The compctition between martensite and omega in quenched Ti-Nb alloys

A careful experimental study of the phase transformations which occur in annealed β phase Ti-Nb alloys during quenching has been completed. The compctition of the α″ and ω phases to form in alloys of 20 to 70 at. pct Nb was investigated as a function of quench rate and alloy composition. Particular attention was paid to the interstitial content and chemical homogeneity of the alloys. The martensitic α″ phase was found only in 20 and 25 at. pct Nb alloys, and then only using fast water quenches of ~300 °C/sec. Under slower quench conditions,e.g., ~0.3 to 3 °C/sec, ω phase precipitates were found in these alloys and in 30 and 35 at. pct Nb alloys. Evidence of “diffuse” ω phase precipitation was observed in alloys up to 50 at. pct Nb. Only alloys of 60 and 70 at. pct Nb were found to retain the single phaseβ structure upon quenching. These results constitute the first part of a study of the stable and metastable equilibria of the Ti-Nb alloy system. Formerly a Graduate Student in the Materials Science Program at the University of Wisconsin-Madison.     

Correlation of structure and superconducting properties in Nb(Cb)-Ti quaternary alloys

The aging behavior of a Nb-Ti alloy containing 60 wt pct Ti and small additions of oxygen and erbium or scandium was characterized and related to superconducting properties. The ternary and quaternary alloys were cold reduced and aged for various times at temperatures between 250° and 1000°C. ω and α phase transformations and oxide precipitation processes were followed by lattice parameter, diffraction intensity, resistivity, and metallographic studies, and correlated with superconducting critical magnetic field and critical current density measurements. The optimum 1 hr aging temperatures for producing ω and α phase precipitation were found to be 400° and 500°C, respectively. Aging at 1000°C produced only oxide precipitation. It was found that oxygen, erbium, and scandium stabilize the α phase but have little effect on ω precipitation. The ω phase proved the most effective fluxoid pinning precipitate. The fine scale dispersoid provided an extremely high number density of pinning sites. Formerly Research Assistant, Department of Metallurgy and Materials Science, M.I.T., Cambridge, Mass. Formerly Research Assistant, Department of Metallurgy and Materials Science, M.I.T. Formerly Research Associate, Department of Metallurgy and Materials Science, M.I.T.     

Characterization of the aging response of a melt-spun Al-Be-Li alloy ribbon

A rapidly solidified Al-10 wt pct Be-3 wt pet Li alloy has been produced by a melt-spinning technique. The aging behavior of the melt-spun ribbon has been characterized by a number of techniques, including differential scanning calorimetry, hardness measurements, and transmission electron microscopy. The results have shown that the aging responses of the ternary Al-Be-Li alloy are very similar to those of Al-Li binary alloys. Specifically, δ′ precipitates are formed at temperatures near 180 °C and they transform to δ at temperatures around 300 °C. Microstructural examination indicates that, in the Al-Be-Li alloy, α-Be particles are present in the matrix as independent dispersoids and, apparently, have little effect on the aging behavior of the Al-Li matrix; no new phases are present in the matrix of specimens heat treated up to 400 °C. The α-Be particles coarsen, however, above a temperature of approximately 300 °C. The growth of α-Be particles follows the classical Ostwald coarsening type of mechanism.     

Decomposition of Fe-Ni martensite: Implications for the low-temperature (≤500 °C) Fe-Ni phase diagram

The low-temperature (<500 °C) decomposition of Fe-Ni martensite was studied by aging martensitic Fe-Ni alloys at temperatures between 300 °C and 450 °C and by measuring the composition of the matrix and precipitate phases using the analytical electron microscope (AEM). For aging treatments between 300 °C and 450 °C, lath martensite in 15 and 25 wt pct Ni alloys decomposed with γ [face-centered cubic (fcc)] precipitates forming intergranularly, and plate martensite in 30 wt pct Ni alloys decomposed with γ (fcc) precipitates forming intragranularly. The habit plane for the intragranular precipitates is {111}_fcc parallel to one of the {110}_bcc planes in the martensite. The compositions of the γ intergranular and intragranular precipitates lie between 48 and 58 wt pct Ni and generally increase in Ni content with decreasing aging temperature. Diffusion gradients are observed in the matrix α [body-centered cubic (bcc)] with decreasing Ni contents close to the martensite grain boundaries and matrix/precipitate boundaries. The Ni composition of the matrix α phase in decomposed martensite is significantly higher than the equilibrium value of 4 to 5 wt pct Ni, suggesting that precipitate growth in Fe-Ni martensite is partially interface reaction controlled at low temperatures (<500 °C). The results of the experimental studies modify the γ/α + γ phase boundary in the present low-temperature Fe-Ni phase diagram and establish the eutectoid reaction in the temperature range between 400 °C and 450 °C. Formerly Research Assistant, Department of Materials Science and Engineering, Lehigh University     

A phenomenological study of the shape memory effect in polycrystalline uranium-niobium alloys

When uranium-niobium alloys containing between 13.9 and 17.9 at. pct Nb are quenched to room temperature from the BCC (γ) phase at elevated temperatures, diffusion-controlled precipitation of the equilibrium phases is prevented and martensitic transformations to transition phases occur instead. Dilatometry was used to detect transformation temperatures and with the help of X-ray diffraction analysis, a metastable phase diagram was established. At room temperature after quenching, alloys containing < 15.2 at. pct Nb were monoclinic (α″) and those with < 16.6 at. pct Nb were tetragonal (γ°). The deformation behavior and shape memory effects (SME) accompanying the reverse martensitic phase transformations in polycristalline specimens were surveyed and characterized phenomelogically. From uniaxial tensile tests at room temperature, macroscopic stress-strain parameters, associated with the reversible deformation modes in the α″ and γ° martensites, were defined and their composition and structural state dependencies delineated. A diffuse maximum manifested in the stress-strain diagrams was identified with the reversible strain limit, which varied inversely and continuously with composition. A concentration-independent value of 693 MPa was found for the plastic yield strength of the alloys. All the alloys exhibited heat-activated shape recovery but the degree depended on structural state and composition. The α″ alloys showed a much larger effect than γ° alloys. Shape recovery occurred in two stages in all alloys. The first stage of recovery accompanied martensite reversion but final reversion to the equilibrium y phase was not accomplished until much higher temperatures were reached. Rapid, low temperature aging reactions were thought to affect the finish of shape recovery and delay it to higher temperatures. Formerly with Metals and Ceramics Division, Oak Ridge National Laboratory     

Microstructural study of the titanium alloy Ti-15Mo-2.7Nb-3Al-0.2Si (TIMETAL 21S)

A relatively new titanium alloy, TIMETAL 21S (Ti-15Mo-2.7Nb-3Al-0.2Si-0.15O (in wt pct)), is a potential matrix material for advanced titanium matrix composites for elevated temperature use. In order to develop a perspective on the microstructural stability of this alloy, the influence of several commonly used heat treatments on the microstructure of TIMETAL 21S was studied using optical and transmission electron microscopy (TEM). Depending on the specific thermal treatment, a number of phases, includingα,ω- type, and silicide, can form in this alloy. It was found that both recrystallized and nonrecrystallized areas could be present in the microstructure of an annealed bulk alloy, but the microstructure of annealed sheet alloy was fully recrystallized. The mixed structure of the bulk alloy, developed as a result of inhomogeneous deformation, could not be removed by heat treatment alone at 900 °C. Athermalω-type phase formed in this alloy upon quenching from the solution treatment temperature (900 °C). Silicide precipitates were also found in the quenched sample. Thermal analysis was used to determine theβ transus and silicide solvus as close to 815 °C and 1025 °C, respectively. In solution-treated and quenched samples, a high-temperature aging at 600 °C resulted in the precipitation ofα phase. The precipitation reaction was slower in the recrystallized regions compared to the nonrecrystallized regions. During low-temperature aging (350 °C), the ellipsoidalω-type phase persisted in the recrystallized areas even after 100 hours, whereas a high density ofα precipitates developed in the nonrecrystallized areas within only 3 hours. The observed behavior in precipitation may be related to the influence of substructure in the nonrecrystallized areas, providing for an enhanced kinetics during aging. Theα precipitates (formed during continuous cooling from the solution treatment temperature, low-temperature aging, and high-temperature aging) always obeyed the Burgers orientation relationship. With respect to the microstructure, TIMETAL 21S is similar to other solute-lean, metastableβ titanium alloys.     

Transformation squence in a Cu-Al-Ni shape memory alloy at elevated temperatures

Precipitation sequences in a Cu-14 pct Al-4 pct Ni (wt pct) shape memory alloy were studied by means of transmission electron diffraction and microscopy as well as X-ray microanalysis techniques. On aging thin foil specimens up to 550 °C in the electron microscope, an as-quenched sample having a mixture of 2H-type and D0₃-type metastable structures transformed to the stable simple cubic γ₂ phase at or above 450 °C. The remaining matrix either showed precipitates of the fcc α-phase on prolonged annealing at 500 to 550 °C for a longer period, or transformed to martensite on cooling below theM _s temperature (~150 °C).     

Kinetics and Equilibrium of Age-Induced Precipitation in Cu-4 At. Pct Ti Binary Alloy

Transformation kinetics and phase equilibrium of metastable and stable precipitates in age-hardenable Cu-4 at. pct Ti binary alloy have been investigated by monitoring the microstructural evolution during isothermal aging at temperatures between 693 K (420 °C) and 973 K (700 °C). The microstructure of the supersaturated solid solution evolves in four stages: compositional modulation due to spinodal decomposition, continuous precipitation of the needle-shaped metastable β′-Cu₄Ti with a tetragonal structure, discontinuous precipitation of cellular components containing stable β-Cu₄Ti lamellae with an orthorhombic structure, and eventually precipitation saturation at equilibrium. In specimens aged below 923 K (650 °C), the stable β-Cu₄Ti phase is produced only due to the cellular reaction, whereas it can be also directly obtained from the intergranular needle-shaped β′-Cu₄Ti precipitates in specimens aged at 973 K (700 °C). The precipitation kinetics and phase equilibrium observed for the specimens aged between 693 K (420 °C) and 973 K (700 °C) were characterized in accordance with a time–temperature–transformation (TTT) diagram and a Cu-Ti partial phase diagram, which were utilized to determine the alloy microstructure, strength, and electrical conductivity.

     

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.     

Deformation characteristics of age hardened Ti-6Al-4V

A commercial Ti−6Al−4V alloy with an equiaxed grain shape was investigated after solution annealing at 810°C and after aging at 550 and 350°C. Age hardening at both temperatures produced significant increases in Young's modulus and yield strength. Finely dispersed α₂(Ti₃Al) precipitates formed within the α phase upon aging at 550°C, but not when aging at 350°C. However, there is evidence of order, probably of oxygen, in the α grains of specimens which were aged at 350°C. The formation of the ordered Ti₃Al precipitates at 550°C and the occurrence of oxygen ordering at 350°C can account for the increases in Young's modulus and yield strength. since January 1977 with General Electric Co., Lighting Research Division, Nela Park, Cleveland, OH. KANAY GAZIOGLU, formerly with DFVLR, is deceased.     

Structure-property relations in a metastable β Ti alloy containing Si

The hardness response, tensile behavior, and phase transformations occurring in a quenched and aged metastable β phase Ti-30 at. pct V-l at. pct Si alloy have been inves-tigated. Upon aging at 570°C, as-quenched samples show a broad hardness peak which is associated with the formation of rod-like, hexagonal (Ti,V)_xSi_y transition phase precipi-tates. The equilibrium silicide is observed upon aging at 570°C in the form of faceted, tetragonal particles. A loss of tensile ductility and a transition to intergranular fracture occurs after extended aging at 570°C and is related to Si segregation to the grain bound-aries. Comparing the behavior of Ti-30V to that of Ti-30V-lSi shows that the presence of Si strongly retards α-phase formation. However, a substantial age hardening re-sponse still occurs upon aging at 450°C, especially after prior cold work (the yield strength increases from 635 to 982 MPa). This hardening response is combined with a retention of a ductile, transgranular fracture even after extended aging at 450°C. Aging first at 570°C followed by aging at 450°C results in an increase in the volume fraction of α-phase formed but a subsequent decrease in ductility and hardness response upon aging at 450°C. These results are discussed in terms of the structure/property relationships which result from the influence of Si on the formation of, a) (Ti.V)_xSi_y precipitates, b) the equilibrium silicide, and c) the α-phase.     

The effect of cold work on the precipitation of Ω and θ′ in a ternary Al-Cu-Mg alloy

The effect of plastic deformation on the microstructural evolution of an Al-5.0Cu-0.5Mg (wt pct) ternary alloy was investigated. Hardness measurements and quantitative precipitate analysis were performed on specimens that were water quenched from a solution heat treatment, stretched either 0 or 6 pct and immediately aged at ambient temperature or artificially aged at 200 °C or 250 °C for times up to 3000 hours. Quantitative transmission electron microscopy (TEM) was used to characterize Ω and θ′ number density, diameter, and thickness as a function of preage mechanical stretch and artificial aging condition. Age hardening curves for naturally and artificially aged specimens revealed an increase in hardness corresponding with a preage stretch. Quantitative TEM verified an increase in the number density and a refinement of precipitates for both Ω and θ′ between the 0 and 6 pct stretch condition for those samples artificially aged. When aged at 200 °C, θ′ exhibited superior coarsening resistance relative to the Ω phase. The quantified Ω coarsening kinetics were greater than similar Ag-containing alloys. To investigate the effects of trace Si additions on subsequent microstructural evolution, a series of Al-Cu-Mg-Si quaternary alloys were produced. The addition of 0.1Si (wt pct) was found to suppress Ω precipitation in most Al-4.0Cu-xMg alloys investigated. These initial results indicate that Ω precipitation may be related to the Mg/Si ratio.     

Precipitation in a Cu-30 Pct Ni-1 Pct Nb alloy

Decomposition of a Cu-30 pct Ni-1 pct Nb alloy on aging in the range of 866 K (600°C) to 1073 K (800°C) was investigated. The initial decomposition, concomitant with age hardening, occurred through the precipitation of body centered tetragonal metastable Ni₃Nb-γ” precipitates on the 100 matrix planes. Equilibrium orthorhombicβ phase formed either through a grain boundary cellular reaction at low temperature (≤973 K (700°C)) or as Widmanstaettenplatelets on the 1ll planes at higher temperatures (≥1073 K (800°C)) with the following crystallographic relationship: (0l0)_β//111_γ [100]β//[1•11]_γ. Based on the observations, a schematic transformation sequence is presented.     

Phase relations in concentrated Ta-Hf and Nb-Hf alloys

The precipitation reactions occurring in concentrated Ta-Hf and Nb-Hf alloys over the temperature range 600° to 1400°C were investigated by X-ray diffraction and transmission microscopy. In Ta-Hf alloys containing 30 to 62 at. pct Hf and Nb-Hf alloys containing 25 to 50 at. pct Hf a stable bcc solid solution exists at elevated temperature. When quenched from the single phase region and aged at lower temperatures both alloys precipitate α phase, an hcp hafnium-rich phase. Transmission microscopy showed that in the Nb-Hf system the precipitation reaction begins with the formation of coherent zones. Further aging causes coarsening into discs, still at least partially coherent. When coherency is lost the α morphology changes to rod-type. The effect of small amounts of oxygen added to these alloys during aging is an increase in the unit cell size of theα phase. Oxygen concentrations up to ~0.5 at. pct do not cause observable changes in the phase relations. These results cannot be reconciled with the miscibility gap-monotectoid type phase diagrams reported by some earlier investigators and indicate that the proper phase diagram for both systems is theβ-isomorphous type.     

Structure and properties of a β solution treated,quenched, and aged si-bearing near-α titanium alloy

The microstructure, tensile properties, and fractographic features of a near-α titanium alloy, IMI 829(Ti-6.1 wt pct Al-3.2 wt pct Zr-3.3 wt pct Sn-1 wt pct Nb-05 wt pct Mo-0.32 wt pct Si) have been studied after aging over a temperature range of 550°C to 950°C for 24 hours following solution treatment in the β phase field at 1050°C and water quenching. Transmission electron microscopy studies revealed that aging at 625°C and above produced discrete silicides at α′ interplatelet boundaries. However, aging at 900°C and above has also resulted in the precipitation of β phase along the lath boundaries of martensite. The silicides have been found to have a hexagonal structure withc=0.36 nm anda=0.70 nm (designated as S₂ by earlier workers). There is a significant improvement in yield and ultimate tensile strength after aging at 625°C, but there is less improvement at higher aging temperatures. The tensile ductility is found to be drastically reduced. While the fracture surface of the unaged specimen shows elongated dimples, the aged samples show a mixed mode of fracture, consisting of facets, featureless parallel bands, and extremely fine dimples.     

Effect of Aging on Microstructure and Shape Memory Properties of a Ni-48Ti-25Pd (At. Pct) Alloy

The microstructure and properties of a precipitation-hardenable Ni-48Ti-25Pd (at. pct) shape memory alloy have been investigated as a function of various aging conditions. Both the hardness and martensitic transformation temperatures increased with increasing aging time up to 100 hours at 673 K (400 °C), while no discernable differences were observed after heat treatment at 823 K (550 °C), except for a slight decrease in hardness. For aging at 673 K (400 °C), these effects were attributed to the formation of nano-scale precipitates, while precipitation was absent in the 823 K (550 °C) heat-treated specimens. The precipitation-strengthened alloy exhibited stable pseudoelastic behavior and load-biased-shape memory response with little or no residual strains. The precipitates had a monoclinic base-centered structure, which is the same structure as the P-phase recently reported in Ni(Pt)-rich NiTiPt alloys. 3D atom probe analysis revealed that the precipitates were slightly enriched in Ni and deficient in Pd and Ti as compared with the bulk alloy. The increase in martensitic transformation temperatures and the superior dimensional stability during shape memory and pseudoelastic testing are attributed to the fine precipitate phase and its effect on matrix chemistry, local stress state because of the coherent interface, and the ability to effectively strengthen the alloy against slip.     

Stress-assisted transformation in Ti-60 wt pct Ta alloys

An investigation of the influence of processing variables on mechanical properties and phase development for a Ti-60 wt pct Ta (Ti-28.5 at. pct Ta) alloy was conducted. The alloy was hot-rolled, subjected to heat-treatment temperatures above the β (bcc) transus (1 hour at 700 °C, 800 °C, or 900 °C), and water quenched. All heat treatment produced a combination of metastable β (bcc) and metastable α″ (orthorhombic martensite), with the amount of retained β essentially independent of heat treatment, ranging from 20 to 33 vol pct. Deformation of as-rolled and heat-treated tension specimens showed an anomalous leveling of the stress-strain curve in the stress-strain curves at low strains. X-ray diffraction (both simple 2ϑ diffractometry and texture analysis) on both deformed and undeformed material determined that the leveling of the stress-strain curve was a result of the β→α″ martensitic transformation. The stress required to initiate the transformation increased with prequench temperature. This was determined to be due to the presence of athermal ω. Grain growth kinetics have been determined in the course of this work.     

Microstructure and high-temperature tensile deformation of tiai(si) alloys made from elemental powders

Two ternary TiAl-based alloys with chemical compositions of Ti-46.4 at. pct Al-1.4 at. pct Si (Si poor) and Ti-45 at. pct Al-2.7 at. pct Si (Si rich), which were prepared by reaction powder processing, have been investigated. Both alloys consist of the intermetallic compounds y-TiAl, α₂-Ti₃Al, and ξ-Ti₅(Si, Al)₃. The microstructure can be described as a duplex structure(i.e., lamellar γ/α₂ regions distributed in γ matrix) containing ξ precipitates. The higher Si content leads to a larger amount of ξ precipitates and a finer y grain size in the Si-rich alloy. The tensile properties of both alloys depend on test temperature. At room temperature and 700 °C, the tensile properties of the Si-poor alloy are better than those of the Si-rich alloy. At 900 °C, the opposite is true. Examinations of tensile deformed specimens reveal ξ-Ti₅(Si, Al)₃ particle debonding and particle cracking at lower test temperatures. At 900 °C, nucleation of voids and microcracks along lamellar grain boundaries and evidence for recovery and dynamic recrystallization were observed. Due to these processes, the alloys can tolerate ξ-Ti₅(Si, Al)₃ particles at high temperature, where the positive effect of grain refinement on both strength and ductility can be utilized.     

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.,