Solvus Line Research Articles

Thermodynamics of the massive transformation

The thermodynamic nature of the massive transformation may be revealed by finding the limiting conditions for the massive mode of growth. A review of the experimental information available indicates strongly that the solvus line rather than theT0 line is the natural limit. Some observations of massive transformation inside the two-phase field may be explained by the effect of coherency strains in the composition spike formed in front of the interface. This spike has a significant thickness in many cases and can explain the role of the solvus. It is exceedingly thin in iron-base alloys, and the role of the solvus may there be explained by higher diffusivity inside the interface. A comparison is made with the transformation of γ in Fe-M-C alloys where the situation is very similar.

An AEM investigation of the formation of precipitate-free zones in an AI-16 Wt Pct Ag alloy: Determination of equilibrium and metastable solvus lines

Precipitation of the equilibrium y-phase at grain boundaries in an aged Al-16 wt pct (∼4.5 at. pct) Ag alloy is responsible for the formation of a solute depleted region, free of metastable Guinier-Preston (GP) zones, in the immediate vicinity of the grain boundary. Solute concentration profiles have been obtained across these grain boundary regions using Analytical Electron Microscopy (AEM) in thin foils of the alloy aged at low temperatures. The Ag concentrations at the grain boundary (between γ precipitates) and at the edge of the precipitate-free zone have been used to determine the solid solubility of Ag in Al (the α/(α + γ) solvus) and the metastable GP zone solvus line, respectively.

Precipitation and Solid Solubility in the System NiO‐Cr2O3

The precipitation of NiCr2O4 and the solid solubility of Cr3+ in NiO were investigated. Homogeneous precipitation occurs in this system, and the decomposition of NiO‐Cr2O3 solid solution involves only one stable equilibrium phase, NiCr2O4. The small lattice misfit between precipitate and matrix results in the formation of randomly distributed spherical precipitates about 3 to 10 nm in diameter in the initial stage of growth. Above that size, precipitates form as ellipsoidal plates lying on (100) with all directions of the precipitate and matrix mutually parallel. Strain‐energy effects provide a rational explanation for the observed precipitate morphology during nucleation and subsequent growth. The precipitates grow along 100 to a size at which coherency is no longer maintained. The loss of coherency is associated with the development of interfacial dislocations, and the critical size at which coherency is lost is ∼300 nm. Dissolution of coherent precipitates and growth of semicoherent precipitates was directly observed to occur si multaneously when both are present. The NiO‐NiCr2O4 two‐phase boundary (the NiO(Cr)(ss) solvus line) was experimentally determined between 950° and 1150°C. Concentration profiles measured at two‐phase interfaces indicate that the growth of the NiCr2O4 phase is controlled by volume diffusion. In the present study, interface concentrations obtained from coherent precipitates are indistinguishable from those obtained from semicoherent precipitates. The maximum solubility of Cr in NiO was found to be 0.98°0.10 at 950°, 1.80°0.15 at 1050°, and 3.60°0.31 at 1150°C.

In situ precipitation of θ on A1 in the Al-Cu system

Precipitation in the Cu-Al system can be in several forms, some of which, such as the θ ", θ', and θ phases of CuAl2, have been extensively investigated by both X-ray and TEM methods (see e.g. 1). In general, the precipitation sequence has been systematically controlled by ageing supersaturated solutions of Cu in Al for specific times at temperatures chosen to lie within the upper and lower solvus lines of the required phase. Specimens for TEM were then prepared by standard techniques. In cases where precipitation was carried out in situ, using a hot-stage in the TEM, nucleation at the foil surfaces has appeared to play a prominent role, and precipitation within the foil is, for the most part, suppressed. In this investigation an additional feature is incorported, namely surface diffusion of Cu into a pure Al film, which then leads to the formation of CuAl2 θ-phase precipitates.

The aging response of zirconium-tin alloys

The potential for precipitation hardening in binary Zr-Sn alloys has been examined. Transmission and scanning electron microscopy, hardness measurements, optical microscopy and X-ray diffraction were used to study alloys with the following nominal compositions: (a) 1.5%, 5%, 7% and 9% Sn, produced from Zr of iodide purity, and (b) 8% Sn, made from sponge purity Zr. Following solution treatments in the α- or β-phases, little or no precipitation response was observable under most aging conditions, including times for up to 1000 h at temperatures in the range 700 K to 1000 K. The only evidence for significant precipitation was in Zr-8% Sn, where recrystallization was induced by aging material that had been β-quenched and then cold-worked. With this treatment, precipitation proceeded in a discontinuous manner by the formation of particles, probably Zr 4Sn, at the grain boundaries during recrystallization. The resulting volume fraction was more than an order of magnitude less than expected from the phase diagram. It is concluded that the solvus line for the α-Zr(Sn) solid solution may be incorrect and the solubility of Sn is higher than previously believed. In addition, it is evident that the diffusional kinetics for phase decomposition are very sluggish. The potential for precipitation strengthening in Zr-rich Zr-Sn alloys by thermal aging is poor.

Determination of the Fe-Ni and Fe-Ni-P phase diagrams at low temperatures (700 to 300 °C)

The α + γ two-phase fields of the Fe-Ni and Fe-Ni (P saturated) phase diagrams have been determined in the composition range 0 to 60 wt pet Ni and in the temperature range 700 to 300 °C. The solubility of Ni in (FeNi)3P was measured in the same temperature range. Homogeneous alloys were austenitized and quenched to form α2, martensite, then heat treated to formα (ferrite) + γ (austenite). The compositions of the α and γ phases were determined with electron microprobe and scanning transmission electron microscope techniques. Retrograde solubility for the α/(α + γ) solvus line was demonstrated exper-imentally. P was shown to significantly decrease the size of the α + γ two-phase field. The maximum solubility of Ni in α is 6.1 ± 0.5 wt pct at 475 °C and 7.8± 0.5 wt pct at 450 °C in the Fe-Ni and Fe-Ni (P saturated) phase diagrams, respectively. The solubility of Ni in α is 4.2 ± 0.5 wt pct Ni and 4.9 ± 0.5 wt pct Ni at 300 °C in the Fe-Ni and Fe-Ni (P saturated) phase diagrams. Ternary Fe-Ni-P isothermal sections were constructed between 700 and 300 °C.

Interpretation of the solvus line for metal-hydrogen systems

Interpretation of the solvus line for metal-hydrogen systems

The constitution of copper-hardened samarium-cobalt permanent magnets

A study has been made of the constitution of copper-hardened samarium-cobalt alloys lying between SmCo 5 and Sm 2Co 17 and containing up to 20 at.% copper. It is found that copper exerts a strong influence; the 2:17 phase is repressed in favour of the 1:5 phase with the peritectic line changing to a eutectic trough. The solvus line of the 1:5 phase is displaced towards the 2:17 phase in the composition range studied. SmCu 5 and the 1:5 phase are mutually soluble above the latter's eutectoid transformation temperature, which falls as the copper level increases. Finally, the cobalt solid solution comes into direct equilibrium with a wide range of composition of the 1:5 phase in the ternary alloys (in the region of 800°C).

A resistometric investigation of the decomposition kinetics of Al-21·8 at.% Zn solid solution at 276 °C

Under the condition of nearly equilibrium concentration of vacancies, time dependence of the amount of isothermal transformation given byy=δR/δRf was investigated whereδRf is the total structural change of resistivity on completion of the whole process andδR is the measured resistivity change. The investigation was done on the 21·8 at.% (40·3wt.%) Zn alloy under the condition of relatively low supersaturation of a few degrees centigrade below the metastable α′R solvus line. The total transformation involves four kinetic stages: the first two stages correspond probably to diffusion-controlled growth of the α′R particles from the supersaturated solid solution and to the ripening of these particles till their conversion to the cubic α′ phase takes place. The last two kinetic stages account analogously for the α′ particles growth and ripening. Both α′R and α′ phases were identified by the transmission electron microscopy. When separating the individual stages, the approximation byy=1−exp [−(mt)n] of the amount of transformationy was used. The approximation allowed to get the starting values of both the time and the change of the structural part of the electrical resistance for individual stages and also to derive the parametersmi, ni which had to be redetermined for the individual separated stages. These data made it possible to synthetize the experimental curves ofδR andy vs. time for the total transformation.

A Study of the Stability of Intermediate Precipitates in an AI–4·07 wt.-%Cu Alloy Using Electrical-Resistivity Measurements

Abstract A new observation is reported on the variation of the electrical resistivity of a quenched Al–4·07 wt.-%Cu alloy that has been aged at T 1 (in the range 110–200° C) and then up-quenched about 20° C to T 2. If sufficient time has elapsed at T 1a maximum is attained in the relation between resistivity and time at T 2 with a height commonly < 5 × 10–9 Ω cm. It is concluded that the resistance maximum arises from re-solution of intermediate precipitates which may be G.P.[1], “or”, when conditions are such that the operative solvus line is crossed during up-quenching. It is difficult to perform a meaningful quantitative analysis of the data in terms of the position of the operative solvus because the resistivity measurements cannot yield the vital information about the local solute concentration round the small proportion of intermediate precipitates, estimated to involve about 0·3% of the total copper content of the alloy, that revert into solution. No re-solution of could be observed by hot-stage tr...

Metastable and equilibrium phases in dilute V-N alloys

Alloys of vanadium containing 0 to 10 at. pct nitrogen were prepared and aged at temperatures from 300 to 950°C. The nitrogen solvus line was determined and the discontinuities are discussed in terms of the phases coexisting with the α-phase at various temperatures. Aging temperatures greater than 550°C produced only V3N in equilibrium with the α-phase. Aging at 550°C and below resulted in a sequence of transformations beginning with the precipitation of V16N from supersaturated α-phase. This transformation was followed by the decomposition of the V16N into two other metastable phases, termed VxN and VγN. Longer aging times at 550°C resulted in dissolution of VγN and resultant large discs of V*N precipitate. Further aging yielded V3N. Except for the α-phase, the phases involved in this sequence have nitrogen atoms which are ordered. The crystal structures of both the vanadium atoms and the nitrogen atoms in VxN and VγN are presented and discussed. VN is shown to be bet while VγN is bcc with respect to the vanadium atoms. The morphology of the precipitates is described and discussed in terms of their crystal structures.

The X-ray diffraction and electron microscopic investigation of stable and metastable equilibria in Al-rich Al-Zn alloys

The solvus line for the metastable cubic α′-phase and for the stable α- and α′-solubility limit for Al-Zn alloys with zinc content from 15 to 50 wt.% were determined. X-ray diffraction on single crystals was used to study the structures of phases developed during the applied heat-treatments. The dissolution of α′-phase at temperatures close below and above the determined solvus temperatures was observed by the transmission electron microscopy. The shift of the known (α+α′)-phase boundary to lower temperatures of about 8 °C was found. Similarly, small changes were obtained in the shape of the curve giving the solid solubility limit of Zn in Al.

Solvus line of the metastable rhombohedral phase in the Al-Zn system for Al-rich alloys

The X-ray diffraction on single crystals and on coarse-grained foils of Al-Zn alloys was used to determine the solvus temperatures of the metastable rhombohedral α′R-phase. The dissolution of α′R-phase was studied simultaneously by application of DTA technique. Results obtained from different heat-treatments of the investigated alloys were critically interpreted in terms of the nucleation theory. The dissolution of large α′R-particles was necessary to obtain the correct α′R-solvus line unaffected by the reversion of the solid solution below the solvus temperatures.

The solubility and trapping of hydrogen in vanadium

Solid solutions of hydrogen in vanadium are affected greatly by the presence of other interstitials carbon, nitrogen and oxygen. The solutions depart drastically from randomness at low temperatures and the solvus line is pushed to higher equilibrium hydrogen concentration as the concentration of these interstitials is increased. Utilization of internal friction and resistivity measurements permits this effect to be measured quantitatively. Two explanations are examined to interpret the results: (1) an effect of mechanical constraint of the matrix on the precipitate which acts to retard its growth. (2) an effect of clusters of hydrogen with C, N or O to alter the equilibrium concentration of hydrogen in the free lattice, thereby altering the effective supersaturation of hydrogen. The first is considered to be inadequate to explain the full magnitude of the effect. The second can explain the effect in detail if the multi-atom clusters are assumed to exist in relatively high concentration, that is, if the binding energy per hydrogen atom is very much greater than kT.

Decomposition of the α-phase in titanium-rich Ti-Al alloys

Decomposition of the α-phase in Ti-Al alloys in the range 6.8 to 19.2 at. pct Al was investi-gated using electrical resistivity measurements and transmission electron microscopy. The solvus line between the α and α+ α2 regions was found to be somewhat higher than that re-ported by Blackburn. Short range ordering both above and below the solvus in 6.8 and 10.0 pct Al alloys was indicated by the resistivity measurements. In alloys containing 14.2 to 19.2 pet Al the precipitation process follows a C-curve behavior with a maximum rate at about 600°C.

Activities and phase boundaries in the Cr-Ni system using a solid electrolyte technique

The solvus lines in the Cr-Ni phase diagram have been determined with a solid electrolyte technique. Also, the solidus and liquidus lines have been calculated by thermodynamic methods and these are compatible with the solid solution phase boundaries which were determined by experiment. Some features of this study correlate quite well with previous investigations. The resultant diagram substantiates the one proposed by Hansen. The solvus lines have been more accurately positioned by these experiments.

Precipitation in LiF crystals doped with MgF2

X-ray, and optical and electron microscopy studies have been made of precipitation in single crystals of LiF doped with MgF2. MgF2 was found to nucleate only with difficulty, which probably accounts for the large MgF2 precipitates that were observed. However, a metastable phase was discovered which readily nucleated, and, at low concentrations (e.g. 0.045 mol % MgF2), it formed even on slow cooling. Its structure appears to be similar to that proposed by Suzuki for the NaCl/CdCl2 system. The morphology of the metastable phase was seen by electron microscopy to be intersecting {100} platelets. On holding crystals containing the metastable phase at temperatures of about 300° C, particles of MgF2 nucleated at, and grew at, the expense of the metastable phase. The observation of precipitation of the metastable phase at low Mg++ ion concentrations is notable, for precipitation has almost certainly occurred during other studies of ionic conductivity, dielectric loss, and colour centres, etc., where crystals have been held at temperatures below 250° C. Solid-solubility measurements have been made to establish the MgF2 solvus line at concentrations up to 6.5 mol % MgF2 and the metastable phase solvus line up to 0.225 mol % MgF2. Kinetic measurements indicate that the growth of the metastable phase is controlled by bulk diffusion of Mg++ ions, whereas the growth of MgF2 appears to be controlled by the motion of vacancy pairs.

Segregation in homogeneous alloys during sintering

The vacancy gradient set up by the sharp curvature of the neck between two sintered particles can under favorable conditions produce a considerable segregation in a completely homogenized solid solution. This effect was demonstrated by the precipitate in Cu-In and Cu-Ag alloys occurring in the neck cavity at a higher temperature than indicated by the solvus line and at a lower one than the solidus temperature. These results are supported by the appearance of side peaks and a shift of the peaks of X-ray lines.

Zirconium-Columbium Diagram

The constitutional diagram presented herein is relatively simple. Complete mutual solid solubility exists for an interval below the solidus line, a continuous curve with a flat minimum near 22 pet Cb and 1740°C. Upon cooling, the solid solution breaks up, except at the columbium-rich side, from two causes: zirconium-rich alloys transform under the influence of the β-α transformation in zirconium; alloys of intermediate composition decompose into two solid solutions below 1000°C. The combined effect is the formation of a eutectoid at a temperature of 610°C and a composition of 17.5 pet Cb. The eutectoid horizontal extends from 6.5 to 87.0 pet Cb. Some age hardening effects have been observed in the zirconium-rich alloys but the positions of the solvus lines remain uncertain.

Chromium-Rich Portion of the Chromium-Nickel Phase Diagram

A more complete determination of the chromium-rich portion of the Cr-Ni phase diagram was made, based in part on resistivity measurements. The existence of a eutectoid reaction at 1215°C, in addition to the previously reported eutectic reaction at 1343°C, was confirmed. Metallographic studies, lineal analysis, and diffusion-couple evidence were utilized to support and extend the findings of the resistivity measurements. Delineation of the a chromium solvus line shows that the maximum solubility of nickel in a chromium decreases rapidly from 25 wt pct Ni at the eutectoid temperature to 16 wt pct Ni at 950°C.