Supercooling effects in Cu-10 Wt Pct Co alloys solidified at different cooling rates

Electromagnetic levitation and electron beam surface melting were employed to study the effects of supercooling and cooling rate on the solidification of Cu-10 wt pct Co alloys. Two major effects were observed in the supercooled alloys: the nucleation of a metastable copper-rich phase which contains 13 wt pct to 20 wt pct Co in samples supercooled between 105 and 150 K and liquid phase separation which occurs in samples supercooled 150 K or more. The microstructure of the electron beam melted surfaces consisted of very fine spheres which were similar to those of the sample supercooled more than 150 K but with a refined microstructure. The results indicate that a dynamic bulk supercooling of 150 K may exist in the molten pool during the solidification of electron beam melted surfaces.     

Effect of the primary phase on grain coarsening in undercooled Fe-Co alloys

Fe-Co alloy melts with Co contents of 10, 30, and 60 at. pct were undercooled to investigate the dependence of the primary phase on grain coarsening. A pronounced characteristic is that the metastable fcc phase in the Fe-10 at. pct Co alloy and the metastable bcc phase in the Fe-30 at. pct Co alloy will primarily nucleate when undercoolings of the melts are larger than the critical undercoolings for the formation of metastable phases in both alloys. No metastable bcc phase can be observed in the Fe-60 at. pct Co alloy, even when solidified at the maximum undercooling of ΔT = 312 K. Microstructural investigation shows that the grain size in Fe-10 and Fe-30 at. pct Co alloys increases with undercoolings when the undercoolings of the melts exceed the critical undercoolings. The grain size of the Fe-60 at. pct Co alloy solidified in the undercooling range of 30 to 312 K, in which no metastable phase can be produced, is much finer than those of the Fe-10 and Fe-30 at. pct Co alloys after the formation of metastable phases. The model for breakage of the primary metastable dendrite at the solid-liquid interface during recalescence and remelting of dendrite cores is suggested on the basis of microstructures observed in the Fe-10 and Fe-30 at. pct Co alloys. The grain coarsening after the formation of metastable phases is analyzed, indicating that the different crystal structures present after the crystallization of the primary phase may play a significant role in determining the final grain size in the undercooled Fe-Co melts.     

Determination of the Fe−Ni phase diagram below 400°C

The phase diagram for the Fe−Ni system below 400°C has been determined experimentally in the composition range from 0 to 52 wt pct Ni using analytical electron microscopy techniques. High spatial resolution X-ray microanalysis and electron diffraction were conducted on the Fe−Ni regions of meteorites. Both stable and metastable phase boundaries were defined. Our phase diagram is consistent with the available theoretical diagram in that firm experimental evidence was found for a miscibility gap and an associated, asymmetrical spinodal decomposition region. The spinodal decomposition resulted in a two-phase, isotropic microstructure, as expected. The miscibility gap is a metastable construction arising from the presence of a tricritical point due to magnetic interactions. Our experimental diagram differs from the theoretical diagram in three ways. First, observations of meteorite structures show that Fe−Ni solid solution containing 4.0 wt pct Ni is in local equilibrium with ordered FeNi containing 51.4 wt pct Ni and not Ni₃Fe as in the theoretical diagram. Second, our miscibility gap below 400°C, located between 11.7 and 51.9 wt pct Ni at 200°C, is wider than the calculated miscribility gap, especially at the high Ni end. Third, we also find evidence for an ordered structure around ∼25 wt pct Ni. This structure may be either Fe₃Ni or a two-phase structure incorporating ordered FeNi.     

Phase stability and yield stress of Ni-base superalloys containing high Co and Ti

Ni-base superalloys containing high Co (>20 wt pct) and Ti (>5.5 wt pct) were designed in order to study the effects of Co16.9 wt pct Ti addition on phase stability and mechanical property. These new alloys, though they contained high Ti, mainly consisted of γ and γ′ phases. Ni₃Ti (η) phase was observed along the grain boundaries in some of the alloys. The formation of η phase was mainly related to the Ti/Al ratio, Ti content, and alloy composition. Tensile and compression tests showed that these new alloys exhibited higher yield stress than that of the baseline alloy, TMW-1(U720LI). The possible strengthening mechanisms were discussed in terms of solid-solution and precipitation strengthening effects by the Co16.9 wt pct Ti additions. Preliminary results show promising trends for the development of new superalloys for turbine disc applications.     

The effect of processing and microstructure development on the slip and fracture behavior of the 2.1 wt pct Li AF/C-489 and 1.8 wt pct Li AF/C-458 Al-Li-Cu-X alloys

Although Al-Li-Cu alloys showed initial promise as lightweight structural materials, implementation into primary aerospace applications has been hindered due in part to their characteristic anisotropic mechanical and fracture behaviors. The Air Force recently developed two isotropic Al-Li-Cu-X alloys with 2.1 wt pct Li and 1.8 wt pct Li designated AF/C-489 and AF/C-458, respectively. The elongation at peak strength was less than the required 5 pct for the 2.1 wt pct Li variant but greater than 10 pct for the 1.8 wt pct Li alloy. The objectives of our investigations were to first identify the mechanisms for the large difference in ductility between the AF/C-489 and AF/C-458 alloys and then to develop an aging schedule to optimize the microstructure for high ductility and strength levels. Duplex and triple aging practices were designed to minimize grain boundary precipitation while encouraging matrix precipitation of the T₁ (Al₂CuLi) strengthening phase. Certain duplex aged conditions for the AF/C-489 alloy showed significant increases in ductility by as much as 85 pct with a small decrease of only 6.5 and 2.5 pct in yield and ultimate tensile strength, respectively. However, no significant variations were found through either duplex or triple aging practices for the AF/C-458 alloys, thus, indicating a very large processing window. Grain size and δ′ (Al₃Li) volume fraction were determined to be the major cause for the differences in the mechanical properties of the two alloys.     

Mutual effects of hydrogenation and deformation in Ti-Nb alloys

Alloying of Ti-based alloys with hydrogen is used to modify the microstructure and improve mechanical properties. In this study, hydrogen charging was performed electrochemically in a 2:1 glycerin-phosphoric acid electrolyte at high fugacities. This research investigated in detail, by means of X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermal desorption spectroscopy (TDS), and microhardness tests, the influence of hydrogen at high fugacities on the phase stability, desorption behavior, and microhardness in Ti-Nb (20 to 45 wt pct Nb) alloys before and after deformation. Hydrogenation of Ti-Nb was found to exhibit a significant effect on the phase stability as well as the microhardness of Ti-Nb alloys. Hydrogenation of Ti-20 wt pct Nb led to precipitation of (Ti,Nb)H _x in the metastable α″ matrix. In Ti-Nb alloys with 40 or 45 wt pct niobium, hydrogen stabilized the bcc β phase, but destabilized the hcp ω phase. With increasing hydrogen content, an expansion of the lattice constant of the β phase occurred, followed by the formation of (Ti,Nb)H _x . The influence of hydrogen on the microhardness of Ti-40 wt pct Nb and Ti-45 wt pct Nb shows only a minor effect, whereas Ti-20 wt pct Nb exhibits significant softening in response to hydrogen charging up to 3000 ppm. The TDS showed that deuterium desorption strongly depends on the niobium content and the deformation treatment prior to or after charging. The observed results should provide further insight on the mutual effects and the resulting micromechanism of hydrogenation and deformation in Ti-Nb alloys. 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.     

Precipitation in rapidly solidified Al-Mn alloys

Precipitation at 450 °C was studied in melt-spun ribbons containing up to 15 wt pct Mn in solid solution in Al. The as-spun ribbons were microsegregation-free at compositions up to 5 wt pct Mn, but in more concentrated alloys a cellular microstructure was present. Upon annealing, four precipitate phases are observed, some of them being found preferentially on cell boundaries and others being found within the cells. Al₆Mn, G, and the Gℍ phase can coexist for long times at 450 °C, but the G phase appears to be slightly more stable. A less stable T phase was detected in Al-5 wt pct Mn foils following short annealing periods. The supersaturation of the Al matrix can persist for many hours in alloys containing up to 3 wt pct Mn, but is essentially gone after 1 hour in alloys with 5 wt pct Mn or more. On leave at the Center for Materials Research, The Johns Hopkins University, Baltimore, MD 21218. R.J.     

Microstructure and its development in Cu-Al-Ni alloys

The microstructure of as-cast Cu-AI-Ni alloys, based on copper containing 9 to 10 wt pct Al and up to 5 wt pct Ni, has been examined. The development of the microstructure on continuous cooling has also been investigated. For alloys with 9.2 to 9.3 wt pct Al, and less than 1 wt pct Ni, the as-cast microstructure consists of proeutectoid α solid solution, α + γ₂ eutectoid, and martensitic β. If the nickel content is more than 2.5 wt pct, the α + γ₂ eutectoid is replaced by α + β ₂ ^′ eutectoid, and no martensitic β is observed in the as-cast alloys. The morphologies of the β ₂ ^′ and γ₂ eutectoid phases are similar; both have the Kurdjumov-Sachs (K-S) orientation relationship with the a phase. Two eutectoid reactions, involving β to α + γ₂ and β to α + β′₂, have been observed in an alloy containing 9.7 wt pct Al and 2.7 wt pct Ni. When both eutectoid reactions occur, the Nishiyama-Wassermann (N-W) orientation relationship exists between γ₂ or β ₂ ^′ and the α phase. During continuous cooling, proeutectoid α solid solution is the first phase to precipitate from the high-temperature β phase. The β to α + β ₂ ^′ eutectoid reaction starts at higher temperatures than the β to α + γ₂ reaction. Tempering of the as-cast alloys results in the elimination of the martensitic β. Y.S. SUN formerly Research Associate with the Manchester Materials Science Centre.     

The impact of cooling rates on the microstructure of Al-U alloys

The impact of cooling rates on the microstructure of Al-U alloys was studied by optical, scanning electron, and transmission electron microscopy. A variety of solidification techniques were employed to obtain cooling rates ranging between 3 × 10⁻² and 10⁶ K/s. High-purity uranium (99.9 pct) and high-purity aluminum (99.99 pct), or “commercially pure” type Al-1050 aluminum alloys were used to prepare Al-U alloys with U concentration ranging between 3 and 22 wt pct. The U concentration at which a coupled eutectic growth was observed depends on the cooling rates imposed during solidification and ranged from 13.8 wt pct for the slower cooling rates to more than 22 wt pct for the fastest cooling rates. The eutectic morphology and its distribution depends on the type of aluminum used in preparing the alloys and on the cooling rates during solidification. The eutectic in alloys prepared from pure aluminum was evenly distributed, while for those prepared from Al-1050, the eutectic was unevenly distributed, with eutectic colonies of up to 3 mm in diameter. Two lamellar eutectic structures were observed in alloys prepared from pure aluminum containing more than 18 wt pct U, which solidified by cooling rates of about 10 K/s. One structure consisted of the stable eutectic between UAl₄ and Al lamella. The other structure consisted of a metastable eutectic between UAl₃ and Al lamella. At least three different eutectic morphologies were observed in alloys prepared from Al-1050.     

X-ray study of the decomposition of metastable Al-rich Al-Fe solid solutions

The extension of the solid-solubility limit obtained in Al-Fe alloys up to 4.4 at. pct Fe by rapid quenching from the melt and the appearance of metastable Al₆Fe phase for 4 to 20 wt pct Fe have been reported in the past. Present experiments with isochronal and isothermal annealing of Al-Fe solid solutions containing 3.6 at. pct Fe have shown that the solutions are very stable at room temperature, and less stable at 200°C, while at 300°C they decompose very rapidly and a metastable Al₆Fe phase appears. This phase is replaced by Al₃Fe after annealing for 10 min at 550°C or for 16 hr at 400°C. The Al₆Fe phase remains stable for 76 hr at 300°C, while taking more than 670 hr to be replaced by Al₃Fe. The author discusses the operation of the “two-piston” quenching apparatus and concludes that the sample thickness cannot serve as a sufficient criterion for quenching effectiveness.     

The role of thermal stabilization in martensite transformations

The variation of the kinetics of the martensite transformation with carbon content and martensite habit plane has been investigated in several Fe−Ni based alloys. Transformation in an Fe-25 wt pct Ni-0.02 wt pct C alloy exhibits predominantly athermal features, but some apparently isothermal transformation also occurs. In a decarburized alloy, on the other hand, the observed kinetic features, such as the dependence ofM _s on cooling rate, were characteristic of an isothermal transformation. In contrast, Fe-29.6 wt pct Ni-10.7 wt pct Co alloys with carbon contents of 0.009 wt pct C and 0.003 wt pct C transform by burst kinetics to {259}_γ plate. At both these carbon levels, theM _b temperatures of the Fe−Ni−Co alloys are independent of cooling rate. It is proposed that the change in kinetic behavior of the Fe-25 pct Ni alloy with the different carbon contents is due to the occurrence of dynamic thermal stabilization in the higher carbon alloy. Dynamic thermal stabilization is relatively unimportant in the Fe−Ni−Co alloys which transform by burst kinetics to {259}_γ plate martensite. P. J. FISHER, formerly with the University of New South Wales D. J. H. CORDEROY, formerly with the University of New South Wales     

Effect of phosphorus on the microstructure and stress rupture properties in an Fe-Ni-Cr base superalloy

This article describes the effect of phosphorus on the microstructure and stress rupture property at 650 °C in an Fe-Ni-Cr base superalloy. The results showed that phosphorus markedly improved the intergranular precipitation in the range of 0.0005 to 0.016 wt pct, which facilitated M₂₃C₆ and M₃B₂ precipitation but inhibited the formation of MC carbide. A too high phosphorus addition (0.051 wt pct P) resulted in an excessive precipitation at grain boundaries, while a too low phosphorus content (0.0005 wt pct P) led to many precipitate-free grain boundaries. Phosphorus also enlarged the size of the γ′ particles and lowered its stability, that η-Ni₃Ti preferred to form in the alloy with 0.051 wt pct P. Due to the improvement of the microstructure, appropriate amount of P content significantly prolonged the rupture life of the alloys in the range of 0.0005 to 0.016 wt pct. The peak value was 660 hours at 0.016 wt pct, more than 4 times that of the alloy with 0.0005 wt pct phosphorus, but phosphorus reduced the fracture elongation. The mechanism by which phosphorus influenced the alloy is discussed.     

The microstructural,mechanical, and fracture properties of austenitic stainless steel alloyed with gallium

The mechanical and fracture properties of austenitic stainless steels (SSs) alloyed with gallium require assessment in order to determine the likelihood of premature storage-container failure following Ga uptake. AISI 304 L SS was cast with 1, 3, 6, 9, and 12 wt pct Ga. Increased Ga concentration promoted duplex microstructure formation with the ferritic phase having a nearly identical composition to the austenitic phase. Room-temperature tests indicated that small additions of Ga (less than 3 wt pct) were beneficial to the mechanical behavior of 304 L SS but that 12 wt pct Ga resulted in a 95 pct loss in ductility. Small additions of Ga are beneficial to the cracking resistance of stainless steel. Elastic-plastic fracture mechanics analysis indicated that 3 wt pct Ga alloys showed the greatest resistance to crack initiation and propagation as measured by fatigue crack growth rate, fracture toughness, and tearing modulus. The 12 wt pct Ga alloys were least resistant to crack initiation and propagation and these alloys primarily failed by transgranular cleavage. It is hypothesized that Ga metal embrittlement is partially responsible for increased embrittlement.     

Evolution of microstructure and tensile strength of rapidly solidified Al-4.7 pct Zn-2.5 pct Mg-0.25 pct Zr-X wt pct Mn alloys

Analytical transmission electron microscopy and thermal analysis of as-extruded Al-4.7 pct Zn-2.5 pct Mg-0.2 pct Zr-X wt pct Mn alloys, with Mn contents ranging from 0.5 to 2.5 wt pct, were carried out to elucidate the microstructural change and accompanying mechanical properties during subsequent heat treatments. The as-extruded alloy was fabricated from rapidly solidified powder and consisted of a fine, metastable manganese dispersoid and the ternary eutectic T phase (Al₂Mg₃Zn₃). Solution heat treatment resulted in the formation of the stable Al₆Mn phase and complete dissolution of the T phase. Formation of stable Al₆Mn was made by two routes: by phase transition from metastable Mn dispersoids which already existed, and from the supersaturated solid solution by homogeneous nucleation. The density of the Al₆Mn phase increased with the addition of manganese, while the shape and average size remained unchanged. A significant increase in the hardness was observed to coincide with the formation of the Al₆Mn phase. Similarly, the tensile strength increased further after the aging treatment, and the increment was constant over the content of Mn in the alloy, which was explained by the contribution from the same amount of precipitates, MgZn₂. Results of thermal analysis indicated that the dissolution of the T phase started near 180 °C and that formation of Al₆Mn occurred at about 400 °C, suggesting that further enhancement of strength is possible with the modification of the heat-treatment schedule.     

Microstructure and yield strength of UDIMET 720LI alloyed with Co-16.9 Wt Pct Ti

We proposed a new method for developing Ni-base turbine disc alloy for application at temperatures above 700 °C by mixing a Ni-base superalloy U720LI with a two-phase alloy Co-16.9 wt pct Ti in various contents. The microstructure and phase stability of the alloys were analyzed using an optical microscope, a scanning electron microscope, energy-dispersive spectroscopy, and an X-ray diffractometer. The yield strength was studied by compression tests at temperatures ranging from 25 °C to 1200 °C. The results show that all the alloys had a dendritic structure. Ni₃Ti (η) phase was formed in the interdendritic region in the alloys with the addition of Co-16.9 wt pct Ti, and its volume fraction increased with the increase in the addition of Co-16.9 wt pct Ti. The results of exposure at 750 °C show that the addition of Co-16.9 wt pct Ti to U720LI had a great effect on suppressing the formation of σ phase due to the reduced Cr content in the γ matrix. Compared to U720LI, the alloys with the addition of Co-16.9 wt pct Ti possessed higher yield strength. The solid-solution strengthening of γ and γ′ and higher volume fraction of γ′ were assumed to cause this strength increase.     

Effect of Co Addition on the Microstructure, Martensitic Transformation and Shape Memory Behavior of Fe-Mn-Si Alloys

The effect of Co addition has been studied in Fe-30Mn-6Si-xCo (x = 0 to 9 wt pct) shape memory alloys in terms of their microstructure, martensitic transformation and shape recovery. Microstructural investigations reveal that in Fe-Mn-Si-Co alloys, the microstructure remains single-phase austenite (??) up to 5 pct Co and beyond that becomes two-phase comprising ?? and off-stoichiometric (Fe,Co)₅Mn₃Si₂ intermetallic ??-phases. The forward ??-?? martensite transformation start temperature (M _S) decreases with the addition of Co up to 5 pct, and alloys containing more than 5 pct Co, show slightly higher M _S possibly on account of two-phase microstructure. Unlike M _S, the ??-?? reverse transformation start temperature (A _S) has been found to remain almost unaltered by Co addition. In general, addition of Co to Fe-Mn-Si alloys deteriorates shape recovery due to decreasing resistance to plastic yielding concomitant with the formation of stress induced ?? martensite. However, there is an improvement in shape recovery beyond 5 pct Co addition, possibly due to the strengthening effect arising from the presence of (Fe,Co)₅Mn₃Si₂ precipitates within the two-phase microstructure and due to higher amount of stress induced ?? martensite.     

Creep and microstructure of magnesium-aluminum-calcium based alloys

This article describes the creep and microstructure of Mg-Al-Ca-based magnesium alloys (designated as ACX alloys, where A stands for aluminum; C for calcium; and X for strontium or silicon) developed for automotive powertrain applications. Important creep parameters, i.e., secondary creep rate and creep strength, for the new alloys are reported. Creep properties of the new alloys are significantly better than those of the AE42 (Mg-4 pct* Al-2 pct RE**) alloy, which is the benchmark creep-resistant magnesium die-casting alloy. Creep mechanisms for different temperature/stress regimes are proposed. A ternary intermetallic phase, (Mg,Al)₂Ca, was identified in the microstructure of the ACX alloys and is proposed to be responsible for the improved creep resistance of the alloys. All concentrations in wt. pct, unless otherwise stated. RE stands for a combination of rare earth elements, i.e., misch metal, in this case.     

Microstructural characterization of rapidly solidified Al-Ta alloys

Two Al-rich Al-Ta alloys containing by weight 3 and 6 pct Ta have been rapidly solidified from the melt using the ‘gun’ technique. The microstructures and the crystal structures of the phases in the as-solidified as well as those formed on subsequent decomposition of the supersaturated solid solution have been characterized. A supersaturated solid solution was obtained in both the alloys in the as-solidified condition indicating a solid solubility extension of Ta in Al to almost 6 wt pct. The supersaturated solid solutions formed in both the alloys have been found to be quite stable up to 673 K (for 1 hour). Annealing at higher temperatures resulted in the formation of rod-shaped precipitates inside the grains and massive precipitates along grain boundaries. The rod-shaped precipitates arranged in a regular pattern constitute a new metastable intermediate phase Al₇Ta having an ordered structure. The massive precipitates which form along grain boundaries constitute the equilibrium Al₃Ta phase with a tetragonal crystal structure. The transformation behavior and the morphology of the transformation products are detailed in this paper.