys with relatively low (<2 at. pct) additions of boron is discussed. Binary Ti-Al alloys containing 49 to 52 at. pct Al form primary α-(Ti) dendrites from the melt, which are subsequently surrounded by γ segregate as the system goes through the peritectic reactionL + α →γ. Alloys between 45 and 49 at. pct Al go through a double peritectic cascade, forming primary β-(Ti) surrounded by α-(Ti) and eventually by γ in the interdendritic spaces. Boron additions to these binary alloys do not change the basic solidifi-cation sequence of the matrix but introduce the refractory compound TiB₂ in a variety of mor-phologies. The boride develops as highly convoluted flakes in the leaner alloys, but needles, plates, and equiaxed particles gradually appear as the B content increases above ∼1 at. pct. Increasing the solidification rate initially promotes the formation of flakes over plates/needles and ultimately gives way to very fine equiaxed TiB₂ particles in the interdendritic spaces of the metallic matrix. Furthermore, the primary phase selection in the 49 to 52 at. pct Al range changes from α-(Ti) to β-(Ti) at supercoolings of the order of 200 K. The different boride morphologies are fully characterized, and their evolution is rationalized in terms of differences in their nucleation and growth behavior and their relationship to the solidification of the inter-metallic matrix. Formerly Research Assistant, University of California-Santa Barbara (UCSB) Formerly Professor of Materials and Dean of the College of Engineering at UCSB     

Thermal equilibria and mechanical stability of Ti3 Al phase in Ti-Mo-Al alloys

The phase diagram of the isopleth section of the Ti-7 at. pct Mo-Al system has been improved and expanded to include alloys up to 25 at. pct aluminum. The mechanical and thermal stability of alloys aged in the two-phase region, β +Ti₃Al, was correlated to the microstructure. X-ray rocking-curve studies of the polycrystalline specimens showed that after 2 pct deformation of a Ti-7 Mo-16 Al alloy theβ matrix became preferentially plastically deformed, while the Ti₃Al particles functioned as hard particles undergoing little lattice distortions. Formerly a Graduate Student. This paper is based on a thesis submitted by T. Hamajima in partial fulfillment of the requirements for a Ph.D. degree from Rutgers University.     

Synthesis of ductile titanium-titanium boride (Ti-TiB) composites with a beta-titanium matrix: The nature of TiB formation and composite properties

The study focused on the in-situ synthesis of titanium (Ti)-titanium boride (TiB) composites with β phase in the matrix by reaction sintering of TiB₂ with Ti and alloying element powders. The goal was to examine the nature of TiB whisker formation in three different kinds of powder mixtures: (1) β-Ti alloy powders and TiB₂; (2) α-Ti powder, a master alloy (Fe-Mo) powder containing the β-stabilizing elements, and TiB₂; and (3) α-Ti powder, a β-stabilizing elemental powder (Mo or Nb), and TiB₂. The effects of powder packing and the relative locations of powder particles on the morphological changes in TiB whisker formation and their growth were studied at processing temperatures ranging from 1100°C to 1300°C. The morphology, size, and distribution of whiskers were found to be influenced by the powder-packing conditions. A large particle-size ratio in bimodally packed mixtures led to the formation of a TiB monolithic layer around β grains. With a relatively finer starting powder, smaller size ratio, and trimodal packing arrangement, the TiB whiskers were found to be distributed more homogeneously in the matrix. The study also used the X-ray direct comparison method and the structure factor for the β phase to determine the volume fraction of TiB phase from X-ray data. Tensile tests and fractographic investigations were carried out on selected composites. The evolution of the composite microstructure, the influence of powder-packing variables, and the morphology and growth of TiB whiskers and their effect on mechanical properties are discussed.     

Abnormal growth of TiB<Subscript>2</Subscript> crystals during sintering of TiB<Subscript>2</Subscript>-Ni<Subscript>3</Subscript>(Al,Ti) cemented borides

TiB₂-Ni₃(Al,Ti) cermets present both normal and abnormal growth of faceted titanium diboride (TiB₂) grains during liquid-phase sintering. Abnormal grain growth (AGG) is preferentially found at high sintering temperatures in specimens processed from powder mixtures with a wide particle size distribution. The WC additions to the initial powder mixtures have proved efficient in reducing the number and size of these large TiB₂ grains. However, the sinterability of these materials is dramatically reduced, which suggests that TiB₂ AGG control is obtained by decreasing TiB₂ dissolution kinetics in the liquid phase. On the other hand, an alternative method based on intensive powder milling not only reduces TiB₂ AGG but also the porosity levels obtained by previous powder processing routes. TiB₂ cermets produced by aggressive milling present a higher amount of alumina particles in the matrix after sintering, which, in addition, appear more homogeneously dispersed in the microstructure. The distortion produced by these particles on the facets of TiB₂ growing grains suggests a possible dragging effect responsible for the AGG reduction found in these cermets. Moreover, aggressive milling removes large TiB₂ particles from the powder mixtures, which could act as seeds for TiB₂ uncontrolled growth. TiB₂-Ni₃(Al,Ti) cermets obtained by intensive milling combine hardness over 20 GPa with K _IC of about 10 MPa √m, data clearly out of the range covered so far by other TiB₂-based materials.     

Development of TiB<Subscript>2</Subscript> reinforced in-situ Ti aluminide matrix composite through reaction synthesis

TiB₂ reinforced in-situ titanium aluminide matrix composite was made through reaction synthesis process using high purity elemental powders of Ti, Al, Cr, Nb and B. XRD of the synthesized block showed presence of mainly Al₃Ti and TiB₂ phases. To obtain γ Ti aluminide based matrix, the material was homogenized in two phase region (α₂+γ). Presence of γ phase matrix alongwith α₂ was confirmed through XRD, SEM and TEM. Uniform distribution of TiB₂ phase was confirmed through elemental mapping and by analyzing specimens of different locations. Differential scanning calorimetry of powder mixture showed presence of endothermic peak for Al melting and exothermic peak of Ti aluminide and TiB₂ formation.     

The forming of Zr3Al-base alloys

Tubes have been formed from the intermediate phase Zr₃Al (L1₂ type) which is of potential use as a structural element in thermal nuclear power reactors. The procedure is to extrude Zr/Zr₂Al two-phase ingots at temperatures above 1270 K in theβ Zr + Zr₂Al two-phase field and then transform the extruded product at lower temperatures to Zr₃Al via the peritectoid transformation Zr + Zr₂Al → Zr₃Al. For small tubes (⪝ 3.2 cm OD) the extrusion constants at 1375 and 1425 K, respectively, are ≃ 400 and ≃ 300 MN/m² for the conditions chosen. The experimental extrusions indicate no fundamental barriers to forming pressure tubes from Zr-Al alloys containing 7.6 to 9.0 wt pct Al.     

Microstructure and phase relations for Ti-Mo-Al alloys

The influence of aluminum additions to a Ti-7 at. pet Mo alloy on the phase equilibria was investigated. The microstructures of the alloys, Ti-7 pct Mo-7 pct Al and Ti-7 pct Mo-16 pct Al, were determined by light and electron microscopy. It was found that with increasing aluminum concentration the formation of the metastable w phase was suppressed. In the Ti-7 pct Mo-16 pct Al alloy the β phase decomposed upon quenching by precipitating coherent, ordered particles having a B2 type of crystal structure (β₂). At low temperatures the equilibrium phases for this alloy were β + α+ β ₂, whereas at high temperature (850° to 950°C) the Ti₃Al phase was in two-phase equilibrium with the β phase. The four-phase equilibrium which exists at a temperature of about 550°C involves the reaction β + Ti₃Al ⇌ α + β₂. G. LUETJERING, formerly Staff Member Materials Research Center, Allied Chemical Corp., Morristown, N. J.,     

Importance of slip mode for dispersion-hardened β-titanium alloys

The mechanical properties of Ti-7 Mo-7 Al and Ti-7 Mo-16 Al (in at. pct) were correlated to the microstructure. The mechanical properties of the alloy with low aluminum content, consisting of α+ β phases, were dependent on the size of the α particles. Although the α phase is softer than the β phase, the small α particles, upon plastic deformation of the alloy, functioned as typical hard agents in a dispersion-hardened system and the volume fraction of the particles controlled the macroscopic ductility. A rapid strain-hardening behavior of the small α particles seemed to be responsible for this effect. Large α particles behaved like soft, incoherent particles, the volume fraction having little effect on the inherent ductility of the alloy. The two phase (β+ Ti₃Al) microstructure of the alloy with high aluminum content resulting from high temperature aging to 900°C exhibited a yield stress of 130 ksi and an elongation to fracture of 5 pct. The ductility of this microstructure was controlled by the volume fraction of the Ti₃Al particles inducing homogeneous slip. The favorable ductility properties of the microstructures with low Ti₃Al volume fraction were lost if the slip mode was changed from homogeneous slip to planar slip. Formerly Staff Member, Materials Research Center, Allied Chemical Corp., Morristown, N. J.     

Microstructural evolution and mechanical properties of AC8A/Al2O3 short-fiber composites after thermal exposure at 150 °C to 350 °C

The microstructural evolution and mechanical properties of an AC8A/12 vol Pct A1₂O₃ (sf) composite fabricated by squeeze casting were characterized. Thermal treatments included the normal T6 temper and thermal exposure at 150 °C, 250 °C, 300 °C, and 350 °C for 400 hours. The predominant strengthening phase in the matrix appeared to be β′ (Mg₂Si) needles. Bulk pure Si particles and dendrites were commonly seen. Large particles, termed asB-type phase, might include hexagonal Al₃(Ni, Cu, Fe, Si, Mg)₂ and orthorhombic Al₃(Ni, Cu, Fe, Si, Mg) phases. Both the Si andB dispersoids were not obviously affected by artificial aging at 150 °C to 350 °C. In certain cases, large cubic β (Mg₂Si) particles, hexagonalQ′ orQ (Al₄Cu₂Mg₈Si₇) precipitates, and numerous small Al particles inside Si dispersoids were also seen. No interfacial reaction product was observed along the fiber/ matrix interface even after long exposure at 350 °C. Amorphous SiO₂ gels, which were used as a binder during fabrication, were occasionally observed. The tensile and fatigue behavior of the AC8A alloys and composites after the preceding thermal exposures were evaluated over the temperature range of 25 °C to 350 °C. The composites showed similar strength as the matrix alloy at room temperature but exhibited higher strength at temperatures above 250 °C, with the sacrifice of the lower ductility. The strength levels of both the alloys and composites were significantly reduced after long thermal exposure, especially for temperatures higher than 250 °C. The loss of strength after long-term exposure at elevated temperatures may be attributed to age-softening of the matrix.     

A structural study of oxidation in a zirconia-toughened alumina fiber-reinforced NiAl composite

A composite of NiAl reinforced with continuous zirconia-toughened alumina (PRD-166) fibers was fabricated by pressure casting. The chemical stability of the composite at 1100 °C in vacuum and air was investigated by optical and transmission electron microscopy and energy-dispersive spectroscopy (EDS). Exposure of the fiber to the molten metal caused ZrO₂ particles in the fiber to move to the surface, thus permitting dissolution of ZrO₂ into the molten metal. The dissolved Zr reacted with A1₂O₃ of the fiber and formed ZrO₂ particles in some regions at the fiber/matrix interface. Vacuum annealing did not result in any noticeable change in the microstructure. Air annealing led to the precipitation of ZrO₂ within the matrix near the fiber/matrix interface. A thin layer of A1₂O₃ was observed to envelop the ZrO₂ particles and cover the fiber. During air annealing, Al oxidized preferentially, thereby continually reducing the Al content of the β-NiAl. This caused a progressive change in the microstructure of the matrix from β-NiAl to premartensitic microstructure, to martensitic structure, followed by nucleation and growth of Ni₃Al, to the development of a two-phase microstructure consisting of Ni₃Al cuboids dispersed in a disordered α-Ni(Al) and, subsequently, the formation of single-phase α-Ni(Al). The orientation relationship between Ni₃Al and NiAl was . Internal oxidation of α-Ni(Al) led to precipitation of A1₂O₃ particles which subsequently reacted with Ni, in the presence of O, to form NiO · A1₂O₃ spinel. The Ni was oxidized to formβ-NiO. Titanium-containing, platelike precipitates with a {111} habit plane were occasionally observed in NiO. Some larger NiTiO₃ particles were also formed within NiO. Diffusion of O through the interphase and grain boundaries of the fiber is believed to be responsible for the rapid oxidation of the composite.     

Transformations in α 2+γ titanium aluminide alloys containing molybdenum: Part II. Heat treatment

The response of as-cast structures of 12 alloys in the Ti-Al-Mo system containing 44 to 50 at. pct Al and 2 to 6 at. pct Mo to simple single step heat treatments in the temperature range 1373 to 1673 K is described. The microsegregation patterns present in the cast structure persist to a large extent after heat treatment, especially below 1673 K. However, tentative conclusions regarding phase equilibria in this temperature and composition range are drawn from the results. High-temperature equilibria are dominated by the β, α+β, and α+γ phase fields, while the β+γ phase field dominates equilibrium below 1473 K. Three major types of transformation behavior are observed: a massive α to γ transformation, which occurs within the α phase on quenching from 1673 and 1573 K in alloys centered around the 48 pct Al composition; a eutectoid transformation from α to B2+γ mixtures, which occurs at 1473 K and below in alloys centered around the 48Al-4Mo and 46Al-6Mo compositions; direct γ precipitation in β, which occurs primarily in the 44Al-6Mo composition at 1273 K and below; and finally growth of γ lamellae in α+γ lamellar structures with B2 precipitation on lamellar interfaces, which occurs over a broad range of alloy compositions and temperatures.     

Formation and stability of a nitride with the structure of beta manganese in Ni-Cr-N ternary system

A nitride with the metal-atom arrangement of β manganese, which is designated as π phase, was confirmed to exist as an equilibrium phase in the ternary Cr-Ni-N system. A Cr-40 mass pct Ni binary alloy was nitrided under a pure nitrogen atmosphere at 1523 K to prepare a Cr-40Ni-5N alloy consisting of a nickel-rich face-centered cubic (fcc) γ phase and a dichromium nitride, Cr₂N, designated as ε phase. Upon aging the alloy at 1273 K, the π phase was formed through a peritectoid reaction between the γ and ε phases. A volume fraction of the π phase reached 95 pct after 3.6 × 10⁵ s, and no more change of the volume fraction was observed, even after 3.6 × 10⁷ s aging. The chemical composition of the π phase was determined to be Cr₁₃Ni₇N₄, whose lattice parameter wasa = 0.6323 nm. The π phase became unstable above 1473 K, which explains a previous unsuccessful attempt to produce the Cr-Ni-N ternary π phase by the replacement of molybdenum in Mo₁₂Ni₈N₄ by chromium at 1473 K.     

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.     

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.     

The precipitation behavior of a Zr-2.5 wt pct Nb alloy

The morphology, crystallography, and nature of precipitates in a quenched and aged Zr-2.5 wt pct Nb alloy has been studied by transmission electron microscopy. The needle-shaped matrix precipitates and equiaxed twin boundary nucleated precipitates produced by aging at 500 °C were the equilibrium Nb-rich β₂ phase. On aging at 600 °C, the matrix precipitation was a mixture of β₂ needles and coarse metastable Zr-rich β₁ particles, while only β₁ particles were found at twin boundaries. The growth direction of the needle-shaped particles, 6.6 deg to 8.2 deg from (1-100)_h, and their orientation relationship can be predicted by an invariant line strain model. The β₁ precipitates have the Burgers orientation relationship. The formation of metastable β₁ and stable β₂ particles is considered from the free energy approach of Menon, Banerjee, and Krishnan.     

Co-deformation processing and modeling of <Emphasis Type="Italic">In-Situ</Emphasis> multiphase composites

A series of in-situ, deformation-processed metal matrix composites were produced by direct powder extrusion of blended constituents. The resulting composites are comprised of a metallic Ti-6Al-4V matrix containing dispersed and co-deformed discontinuously reinforced-intermetallic matrix composite (DR-IMC) reinforcements. The DR-IMCs are comprised of discontinuous TiB₂ particulate within a titanium trialuminide or near-γ Ti-47Al matrix. Thus, an example of a resulting composite would be Ti-6Al-4V+40 vol pct (Al₃Ti+30 vol pct TiB₂) or Ti-6Al-4V+40 vol pct (Ti-47Al+40 vol pct TiB₂), with the DR-IMCs having an aligned, high aspect ratio morphology as a consequence of deformation processing. The degree to which both constituents deform during extrusion has been examined using systematic variations in the percentage of TiB₂ within the DR-IMC, and by varying the percentage of DR-IMC within the metal matrix. In the former instance, variation of the TiB₂ percentage effects variations in relative flow behavior; while in the latter, varying the percentage of DR-IMC within the metallic matrix effects changes in strain distribution among components. The results indicate that successful co-deformation processing can occur within certain ranges of relative flow stress; however, the extent of commensurate flow will be limited by the constituents’ inherent capacity to plastically deform.     

Microstructure evolution in tial alloys with b additions: Conventional solidification

Solidification microstructures of arc-melted, near-equiatomic TiAl alloys containing boron additions are analyzed and compared with those of binary Ti-Al and Ti-B alloys processed in a similar fashion. With the exception of the boride phase, the matrix of the ternary alloy consists of the same α₂ (DO₁₉) and γ (Ll₀) intermetallic phases found in the binary Ti-50 at. pct Al alloy. On the other hand, the boride phase, which is TiB (B27) in the binary Ti-B alloys, changes to TiB₂ (C32) with the addition of Al. The solidification path of the ternary alloys starts with the formation of primary α (A3) for an alloy lean in boron (∼1 at. pct) and with primary TiB₂ for a higher boron concentration (∼5 at. pct). In both cases, the system follows the liquidus surface down to a monovariant line, where both α and TiB₂ are solidified concurrently. In the final stage, the α phase gives way to γ, presumably by a peritectic-type reaction similar to the one in the binary Ti-Al system. Upon cooling, the α dendrites order to α₂ and later decompose to a lath structure consisting of alternating layers of γ and α₂.     

Aging behavior of an Al−Li−Cu−Mg−Zr alloy

The aging processes in an Al-Li-Cu-Mg-Zr alloy have been examined by means of electrical resistivity measurements and high-resolution transmission electron microscopy coupled with an image-processing system. The specimens quenched in iced brine after solution treatment, were reheated at a constant rate of 1 K/min up to 773 K. Six reactions were clearly separated in the temperature derivative of the resistivity/temperature curve, i.e., there was a slight increase at temperatures around 333 K, a large decrease at around 368 K, a significant decrease at around 448 K, a large increase at around 538 K, a remarkable decrease at around 568 K, and a final broad increase at around 623 K. Each reaction observed by the electrical resistivity measurement was examined metallographically. In the as-quenched specimen, spherical undissolved β′ (AlZr₃ L1₂ structure) particles dispersed, but the matrix was already ordered congruently into an L1₂ structure. The first reaction at around 333 K is probably due to the increase of the degree in the congruent ordering, but the second one, at around 368 K, is thought to rise from the rearrangements of antiphase domain, boundaries (APDBs) such as the partition of Li atoms between an APDB and the matrix, the APDBs lying parallel to the {100} and {110} planes. Reheating to temperatures around 448 K induces the phase separation, with well-defined interfaces into Li-rich, ordered δ′ (L1₂) and Li-poor, less-ordered regions, and the Ost-wald ripening of the ordered regions follows. The reactions at 538, 568, and 633 K were identified as the dissolution of δ′ particles into the matrix, the precipitation of δ (AlLi B32) and S′ (Al₂CuMg orthorhombic) particles, and the dissolution of both δ and S compounds into the matrix, respectively. SADAYOSHIITO, formerly Graduate Student, Department of Materials Science and Engineering, Ehime University     

Microstructural and Mechanical Characterization of Two Aluminium Based In Situ Composite Foams

In situ composites are multiphase materials where the reinforcing phase is synthesized within the matrix during composite fabrication. The present paper deals with the processing, microstructural and mechanical characterization of Al?C7Si?C0.3Mg?C10TiB₂ and Al?C4Cu?C10TiB₂ foams. Composite foams with very low relative density (??_r?=?0.17?C0.37) and foams containing uniform cell sizes were successfully processed. Since the TiB₂ particle sizes are less than 2???m and have a good wetting behaviour, TiB₂ can be very good foam stabilizers. Microstructural characterization of the cell walls showed significant grain refinement since TiB₂ is a grain refiner. Elemental mapping clearly showed TiB₂ particles at inter dendritic boundaries. Compression testing of the processed foams showed some interesting features. Stress?Cstrain curve showed a lot of serrations which indicated brittle fracture of the cell walls and edges. Hence, it is observed that a balance should be attained between the grain refinement of ??-Al grains and the amount of TiB₂ particles to obtain desirable mechanical properties. Energy absorbed by the processed foams was calculated and they were observed to be close to that of the commercially available ALPORAS foams.