Morphology and structure of bainite plates in the β’ phase of an Ag-45 at. pct Cd alloy

The morphology and structure of bainite plates which formed in the ordered bcc β’ phase of a Ag-45 at. pct Cd alloy at temperatures 160 to 300°C were studied by X-ray diffraction and transmission electron microscopy. Initially, the plates formed with a 3R stacking fault modulation of the fcc structure and contained a high density of random stacking faults. The stacking faults annealed out during a prolonged isothermal treatment, the structure gradually changing to a regular fcc. The orientation relationship between the bcc matrix and the fcc bainite was as follows: [1•11]_b 0.7 deg from [0•11]_f, [110]_b 1.1 deg from [l00]_f and [011]_b 4.3 deg from the stacking fault plane pole [111]_f. The habit plane of the bainite plates, determined by two surface trace analysis, was close to (3, 11, 12)_b. The surface relief of the plates, examined by interference microscopy, was in the form of a simple tilt indicating an invariant plane strain transformation. The features of the transformation agreed with the predictions of the Bowles-MacKenzie theory of martensite formation. It was concluded that the morphology, structure and other characteristics of the freshly formed bainite plates were consistent with their formation by a thermally activated martensitic process. formerly Research Associate, Department of Metallurgy, University of British Columbia, Vancouver, B.C., Canada.     

Determining interphase boundary orientations from near-coincidence sites

A transmission electron microscope (TEM) study was made of the interphase boundary structure of delta plates precipitated from the gamma phase in alloy 718. A variety of interfacial defects were examined and identified. These results, together with available data obtained from bcc laths in fcc Ni-Cr alloys, were used to develop a method for predicting precipitate orientation relationships and boundary orientations. The method employs a geometric matching approach in three dimensions based upon the concept of near-coincidence sites. It is suggested that precipitates in a given system select an orientation relationship which produces the greatest areal density of near-coincidence sites and that the habit plane adopts an orientation that yields the greatest area of boundary containing contiguous near-coincidence sites. This article is based on a presentation made in the symposium “Kinetically Determined Particle Shapes and the Dynamics of Solid:Solid Interfaces,” presented at the October 1996 Fall meeting of TMS/ASM in Cincinnati, Ohio, under the auspices of the ASM Phase Transformations Committee.     

Intrinsic ledges at interphase boundaries and the crystallography of precipitate plates

The structure of intrinsic ledges at interphase boundaries has been interpreted with extended O-lattice/DSC-lattice approaches. The distribution of structural ledges can be predicted if the spacing difference between parallel matrix and product planes is treated as a measure of the relaxed coincidence condition. A small rotation away from the low-index planar parallelism introduces a series of interfacial dislocations that cancels the spacing difference, resulting in a lattice invariant line. Misfit-compensating ledges at bcc: hcp interfaces are produced as a ledged interface intersects additional O-points that are recognized with the incorporation of previously omitted bcc atom positions into the O-lattice construction. Energetic consideration suggests that structural interfacial energy may decrease when a flat interface becomes ledged with misfit-compensating ledges. Burgers vectors associated with structural ledges and misfit-compensating ledges are displacement shift complete (DSC) lattice vectors. Precipitate and martensite crystallography may both include a lattice invariant line, but they are involved in different interphase boundary characteristics. Assumptions and implications in precipitate and martensite crystallography are discussed in the framework of the O-lattice theory and phenomenological theory of martensite crystallography. This article is based on a presentation made at the Pacific Rim Conference on the “Roles of Shear and Diffusion in the Formation of Plate-Shaped Transformation Products,” held December 18-22, 1992, in Kona, Hawaii, under the auspices of ASM INTERNATIONAL’S Phase Transformations Committee.     

Growth of δ′ on dislocations in a dilute Al-Li alloy

The quantitative results of δ′ growth kinetics on dislocations in an Al-2.27 wt pct Li alloy demonstrate that at temperatures greater than 0.5 of the homologous melting temperature, T _m , volume diffusion is the dominant mechanism. However, a small contribution, approximately one atom in 300, is made by pipe diffusion through the dislocation. This can be established by careful examination of dislocation climb associated with precipitate growth. An analysis based on the δ′ growth kinetics and diffusion equation gives an activation energy of 0.56 eV for Li pipe diffusion. At temperatures <0.5 T _m , δ′ precipitate growth is faster when associated with dislocations, and here, pipe diffusion is necessary to account for the kinetics observed. This article is based on a presentation made in the symposium “Kinetically Determined Particle Shapes and the Dynamics of Solid:Solid Interfaces,” presented at the October 1996 Fall meeting of TMS/ASM in Cincinnati, Ohio, under the auspices of the ASM Phase Transformations Committee.     

A new phase in an Fe-9.0AI-29.5Mn-1.2Si alloy

A new type of precipitate (designated L phase) was observed within the ferrite matrix in an Fe-9.0Al-29.5Mn-l.2Si alloy after being aged at 550 °C. Transmission electron microscopy examinations indicated that the L-phase precipitate has a monoclinic structure with lattice parametersa = 0.656 nm,b = 0.797 nm,c = 0.637 nm, and β = 109.4 deg, and the orientation relationship between the L-phase precipitate and the ferrite matrix could be stated as follows: (-101)_α//(001)_L-phase (-110)_α//(-111)_L-phase with a deviation of 0.3 deg (01l)_α//(131)_L-phase with a deviation of 1.5 deg The L phase has never been observed in various Fe-Al-Mn, Fe-Mn-Si, Fe-Al-Si, and Mn-Al-Si alloy systems before.     

Precipitation-strengthened austenitic Fe−Mn−Ti alloys

The precipitation of intermetallic compounds in the Fe−20Mn−2Ti and Fe−28Mn−2Ti alloy systems has been investigated over the temperature range 700 to 900°C by hardness measurements, optical and scanning electron microscopy, and X-ray diffraction. In both systems only the equilibrium Laves phase was observed. The precipitate was identified as C14(MgZn₂) type hexagonal Laves phase with a chemical composition close to Fe₂ (Ti, Mn). In an as-annealed sample precipitation occurred in a heterogeneous manner, predominantly along grain boundaries. The effect of a cold deformation between the solution annealing and aging processes was also investigated. In addition to a high density of dislocations, martensitic phases were induced by deformation: a γ→∈ transformation occurred in the Fe−28Mn−2Ti alloy while a γ→α′ transformation was predominant in the Fe−20Mn−2Ti alloy. Subsequent aging was conducted at temperatures above theA _f . A large number of very fine precipitates formed randomly in the matrix after a short aging period. This cold work plus aging treatment resulted in an increase in yield strength. The enhancement of mechanical properties is due to the randomly distributed precipitates combined with the high defect density and fine substructure.     

Processing effects on the grain-boundary character distribution of the orthorhombic phase in Ti-Al-Nb alloys

The grain-boundary character distribution of the orthorhombic (O) phase in Ti₂AlNb intermetallic alloys was investigated. The alloys were thermomechanically processed either above or below the bcc transus temperature. Using electron backscattered diffraction, the twin-related O-phase variant interfacial planes were identified and quantified. For the subtransus-processed samples, the equiaxed-O/equiaxed-O grain boundaries tended to primarily prefer 65-deg misorientations and secondarily prefer 90-deg boundaries. Of the 65-deg misoriented boundaries, which were preferentially rotated about [001], ∼40 pct contained (110) twin-related interfacial planes. The observations were rationalized by the α ₂-to-O phase transformation. It is suggested that for subtransus processing within the α ₂-containing phase regimes, the resulting heat-treated O+bcc microstructures evolve such that the O/O boundaries tend to exhibit distinct twin-related variants with misorientations between 55 and 65 deg. For a supertransus-processed alloy, it was found that approximately equal distributions of the six resolvable O variants were formed from the dominant parent bcc orientation. The resulting O/O boundaries tended to cluster at near-90-deg misorientations, which can be explained by the bcc/O orientation relationship. It is suggested that whenever the O phase primarily transforms from the bcc structure, the resulting O+bcc microstructures evolve such that the O/O boundaries tend to exhibit misorientations near 90 deg.     

Nucleation kinetics of Ti carbonitride in microalloyed austenite

Ti(CN) precipitation data determined by stress relaxation are analyzed using the classical theory of diffusion controlled nucleation. For this purpose, the interface energy γ and strain energy ΔG _ɛ accompanying nucleus formation are estimated using a new approach, and the driving force for Ti(CN) nucleation is calculated with the aid of a thermodynamic model. The analysis indicates that the critical nucleus is richer in N than the bulk precipitate at equilibrium at a given holding temperature. The results also show that trace amounts of nitrogen dissolved in austenite can significantly increase the chemical driving force for Ti(CN) nucleation and thereby accelerate the rate of precipitation. On the basis of this analysis, a kinetic model is developed for predicting start times (P _S) for the strain-induced precipitation of Ti(CN) in austenite. Such predictions are in reasonably good agreement with measuredP _S times.     

Neutron diffraction study of austempered ductile iron

Crystallographic properties of an austempered ductile iron (ADI) were studied by using neutron diffraction. A quantitative phase analysis based on Rietveld refinements revealed three component phases, α-Fe (ferrite), γ-Fe (austenite), and graphite precipitate, with weight fractions of 66.0, 31.5, and 2.5 pct, respectively. The ferrite phases of the samples were found to be tetragonal,14/mmm, with ac/a ratio of about 0.993, which is very close to the body-centered cubic (bcc) structure. The austenite phase had C atoms occupying the octahedral site of the face-centered cubic (fcc) unit cell with about 8 pct occupancy ratio. A strong microstrain broadening was observed for the two Fe phases of the samples. The particle sizes of the acicular ferrite phase were studied by using small angle neutron scattering. The analysis suggested a mean rod diameter of 700 A. The scattering invariant predicts a ferrite volume fraction consistent with the powder diffraction analysis. A textbook case of nodular graphite segregation, with average diameters ranging from 10 to 20 μm, was observed by optical micrography.     

On the equilibrium silicide in beta Ti-V alloys containing Si

The addition of small amounts of silicon (usually less than R ~1 at. pct) to strengthen α(hcp)-β(bcc) and martensitic Ti alloys is well established. The equilibrium suicide formed in these alloys has been identified as hexagonal Ti₅Si₃ (Refs. 1, 2) (or (Ti,Zr)₅Si₃ phase in alloys containing Zr), although there is also a report of a tetragonal Ti₃Si phase. The use of Si to age harden β Ti alloys, specifically Ti-V-Si alloys, has also been reported. While the precipitation sequence in these alloys involves an identifiable hexagonal     

Precipitation strengthening in K5-series γ-TiAl alloyed with silicon and carbon

Interstitial additions and precipitation hardening in fully lamellar gamma TiAl have been investigated in recent years, with a prime objective of improving the high-temperature creep resistance. As a result of this alloy development effort, the alloy system K5 (Ti46Al-2Cr-3Nb-0.2W) was found to show remarkably improved creep resistance when reinforced with C or C+Si additions and then aged appropriately. Precipitation strengthening is the proposed mechanism accounting for the observed creep strengthening of K5SC alloys, with emphasis being paid on the effect of B2 particles, ζ-type silicides, and H-type carbide precipitates delineating γ/γ interfaces. In this study, the creep-deformed microstructures of fully lamellar K5 (S-C)-type alloys in aged and unaged conditions were characterized using detailed electron microscopy, involving high-resolution imaging techniques and in-situ heating studies. Overall, the presence of these particles and their relative distribution result in strengthening of the lamellar structure. The particular effect of each type of precipitate (silicides vs carbides) on creep has been assessed. New information about the nature of the light-element precipitation processes has been obtained by studying the nucleation and growth of the carbide and silicide precipitates at the expense of dissolving α ₂ laths during aging. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.     

Calorimetrically measured enthalpies for the reaction of Hf with H(D)2 (g)

The enthalpy for the direct reaction of H₂ (g) with Hf has been measured by calorimetry for the first time at both moderate, 334 K, and elevated, 919 K, temperatures. The enthalpy for the reaction: 1/2 H₂ (g) + 1/(b - a)HfH_a(α) → 1/(b- a)HfH_b(δ) is -70 ± 2.0 kJ/mol H at 334 K over a range of H contents from (H/Hf) = 0.5 to 1.5 with similar values found for D. The quantities α and δ are the coexisting phases anda andb are the corresponding (H/ Hf ) ratios, respectively. The magnitude of the enthalpy decreases from (H/Hf) = 0 to 0.5 and is then stable from 0.5 to 1.7. The value of δH _f ^‡ (HfH_1.5) = -107.5 kJ/mol and δH _f ^‡ (HfH_2.0) = -142.0 kJ/mol. In the elevated temperature range, calorimetric and equilibrium hydrogen pressure were determined over the range of H contents from 0 to 1.6. The enthalpy for the plateau reaction is -74.5 kJ/mol H and after the two-phase region, |δH_H| increases with the increase of (H/Hf) passing through a maximum at about (H/Hf) = 1.3. Formerly Graduate Student, Department of Chemistry, University of Vermont     

Phase transformations in the Al-Li-Hf and Al-Li-Ti systems

Phase transformations in Al-1, 9Li-1.6Hf and Al-1.5Li-0.7Ti (wt pct) alloys have been analyzed thoroughly using various electron microscopy techniques. An Ll₂-ordered ternary Al₃ (Li,X) phase, orά (X), forms in both alloys during heat treatment. Theά forms as spherical precipitates by normal nucleation and growth and in the Hf alloy as filaments by discontinuous precipitation. The X:Li ratio in these precipitates is found to be a function of the precipitation mode in the Hf alloy.ά is coherent with the Al matrix and serves as a preferred nucleation site for the precipitation of the Al₃Li orδ' phase when the alloy is aged at 190°C. Theδ', also Ll₂-ordered, grows as continous envelopes which completely enclose theα' phase. An altered morphology predominates forα'(Ti) after extended heat treatment. The major feature of the altered morphology is the incorporation of spiraled dislocations at theα'/matrix interface, which act to ease the misfit between theα' (Ti) and the matrix.δ' forms as caps on the alteredα'(Ti) in areas away from the dislocations. Disruption of theα' envelopes is attributed to the strain fields and loss of coherency associated with the presence of interfacial dislocations and a possible local shift from Ll₂ to DO₂₂ ordering in theα'(Ti) precipitates.     

Morphology and properties of low-carbon bainite

Morphology of low-carbon bainite in commercial-grade high-tensile-strength steels in both isothermal transformation and continuous cooling transformation is lathlike ferrite elongated in the 〈11l〉_b direction. Based on carbide distribution, three types of bainites are classified: Type I, is carbide-free, Type II has fine carbide platelets lying between laths, and Type III has carbides parallel to a specific ferrite plane. At the initial stage of transformation, upper bainitic ferrite forms a subunit elongated in the [−101]f which is nearly parallel to the [lll]_b direction with the cross section a parallelogram shape. Coalescence of the subunit yields the lathlike bainite with the [−101]_f growth direction and the habit plane between (232)_f and (lll)_f. Cementite particles precipitate on the sidewise growth tips of the Type II bainitic ferrite subunit. This results in the cementite platelet aligning parallel to a specific ferrite plane in the laths after coalescence. These morphologies of bainites are the same in various kinds of low-carbon high-strength steels. The lowest brittle-ductile transition temperature and the highest strength were obtained either by Type III bainite or bainite/martensite duplex structure because of the crack path limited by fine unit microstructure. It should also be noted that the tempered duplex structure has higher strength than the tempered martensite in the tempering temperature range between 200 °C and 500 °C. In the case of controlled rolling, the accelerated cooling afterward produces a complex structure comprised of ferrite, cementite, and martensite as well as BI-type bainite. Type I bainite in this structure is refined by controlled rolling and plays a very important role in improving the strength and toughness of low-carbon steels. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

Calorimetrically measured enthalpies for the reaction of Hf with H(D) 2 (g)

The enthalpy for the direct reaction of H₂ (g) with Hf has been measured by calorimetry for the first time at both moderate, 334 K, and elevated, 919 K, temperatures. The enthalpy for the reaction: 1/2 H₂ (g) + 1/(b - a)HfH_a(α) → 1/(b - a)HfH_b(δ) is -70 ± 2.0 kJ/mol H at 334 K over a range of H contents from (H/Hf) = 0.5 to 1.5 with similar values found for D. The quantities α and δ are the coexisting phases anda andb are the corresponding (H/ Hf) ratios, respectively. The magnitude of the enthalpy decreases from (H/Hf) = 0 to 0.5 and is then stable from 0.5 to 1.7. The value of ΔH°_f (HfH_1.5) = -107.5 kJ/mol and ΔH°_f (HfH_2.0) = -142.0 kJ/mol. In the elevated temperature range, calorimetric and equilibrium hydrogen pressure were determined over the range of H contents from 0 to 1.6. The enthalpy for the plateau reaction is -74.5 kJ/mol H and after the two-phase region, |ΔH_H| increases with the increase of (H/Hf) passing through a maximum at about (H/Hf) = 1.3. Formerly Graduate Student, Department of Chemistry, University of Vermont     

Part III. The tensile behavior of Ti-Al-Nb O+Bcc orthorhombic alloys

The tensile behavior of Ti-Al-Nb alloys with Al concentrations between 12 and 26 at. pct and Nb concentrations between 22 and 38 at. pct has been investigated for temperatures between 25 °C and 650 °C. Several microstructural features were evaluated in an attempt to identify microstructure-property relationships. In particular, the effects of the phase volume fraction, composition, morphology, and grain size were examined. In addition, the constitutive properties were evaluated using single-phase microstructures, and the results provided insight into the microstructure-property relationships of the two-phase orthorhombic (O)+body-centered-cubic (bcc) microstructures. The disordered fully-bcc (β) Ti-12Al-38Nb microstructure, produced through heat treatment above the β-transus, exhibited a room-temperature (RT) elongation of more than 27 pct and the lowest yield strength (YS-553 MPa) of all the alloys studied. The ordered fully-bcc (B2) microstructures, produced through supertransus heat treatment of near-Ti₂AlNb alloys, exhibited fracture strengths up to 672 MPa and low elongations-to-failure (ε _f≤0.6 pct). Thus, increasing the Al content, which favors ordering of the bcc structure, significantly reduces the ductility of the bcc phase. Similar to the ordered B2 microstructure, the ordered fully-O Ti₂AlNb microstructures exhibited intermediate RT strength (≤704 MPa) and ε _f (≤1 pct). The O+bcc microstructures tended to exhibit strengths greater than both the fully-O and fully-bcc microstructures, and this was attributed to the finer grain sizes in the two-phase microstructures compared to their single-phase counterparts. A RT of 1125 MPa was measured for the finest-grained two-phase microstructure. The O+bcc microstructures containing greater bcc-phase volume fractions tended to exhibit greater elongations yet poorer elevated-temperature strengths. A higher Al content typically resulted in larger elevated-temperature strengths. For the Ti-12Al-38Nb bcc-dominated microstructures, fine O platelets, which precipitated during aging, provided significant strengthening and a reduction in ε _f for the Ti-12Al-38Nb alloy. However, large RT elongations (ε _f>12 pct) were maintained for aged Ti-12Al-38Nb microstructures, which contained 28 vol pct O phase. Morphology did not appear to play a dominant role, as fully-lath and fully-equiaxed two-phase microstructures containing the same phase volume fractions exhibited similar RT tensile properties. The slip and cracking observations provided evidence for the ductile and brittle characteristics of the single-phase microstructures, and the slip compatibility exhibited between the two phases is an important part of why O+bcc microstructures achieve attractive strengths and elongations. The YS vs temperature behavior is discussed in light of other Ti-alloy systems.     

Microstructural evolution during thermomechanical processing of a Ti-Nb interstitial-free steel just below the Ar 3 temperature

Laboratory thermomechanical processing (TMP) experiments have been carried out to study the austenite transformation characteristics, precipitation behavior, and recrystallization of deformed ferrite for an interstitial-free (IF) steel in the temperature range just below Ar ₃. For cooling rates in the range 0.1 °C s⁻¹ to 130 °C s⁻¹, austenite transforms to either polygonal ferrite (PF) or massive ferrite (MF). The transformation temperatures vary systematically with cooling rate and austenite condition. There is indirect evidence that the transformation rates for both PF and MF are decreased by the presence of substitutional solute atoms and precipitate particles. When unstable austenite is deformed at 850 °C, it transforms to an extremely fine strain-induced MF. Under conditions of high supersaturation of Ti, Nb, and S, (Ti,Nb) _x S _y precipitates form at 850 °C as coprecipitates on pre-existing (Ti,Nb)N particles and as discrete precipitates within PF grains. Pre-existing intragranular (Ti,Nb) _x S _y precipitates retard recrystallization and grain coarsening of PF deformed at 850 °C and result in a stable, recovered subgrain structure. The results are relevant to the design of TMP schedules for warm rolling of IF steels.     

An electron microscopy study of carbide precipitation in vanadium

The precipitation of carbon from supersaturated solid solution in vanadium has been investigated using transmission electron microscopy techniques on thin foils. High purity vanadium strip was doped to a carbon content of about 0.2 at. pct and rapidly quenched in high vacuum, followed by aging treatments for various times in the temperature range from 300° to 600°C. The strip was then thinned for transmission electron microscopy. The carbon is observed to precipitate initially as very finely dispersed carbides visible through structure factor contrast. With increasing aging the precipitate distribution coarsens, and the carbides appear as very thin coherent platelets on {310} planes, showing a pronounced displacement fringe contrast. The coherent precipitate appears to have a bcc structure closely related to that of the matrix. With further aging these platelets are observed to thicken and partially lose coherency, punching out prismatic dislocation loops and helices having axes in 〈111〉 directions. This semicoherent precipitate is found to be the hexagonal V₂C phase described by other researchers, and its orientation relationship with the matrix may be expressed: (110)v‖(00.1)v₂c, [•111]v‖[•110] v₂C. The dislocation loops are a result of the specific volume difference between the V₂C precipitation and the matrix, and have Burgers vector b = a/2〈111〉. Formerly Research Assistant, Department of Metallurgy and Mining Engineering, University of Illinois at Urbana-Champaign, Urbana Ill.     

Decagonal quasicrystal and related crystalline phases in Mn−Ga alloys with 52 to 63 a/o Ga

A decagonal quasicrystal (DQC) and six related intermetallic phases with large unit cells have been found in binary Mn−Ga alloys with 52 to 63 at. pct Ga by means of transmission electron microscopy (TEM). As does the Al−Mn DQC, the Ga−Mn DQC also has a periodicity of 1.25 nm along its tenfold axis. However, its Mn content, determined by electron microprobe X-ray analysis (about 45 to 50 at. pct Mn), is much higher than that of the Al−Mn DQC (about 20 to 30 at. pct Mn). The compositions of the intermetallic phases are about 53, 56, 58, and 62 at. pct Ga, corresponding respectively to the unknown structures of MnGa (50.7 to 53.4 at. pct Ga), Mn₅Ga₆ (55 at pct Ga), Mn₅Ga₇ (57.9 at. pct Ga), and Mn₃Ga₅ (62.9 at. pct Ga) given in the binary Mn−Ga phase diagram (Metals Hand-book, T.B. Massalski, J.L. Murray, L.H. Benneft, and H. Baker, eds., ASM, Metals Park, OH, 1986, vol. 2, p. 1144). Their lattice types have been determined by selected area electron diffraction. The ferromagnetic Mn₃Ga₅ is tetragonal, a=1.25 nm and c=2.50 nm; Mn₅Ga₇ is orthorhombic, a=4.57 nm, b=1.25 nm, and c=1.44 nm; Mn₅Ga₆ has two different but closely related orthorhombic unit cells, a=1.26 nm, b=1.25 nm, and c=1.48 nm as well as a=0.77 nm, b=1.25 nm, and c=2.36 nm; MnGa also has two different and related unit cells, one orthorhombic with a=2.04 nm, b=1.25 nm, and c=1.48 nm and the other monoclinic with a=2.59 nm, b=1.25 nm, c=1.15 nm, and β≈=110 deg. All these orthorhombic phases have b=1.25 nm, being the same as the periodicity along the tenfold axis of the Ga−Mn and Al−Mn DQCs. Moreover, all these six intermetallic phases give electron diffraction patterns displaying a pseudo-tenfold distribution of strong diffraction spots and are considered to be crystalline approximants of the Ga−Mn DQC.