Part I. The microstructural evolution in Ti-Al-Nb O+Bcc orthorhombic alloys

Phase transformations and the resulting microstructural evolution of near-Ti₂AlNb and Ti-12Al-38Nb O+bcc orthorhombic alloys were investigated. For the near-Ti₂AlNb alloys, the processing temperatures were below the bcc transus, while, for Ti-12Al-38Nb, the processing temperature was supertransus. Phase evolution studies showed that these alloys contain several constituent phases, namely, bcc, O, and α ₂; when present, the latter was in small quantities compared to the other phases. The transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray investigations of samples that were solutionized and water quenched were used to estimate the phase fields, and a pseudobinary diagram based on Ti=50 at. pct was modified. The aging-transformation behavior was studied in detail. For solutionizing temperatures between 875 °C and the bcc transus, the phase composition and volume fraction of the near-Ti₂AlNb alloys adjusted through relative size changes of the equiaxed B2, O, and α ₂ grains. The aging behavior followed three distinct transformation modes, dependent on the solutionizing and aging temperatures. Widmanstatten formation was observed when a new phase evolved from a parent phase. Thus, Widmanstatten O phase precipitated within the B2 phase for supertransus fully B2 microstructures, as well as for substransus α ₂+B2 microstructures. Similarly, Widmanstatten B2 phase can form from a fully O microstructure, a transformation that has not been observed before. In the case of equiaxed O+B2 solutionized and water-quenched microstructures, Widmanstatten O-phase formation occurred only below 875 °C. For the subtransus-solutionized and water-quenched microstructures, a second aging transformation mode, cellular precipitation, was dominant below 750 °C. This involved formation of coarse and lenticular O phase that grew into the prior B2 grains from the grain boundaries. A third transformation mode involved composition-invariant transformation, where the fully B2 supertransus-solutionized and water-quenched microstructure transformed to a fully O microstructure at 650 °C. This microstructure reprecipitated B2 phase out of the O phase with continued aging time. For Ti-12Al-38Nb, Widmanstatten O precipitation remained the only transformation mode. It is shown that subtransus processing offers flexibility in controlling microstructures through postprocessing heat treatments.     

Part II. The creep behavior of Ti-Al-Nb O+bcc orthorhombic alloys

The intermediate-temperature (650 °C to 760 °C) creep behavior of orthorhombic (O)+bcc alloys containing 50 at. pct Ti was studied. Ti-25A1-25Nb, Ti-23Al-27Nb, and Ti-12Al-38Nb ingots were processed and heat treated to obtain a wide variety of microstructures. Creep deformation mechanisms and the effects of grain size, phase volume fraction, tension vs compression and aging on creep rates were examined. Unaged microstructures, which transformed during the creep experiments, exhibited larger primary creep strains than transformed microstructures, which were crept after long-term aging. The deformation observations and calculated creep exponents and activation energies suggested that separate creep mechanisms, dependent on the applied stress level, were dominating the secondary creep behavior. Coble creep characteristics, including relatively low activation energies and dislocation densities as well as stress exponents close to unity, were exhibited at low applied stresses. Experiments on fiducially marked specimens indicated that grain-boundary sliding was occurring for intermediate applied stresses. In this regime, the minimum creep rates were proportional to the applied stress squared and inversely proportional to the grain size. Dislocation-controlled creep characteristics, including stress exponents of greater than or equal to 3.5 and relatively high activation energies and dislocation densities, were exhibited at high stresses. Overall, the minimum creep rates were dependent on microstructure and stress. Within the low-to-intermediate stress regimes, subtransus processed and heat-treated microstructures, which contained much finer grain sizes than supertransus microstructures, exhibited the poorest creep resistance. The influence of grain size was not as significant within the high-stress regime. It is shown that for low-to-intermediate stress levels, grain size is the dominant microstructural feature influencing the creep behavior of O+bcc alloys.     

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.     

Microscopy and tensile behavior of melt-spun Ni-Al-Fe alloys

Microscopy and room-temperature tensile tests were performed on as-spun and annealed ribbons of Ni-20 (at. pet) Al-Fe alloys containing 20 to 40Fe. The ribbons had the duplex structures consisting of grains of ordered bec β-NiAl and grains of disordered fee γ-Ni, which contains precipitates of γ′-Ni₃Al. The 25 to 30Fe alloys exhibited high ductility (∼10 pet elongation) in both the as-spun and annealed conditions. These results indicate that rapid solidification-induced effects, such as the suppression of ordering, do not enhance ductility as previously reported. The ductile alloys were found to contain high dislocation densities in both they and β grains, with no evidence of stress-induced martensite formation in the β phase. Dislocation analysis revealed that the vast majority of dislocations in theβ had ≤100≥Burgers vectors; however, ≤111≥ dislocations were also observed. Additionally, slip bands were frequently observed meeting at γ-β grain boundaries. Since they tend to align across the interphase grain boundary, deformation transfer between γ and β is inferred. The deformation transfer was found to be facilitated by a specific orientation relationship between the grains. The unusual deformation of ββby ≤111≥ slip and by deformation transfer from neighboring grains may be responsible for the high ductility.     

The surface tension of liquid silver alloys: Part II. Ag−O alloys

The surface tensions of liquid Ag-O alloys have been determined by the sessile drop method. The surface activity of oxygen, as measured by ?(dσ/dX _O)X_O→0_j, where σ is the surface tension of the metal andX _O the mole fraction of oxygen, is quite large and equals 3.80×10⁵ dyne per cm at 980°C and 1.35×10⁵ dyne per cm at 1108°C. The heat of adsorption of oxygen is estimated to be of the order of 30 kcal per mole. Application of the monolayer approximation shows that liquid silver becomes saturated with oxygen when each adsorbed oxygen atom occupies an area of 33±5Ų. Small additions of platinum to silver do not change the characteristics of the adsorption of oxygen appreciably. An analysis of the data is consistent with the conclusion that saturation of the surface of liquid silver with oxygen results from the formation of an ionic two-dimensional compound at the surface. This hypothesis is tested in the case of several other systems and yields satisfactory results. The structure of these compounds is discussed. In the case of the Ag-O system, it appears to correspond to the stoichiometry Ag₃O.     

The formation of Hcp and Bcc phases in austenitic iron alloys

The formation of the hcp (∈) and bcc (α) structures in pure iron under high pressure conditions, as well as the morphological and crystallographic aspects of martensitic transformation to these structures at atmospheric pressure in iron alloys, are reviewed., It is concluded that the unique features of α- or lath martensite formation are not dependent upon the presence of the ∈ phase. Application of the phenomenological theory of martensitic transformation has not successfully rationalized the crystallography of lath-martensite. The criterion for ∈- and α-phase formation is established using the regular solution approximation and appropriate lattice stability parameters. In particular, the ∈ phase can be formed in Fe?Ni?Cr compositions through stress-induced transformation attending α-martensite formation. Further consideration suggests that the ∈→α transformation is not expected at atmospheric pressure at temperatures below approximately 500°K in the alloys considered. Thus, two martensitic transformations, γ→∈ and γ→α, can occur jointly in certain alloys.     

Thermodynamic activities in the alloys of the Ti-Al-Nb system

The vaporization of the solid alloys of the Ti-Al-Nb system has been extensively studied, with emphasis on the α ₂-Ti₃Al, (α ₂-Ti₃Al+γ-TiAl), and γ-TiAl phase fields, by using Knudsen effusion mass spectrometry at temperatures between 1170 and 1635 K. Twenty-four alloy samples of different compositions, were investigated and their Ti and Al partial pressures were determined. The thermodynamic activities of Ti and Al, as well as partial enthalpies and entropies of mixing, were evaluated for the mean temperature of 1473 K. The data obtained are discussed with respect to their significance for high-temperature corrosion of titanium aluminides and the solubility behavior of Nb in the γ-TiAl phase.     

Transformations in α 2+γ titanium aluminide alloys containing molybdenum: Part I. Solidification behavior

The solidification structures of 12 alloys in the Ti-Al-Mo system with Al contents ranging from 44 to 50 at. pct and Mo contents ranging from 2 to 6 at. pct have been characterized metallographically and, for composition gradients, by electron probe microanalysis. All alloys solidify dendritically through sequences that fall into four distinct categories: alloys that solidify completely into the β phase field, alloys that solidify first into an L+β phase field and finally into an α+β phase field through an L+β+γ region, alloys that solidify first into a L+β phase field and finally form α+β+γ structures, and alloys that solidify into a L+α phase field. Postsolidification transformations occur as consequence of transitions from the high-temperature β, α+β, and α+γ phase fields to low-temperature α+β+γ or β+γ region. A variety of phase distributions result, such as Widmanstätten α ₂+B2 structures from the α → α+β reaction followed by ordering, lamellar α ₂+γ structures from the α → α+γ reaction followed by ordering of the α phase, eutectoid B2+γ structures from the α → B2+γ reaction, and platelike γ structures from the β → β+γ reaction. The former are observed in Al lean alloys, while the latter are present in Mo rich alloys.     

Solidification of undercooled Fe-Cr-Ni alloys: Part I. Thermal behavior

Solidification of undercooled Fe-Cr-Ni alloys was studied by high-speed pyrometry during and after recalescence of levitated, gas-cooled droplets. Alloys were of 70 wt pct Fe, with Cr varying from 15 to 19.7 wt pct, balance was Ni. Undercoolings were up to about 300 K. Alloys of Cr content less than that of the eutectic (18.1 wt pct) have face-centered cubic (fee) (austenite) as their equilibrium primary phase, and alloys of higher Cr content have body-centered cubic (bcc) (ferrite) as their equilibrium primary phase. However, except at low undercoolings in the hypoeutectic alloys, all samples solidified with bcc as the primary phase; the bcc then transformed to fcc during initial recalescence for the lower Cr contents or during subsequent cooling for the higher Cr contents. The bcc-to-fcc transformation, whether in the semisolid or solid state, was detected by a second recalescence. In the hypoeutectic alloys, the growth of primary metastable bcc apparently results from preferred nucleation of bcc. The subsequent nucleation of fcc may occur at bcc/bcc grain boundaries. Formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology     

Mechanical behavior of Al-Li-SiC composites: Part I. Microstructure and tensile deformation

The microstructure and tensile properties of an 8090 Al-Li alloy reinforced with 15 vol pct SiC particles were investigated, together with those of the unreinforced alloy processed following the same route. Two different heat treatments (naturally aged at ambient temperature and artificially aged at elevated temperature to the peak strength) were chosen because they lead to very different behaviors. Special emphasis was given to the analysis of the differences and similarities in the microstructure and in the deformation and failure mechanisms between the composite and the unreinforced alloy. It was found that the dispersion of the SiC particles restrained the formation of elongated grains during extrusion and inhibited the precipitation of Al₃Li at ambient temperature. The deformation processes in the peak-aged materials were controlled by the S′ precipitates, which acted as barriers for dislocation motion and homogenized the slip. Homogeneous slip was also observed in the naturally aged composite, but not in the unreinforced alloy, where plastic deformation was concentrated in slip bands. The most notorious differences between the alloy and the composite were found in the fracture mechanisms. The naturally aged unreinforced alloy failed by transgranular shear, while the failure of the peak-aged alloy was induced by grain-boundary fracture. The fracture of the composite in both tempers was, however, precipitated by the progressive fracture of the SiC reinforcements during deformation, which led to the early failure at the onset of plastic instability.     

The effect of lamellar morphology on tensile and high-cycle fatigue behavior of orthorhombic Ti-22Al-27Nb alloy

The room-temperature tensile and high-cycle fatigue (HCF) behavior of orthorhombic Ti-22Al-27Nb alloy with varying lamellar morphology was investigated. Varying lamellar morphology was produced by changing the cooling rate after annealing in the single B2 phase region. A slower cooling rate of 0.003 K/s, for example, resulted in several large packets or colonies of similarly aligned O-phase lamellae and a nearly continuous massive α ₂ phase at the prior B2 grain boundaries, while a faster cooling rate of 0.1 K/s led to the refinement of colony sizes and the O-phase lamellae. The interface of O-phase lamellae and B2 phases was semicoherent. Water quenching produced a very fine tweed-like microstructure with a thin continuous O phase at the prior B2 grain boundaries. The 0.2 pct yield stress, tensile strength, and HCF strength increased with increasing cooling rate. For example, the tensile strength and HCF strength at 10⁷ cycles of 0.003 and 0.1 K/s-cooled were 774 and 450 MPa, and 945 and 620 MPa, respectively. Since the fatigue ratio, which is the ratio of HCF strength at 10⁷ cycles to tensile strength, did not show a constant value, but instead increased with increasing cooling rate, part of the fatigue improvement was the result of improved resistance to fatigue associated with the microstructural refinement of the lamellar morphology. Fatigue failure occurred by the subsurface initiation, and every initiation site was found to contain a flat facet. Concurrent observation of the fatigue initiation facet and the underlying microstructure revealed that the fatigue crack initiated in a shear mode across the colony, irrespective of colony size, indicating that the size of the initiation facet corresponded to that of the colony. Therefore, the colony size is likely a major controlling factor in determining the degree of fatigue improvement due to the microstructural refinement of lamellar morphology. For the water-quenched specimens, fatigue crack initiation appeared to be associated with shear cracking along the boundary between the continuous grain boundary O phase and the adjacent prior B2 grain.     

The influence of coherency strain on the elevated temperature tensile behavior of Ni-15Cr-AI-Ti-Mo alloys

The effect of coherency strain on elevated temperature tensile strength was examined in a model, two-phase y’-strengthened Ni-15Cr-Al-Ti-Mo alloy series. The temperature dependence of coherency strain as represented by the γ-γ’ mismatch was determined over the temperature range 25 to 800 °C. The flow stress incrementAσ _γ, due to precipitation of γ’, was found to correlate well to the magnitude of the γ-γ’ mismatch over the same temperature interval. The correlation was strongest for high misfit alloys regardless of the Antiphase Boundary Energy (APBE). The predominance of by-pass type dislocation-particle interactions in high coherency alloys confirms that strengthening is primarily due to coherency strains. Conversely, alloys with low misfit exhibit two distinct particle shear mechanisms believed to be dependent upon the relative APBE and matrix stacking fault energy of the alloy.     

Solidification of undercooled Fe-Cr-Ni alloys: Part III. Phase selection in chill casting

In Parts I and II of this series of articles, it was shown that a range of levitation-melted Fe-Cr-Ni alloys, both hypoeutectic and hypereutectic, all solidified with the hypereutectic phase (bcc) as their primary phase, except for the hypoeutectic alloys at low undercoolings. In this article, the effect of heat extraction on phase formation is studied by chill casting the undercooled alloys before nucleation. Two of the previously studied alloys are examined; one hypoeutectic and the other hypereutectic. Chill substrates employed were copper, stainless steel, alumina, zirconia, and a liquid gallium-indium bath. Contrary to the case of levitation melting and solidification, it is found that the dominant primary phase to solidify in both alloys, independent of chill substrate, is the hypoeutectic phase (fcc). It is concluded that chilling the undercooled melt results in nearly concurrent nucleation of bcc and fcc. Two different mechanisms are considered as possible explanations of the subsequent fcc phase selection during growth. These are termed “growth velocity” and “phase stability” mechanisms. Evidence favors a phase stability mechanism, in which the bcc phase massively transforms to fcc early in solidification so that fcc then grows without competition. It is suggested that this mechanism may also explain structures observed in welds and other rapid solidification processes.     

Solidification behavior, microstructure and tensile properties of ZK60-Er magnesium alloys

ZK60-Er (erbium) alloys were made by melting ZK60 and Mg-Er magnesium alloys (20 wt.% Er) in an electric resistance furnace. The contents of Er were 0, 0.5, 1, 2, 3 wt.%, respectively. The influence of Er on solidification behavior, microstructure, corrosion resistant and mechanical properties of ZK60 magnesium alloy was studied. The results showed that long rod-like γ phase (ErZn5) formed during solidification increased with increasing Er content in the range investigated, which resulted in the decrease of the amount of galvanic couplings between phase particles and alloy matrix and the marked improvement of corrosion resistant. It was also found that elongation of the alloys decreased with increasing Er content, but tensile strength of the alloys were improved by the addition of Er due to the strengthening effect of γ phases distributing along grain boundaries.     

The surface tension of liquid silver alloys: Part I. Silver-gold alloys

The sessile drop technique has been used to determine the surface tension of silver-gold alloys. The techniques of data reduction are discussed and an improved one is presented. The measurements are in good agreement with previous data reported in the literature for pure silver and gold. The surface tension of alloys at 1108‡C was found to behave according to the prediction of the perfect solution model. Formerly Graduate Student, Department of Metallurgy and Materials Science, Carnegie-Mellon University     

Phase chemistry and precipitation reactions in maraging steels: Part III. Model alloys

This article describes studies of phase transformations during aging in a variety of model maraging steels. Atom-probe field-ion microscopy (APFIM) was the main research technique employed. Thermochemical calculation was also used during the course of the work. The composition and morphology of precipitates were compared in several maraging systems aged at different temperatures for different times to investigate the aging sequence. The APFIM results are compared with studies by other workers using different experimental techniques. In Fe-Ni(-Co)-Mo model alloys, ω phase and Fe₇Mo₆ μ phase have been found to contribute to age hardening at different stages of aging; no evidence was found for the existence of Mo-rich clusters in the as-quenched Fe-Ni-Co-Mo alloy. In a high-Si Cr-containing steel, Ti₆Si₇Ni₁₆ G phase and Ni₃Ti have been found to contribute to age hardening; reverted austenite was found after aging for 5 hours at 520 °C. In a Mn-containing steel, Fe₂Mo Laves phase and a structurally uncertain phase with a composition of Fe₄₅Mn₃₂Co₅Mo₁₉ have been found to contribute to age hardening. Formerly Graduate Student with the Department of Materials, Oxford University.     

Structure and properties of rapidly solidified dispersion strengthened titanium alloys: Part II. tensile and creep properties

The effects of incoherent dispersoids on tensile and creep properties were determined in rapidly solidified Ti-Er and Ti-Nd alloys. Uniform distributions of. fine incoherent dispersoids in Ti matrix were produced by rapid solidification at cooling rates > 10³ °C per second and subsequent annealing at 700 to 800°C of Ti-1.0Er, Ti-2.0Er, Ti-1.5Nd, and Ti-3.0Nd alloys. The rapidly solidified particulates consolidated by vacuum hot pressing were isothermally forged, rolled, and annealed to produce fully recrystallized microstructures. The incoherent dispersoids in Ti-Er and Ti-Nd alloys increase by 40 to 110 pct the yield strength and ultimate tensile strength of Ti with no significant loss in ductility. The strength increments were analyzed in terms of the superposition of dispersion-, solid solution-, and fine grain-strengthening. Dispersion strengthening is offset to some extent by the reduction in interstitial oxygen solid solution strengthening caused by the scavenging of oxygen by Er and Nd. The dispersoids decrease the creep rates and increase the stress rupture lifetimes of Ti at 482 to 700 °C.