Determination of the Phase Equilibria in the Mn-Sn-Zn System at 500 °C

The isothermal section of the Mn-Sn-Zn system at 500 °C was determined with 20 alloys. The alloys were prepared by melting the pure elements in evacuated quartz capsules. The alloy samples were examined by means of X-ray diffraction (XRD) and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy. A new ternary phase Mn₄Zn₈Sn (λ) was found to have a bcc structure with a lattice parameter a = 0.92508 (5) nm. Its composition range spans 25 to 35 at. pct Mn, 4 to 8 at. pct Sn, and 55 to 70 at. pct Zn. The Zn is substituted for Mn in Mn₃Sn, Mn₂Sn, and Mn₃Sn₂. The solubility of Zn in Mn₃Sn, Mn₂Sn, and Mn₃Sn₂ was measured to be about 17, 12, and 4 at. pct, respectively. The phase boundaries of the liquid and β-Mn phases were well established. The following 3 three-phase equilibria were well determined: (1) β-Mn + ε-MnZn₃ + Mn₃Sn, (2) λ + Mn₃Sn + Mn₂Sn, and (3) L + λ + Mn₂Sn. The additional 5 three-phase equilibria, which are ε-MnZn₃ + λ + Mn₃Sn, ε ₁-MnZn₃ + ε-MnZn₃ + λ, ε ₁-MnZn₃ + λ + L, Mn₂Sn + L + MnSn₂, and Mn₃Sn₂ + MnSn₂ + Mn₂Sn, were deduced and shown with dashed lines in the present isothermal section.     

Constitution of the Lead-Tin-Strontium System up to 36 Atomic Percent Sr

The constitution of the Pb-Sn-Sr system from the Pb-Sn binary up to 36 at. pct Sr was determined by differential thermal analysis, metallography, microprobe analysis, and X-ray diffraction. Pb₃Sr forms a continuous series of solid solutions with Sn₃Sr, and is referred to here as the8 phase. Sn₄Sr was the only other intermetallic phase found and is designated here as γ. A eutectic-like trough is formed between (Pb) and δ. It originates at 1.0 at. pct Sr and 324.5 °C (the (Pb)/Pb₃Sr eutectic) and falls monotonically to ~75 at. pct Pb, 24.5 at. pct Sn, and 0.45 at. pct Sr at 283 °C. At 283 °C, a Class II, four-phase reaction occurs: L + δ→ (Pb) + γ. A eutectic-like trough between (Pb) and γ falls from the four-phase plane at 283 °C to the ternary eutectic at ~26 at. pct Pb, ~74 at. pct Sn and <0.3 at. pct Sr at 182 °C. The ternary eutectic reaction is L → (Pb) + (Sn) + γ.     

Isothermal section of the ternary Sn-Cu-Ni system and interfacial reactions in the Sn-Cu/Ni couples at 800 °C

The isothermal section of the Sn-Cu-Ni system at 800 °C has been experimentally determined. There is no ternary compound. A solid solution with a very wide compositional range, the γ phase is formed between the Ni₃Sn(H) phase and Cu₄Sn(H) phase; however, both of these two binary phases are not stable at 800 °C. The binary Ni₃Sn₂ phase also has extensive ternary solubility. The homogeneity ranges of both the γ and Ni₃Sn₂ phases are very large in parallel to the Cu-Ni side, but relatively narrow along the Sn direction. This phenomenon indicates that Cu and Ni are exchangeable in both phases. Three kinds of reaction couples, Sn-55 at. pct Cu/Ni, Sn-65 at. pct Cu/Ni, and Sn-75 at. pct Cu/Ni, were prepared and reacted at 800 °C for 5 to 20 minutes. The reaction paths are liquid/Ni₃Sn₂/γ/Ni₃Sn(L)/Ni for the Sn-55 at. pct Cu/Ni and Sn-65 at. pct Cu/Ni couples, and the reaction path is liquid/γ/Ni₃Sn(L)/Ni for the Sn-75 at. pct Ni couples.     

Liquidus Projection of the Ag-Sn-Te Ternary System

The Ag-Sn-Te ternary system is of interest to thermoelectric applications and its liquidus projection is determined in this study. Forty Ag-Sn-Te ternary alloys are prepared and their primary solidification phases are determined. These different primary solidification phase regions include three terminal solid solutions: Ag, Sn, and Te; six binary intermediate phases: SnTe, β-Ag₅Te₃, Ag_1.9Te, Ag₂Te (assuming no phase transformation), ζ-Ag₄Sn, and ε-Ag₃Sn; and one ternary compound, AgSnTe₂. These data, together with the phase diagrams of the three constituent binary systems, are employed to construct the univariant lines of the liquidus projection. The temperature-descending directions of these univariant lines are determined using thermal analysis results and mass balance concept. The types of invariant reactions and the reaction temperatures are determined from the temperature-descending directions of the univariant lines and by thermal analysis. There are two Class I reactions, five Class II reactions, and one Class III reaction. The invariant reaction with the highest reaction temperature is L + Ag = Ag₂Te + ε-Ag₃Sn, at 992.7 ± 4 K (719.5 ± 4 °C), and that with the lowest reaction temperature is L = Sn + ε-Ag₃Sn + SnTe, at 494.2 ± 4 K (221 ± 2 °C).     

Reaction Scheme and Liquidus Surface of the Ternary System Aluminum-Chromium-Titanium

The constitution of the ternary system Al-Cr-Ti is investigated over the entire composition range using X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), differential thermal analysis (DTA) up to 1500 °C, and metallography. Solid-state phase equilibria at 900 °C are determined for alloys containing ≤75 at. pct aluminum and at 600 °C for alloys containing >75 at. pct Al. A reaction scheme linking these solid-state equilibria with the liquidus surface is presented. The liquidus surface for ≤50 at. pct aluminum is dominated by the primary crystallization field of bcc β(Ti,Cr,Al). In the region >50 at. pct Al, the ternary L1₂-type phase τ forms in a peritectic reaction p _max at 1393 °C from L + TiAl. Furthermore, with the addition of chromium, the binary peritectic L + α(Ti,Al) = TiAl changes into an eutectic L = α(Ti,Al) + TiAl. This eutectic trough descends monotonously through a series of transition reactions and ternary peritectics to end in the binary eutectic L = Cr₇Al₄₅ + (Al).     

Solidification of undercooled Sn-Sb peritectic alloys: Part II. Heterogeneous nucleation

The undercooling behavior of fine droplet samples of Sn-rich, Sn-Sb alloys was investigated using differential thermal analysis (DTA). Undercooling levels measured during cooling from the liquid state follow the trend of the intermetallic phase liquidus, suggesting that solidification of all droplet samples (even those which solidify to yield a supersaturatedβ-tin product) was probably initiated with formation of primary intermetallic phase. Heterogeneous nucleation thermal cycling treatments were then used to measure the relative catalytic potency of primary intermetallic phases in this system for nucleation ofβ-tin during cooling. Crystallization reactions below the equilibrium peritectic temperature of 250 °C, at 187 °C and 230 °C, have been interpreted as corresponding to nucleation ofβ on Sn₃Sb₂ and SnSb substrates, respectively. The behavior observed in the Sn-Sb system can be generalized to guide the interpretation of heterogeneous catalysis and the analysis of solidification pathways in other peritectic alloy systems. Formerly Graduate Student, Department of Materials Science and Engineering, University of Wisconsin-Madison     

Diffusion Parameters and Growth Mechanism of Phases in the Cu-Sn System

The tracer diffusion coefficients of the elements as well as the integrated interdiffusion coefficients are determined for the Cu₃Sn and Cu₆Sn₅ intermetallic compounds using incremental diffusion couples and Kirkendall marker shift measurements. The activation energies are determined for the former between 498 K and 623 K (225 °C and 350 °C) and for the latter between 423 K and 473 K (150 °C and 200 °C). Sn is found to be a slightly faster diffuser in Cu₆Sn₅, and Cu is found to be the faster diffuser in Cu₃Sn. The results from the incremental couples are used to predict the behavior of a Cu/Sn couple where simultaneous growth of both intermetallics occurs. The waviness at the Cu₃Sn/Cu₆Sn₅ interface and possible reasons for not finding Kirkendall markers in both intermetallics in the Cu/Sn couple are discussed.     

Isothermal Phase Transformation in Biomedical Co-29Cr-6Mo Alloy without Addition of Carbon or Nitrogen

The isothermal phase transformation behavior in a biomedical Co-29Cr-6Mo alloy without carbon or nitrogen was investigated during aging at temperatures between 973 K and 1273 K (700 °C and 1000 °C) for up to 90 ks. Transformation from the γ to the ε phase did not occur at 1273 K (1000 °C) as the γ phase was more stable than the ε phase, and the σ phase precipitated at the γ grain boundaries. At 1173 K (900 °C), a γ → ε ₁ phase transformation occurred by massive precipitation. Prolonged annealing at 1173 K (900 °C) led to a lamellar structure of ε ₂ and σ phases at ε ₁/ε ₁ boundaries by a discontinuous/cellular reaction, expressed by the reaction equation ε ₁ → ε ₂ + σ. After decreasing the aging temperature to 973 K (700 °C), transformation from the γ to the ε phase occurred mainly by isothermal martensitic transformation, but a lathlike massive ε ₁ phase and ε ₂/σ lamellar colonies were also observed at the original γ-grain boundaries. It is likely that not adding carbon results in the promotion of the massive transformation and the precipitation of the σ phase during isothermal aging in the Co-29Cr-6Mo alloy system, whose composition corresponds to the ASTM F75 standard for metallic materials for surgical implantation. The resultant isothermal transformation behavior of the present alloy is described on the basis of thermodynamic calculations using Thermo-Calc.     

High-Temperature Oxidation of Alloy 617 in Helium Containing Part-Per-Million Levels of CO and CO<Subscript>2</Subscript> as Impurities

The objective of this study was to determine the mechanisms of carburization and decarburization of alloy 617 in impure helium. To avoid the coupling of multiple gas/metal reactions that occurs in impure helium, oxidation studies were conducted in binary He + CO + CO₂ gas mixtures with CO/CO₂ ratios of 9 and 1272 in the temperature range 1123 K to 1273 K (850 °C to 1000 °C). The mechanisms were corroborated through measurements of oxidation kinetics, gas-phase analysis, and surface/bulk microstructure examination. A critical temperature corresponding to the equilibrium of the reaction 27Cr + 6CO ↔ 2Cr₂O₃ + Cr₂₃C₆ was identified to lie between 1173 K and 1223 K (900 °C and 950 °C) at CO/CO₂ ratio 9, above which decarburization of the alloy occurred via a kinetic competition between two simultaneous surface reactions: chromia formation and chromia reduction. The reduction rate exceeded the formation rate, preventing the growth of a stable chromia film until carbon in the sample was depleted. Surface and bulk carburization of the samples occurred for a CO/CO₂ ratio of 1272 at all temperatures. The surface carbide, Cr₇C₃, was metastable and nucleated due to preferential adsorption of carbon on the chromia surface. The Cr₇C₃ precipitates grew at the gas/scale interface via outward diffusion of Cr cations through the chromia scale until the activity of Cr at the reaction site fell below a critical value. The decrease in activity of chromium triggered a reaction between chromia and carbide: Cr₂O₃ + Cr₇C₃ → 9Cr+3CO, which resulted in a porous surface scale. The results show that the industrial application of the alloy 617 at T > 1173 K (900 °C) in impure helium will be limited by oxidation.     

Dissolution and Interface Reactions between Palladium and Tin(Sn)-Based Solders: Part II. 63Sn-37Pb Alloy

The interface microstructures and dissolution behavior were studied, which occur between 99.9 pct Pd substrates and molten 95.5Sn-3.9Ag-0.6Cu (wt pct, Sn-Ag-Cu) solder. The solder bath temperatures were 513 K to 623 K (240 °C to 350 °C). The immersion times were 5 to 240 seconds. The IMC layer composition exhibited the (Pd, Cu)Sn₄ (Cu, 0 to 2 at. pct) and (Pd, Sn) solid-solution phases for all test conditions. The phases PdSn and PdSn₂ were observed only for the 623 K (350 °C), 60 seconds test conditions. The metastable phase, Pd₁₁Sn₉, occurred consistently for the 623 K (350 °C), 240 seconds conditions. Palladium-tin needles appeared in the Sn-Ag-Cu solder, but only at temperatures of 563 K (290 °C ) or higher, and had a (Pd, Cu)Sn₄ stoichiometry. Palladium dissolution increased monotonically with both solder bath temperature and exposure time. The rate kinetics of dissolution were represented by the expression At ⁿ exp(∆H/RT), where the time exponent (n) was 0.52 ± 0.10 and the apparent activation energy (∆H) was 44 ± 9 kJ/mol. The IMC layer thickness increased between 513 K and 563 K (240 °C and 290 °C) to approximately 3 to 5 μm, but then was less than 3 μm at 593 K and 623 K (320 °C and 350 °C). The thickness values exhibited no significant time dependence. As a protective finish in electronics assembly applications, Pd would be relatively slow to dissolve into molten Sn-Ag-Cu solder. The Pd-Sn IMC layer would remain sufficiently thin and adherent to a residual Pd layer so as to pose a minimal reliability concern for Sn-Ag-Cu solder interconnections.     

Crystallographic Texture and Volume Fraction of <Emphasis Type="Italic">α</Emphasis> and <Emphasis Type="Italic">β</Emphasis> Phases in Zr-2.5Nb Pressure Tube Material During Heating and Cooling

The phase transformations in an as-received Zr-2.5Nb pressure tube material were characterized in detail by neutron diffraction. The texture and volume fraction of α and β phases were measured on heating at eight different temperatures 373 K to 1323 K (100 °C to 1050 °C) traversing across the α/(α + β) and (α + β)/β solvus lines, and also upon cooling at 1173 K and 823 K (900 °C and 550 °C). The results indicate that the α-phase texture is quite stable, with little change in the {0002} and { 11[`2]0 } \left\{ {11\bar{2}0} \right\} pole figures during heating to 1123 K (850 °C). The β-phase volume fraction increased while a slight change in texture was observed until heating reached 973 K (700 °C). On further heating to 1173 K (900 °C), there appears a previously unobserved α-phase texture component due to coarsening of the prior primary α grains; meanwhile the transformed β-phase texture evolved markedly. At 1323 K (1050 °C), the α phase disappeared with only 100 pct β phase remaining but with a different texture than that observed at lower temperatures. On cooling from the full β-phase regime, a different cooldown transformed α-phase texture was observed, with no resemblance of the original texture observed at 373 K (100 °C). The transformed α-phase texture shows that the {0002} plane normals are within the radial-longitudinal plane of the pressure tube following the Burgers orientation relationship of (110)<sub>bcc</sub>//(0002)<sub>hcp</sub> and [[`1]11]_\textbcc //[11[`2]0]_\texthcp [\bar{1}11]_{\text{bcc}} //[11\bar{2}0]_{\text{hcp}} with a memory of the precursor texture of the primary α grains observed on heating at 1173 K (900 °C).     

The growth of Cu-Sn intermetallics at a pretinned copper-solder interface

This article reports a comparative study of the formation and growth of intermetallic phases at the interface of Cu wetted with a thick solder joint or a thin, pretinned solder layer. The η phase (Cu₆Sn₅) forms when Cu is wet with eutectic solder at temperatures below 400 °C. The intermetallic layer is essentially unaffected by aging at 70 °C for as long as 13 weeks. On aging a eutectic joint at 170 °C, the η-phase intermetallic layer thickens and ε phase (Cu₃Sn) nucleates at the Cu/intermetallic interface and grows to a thickness comparable to that of the η phase, while a Pb-rich boundary layer forms in the solder. The aging behavior of a thin, pretinned eutectic layer is qualitatively different. At 170 °C, the Sn in the eutectic is rapidly consumed to form η-phase intermetallic, which converts to ε phase. The residual Pb withdraws into isolated islands, and the solderability of the surface deteriorates. When the pretinned layer is Pb-rich (95Pb-5Sn), the Sn in the layer is also rapidly converted into η phase, in the form of dendrites penetrating from the intermetallic at the Cu interface and discrete precipitates in the bulk. How ever, the development of the intermetallic largely ceases when the Sn is consumed; ε phase does not form, and the residual Pb remains as an essentially continuous layer, preserving the solderability of the sample. These observations are interpreted in light of the Cu-Sn and Pb-Sn phase diagrams, the temperature of initial wetting, and the relative diffusivities of Cu and Sn in the solder and intermetallic phases. A.J. SUNWOO, Formerly with the Lawrence Berkeley Laboratory, Berkeley, CA,     

Microstructure and phase relations for Ti-Mo-Al alloys

The influence of aluminum additions to a Ti-7 at. pet Mo alloy on the phase equilibria was investigated. The microstructures of the alloys, Ti-7 pct Mo-7 pct Al and Ti-7 pct Mo-16 pct Al, were determined by light and electron microscopy. It was found that with increasing aluminum concentration the formation of the metastable w phase was suppressed. In the Ti-7 pct Mo-16 pct Al alloy the β phase decomposed upon quenching by precipitating coherent, ordered particles having a B2 type of crystal structure (β₂). At low temperatures the equilibrium phases for this alloy were β + α+ β ₂, whereas at high temperature (850° to 950°C) the Ti₃Al phase was in two-phase equilibrium with the β phase. The four-phase equilibrium which exists at a temperature of about 550°C involves the reaction β + Ti₃Al ⇌ α + β₂. G. LUETJERING, formerly Staff Member Materials Research Center, Allied Chemical Corp., Morristown, N. J.,     

Thermal properties and interfacial reaction between the Sn-9Zn-<Emphasis Type="Italic">x</Emphasis>Ag lead-free solders and Cu substrate

The thermal properties and interfacial reaction between the Sn-9Zn-xAg lead-free solders and Cu substrate, such as solidus and liquidus temperatures, heat of fusion, intermetallic compounds, and adhesion strength, have been investigated. Two endothermic peaks appear in the DSC curve when the Ag content in the Sn-9Zn-xAg solder alloy is above 1.5 wt pct. The solidus temperatures of the Sn-9Zn-xAg solder alloys are around 197 °C, but the liquidus temperatures decrease from 225.3 °C to 221.7 °C and 223.6 °C with increasing the Ag content in the solder alloy from 1.5 to 2.5 and 3.5 wt pct, respectively. Three intermetallic compounds, namely, Cu₆Sn₅, Cu₅Zn₈, and Ag₃Sn are observed at the Sn-9Zn-xAg/Cu interface. The Cu₅Zn₈ is formed close to the Cu substrate, Ag₃Sn is adjacent to it, and Cu₆Sn₅ is nearest the Sn-9Zn-1.5Ag solder alloys. A bi-structural Cu₆Sn₅ layer with hexagonal η-Cu₆Sn₅ and monoclinic η′-Cu₆Sn₅ is found at the Sn-9Zn-1.5Ag/Cu interface due to Ag dissolution. A maximum adhesion strength of 10.7±0.8 MPa is obtained at the Sn-9Zn-2.5Ag/Cu interface as soldered at 250 °C for 30 seconds.     

Microstructural development of a gas-atomized and hot-pressed super-α2 alloy

A variety of heat treatments have been employed to explore the microstructure in Ti-25Al-10Nb-3V-lMo alloy prepared by gas atomization and hot pressing. These treatments include quenching by oil cooling and water cooling and aging at temperatures between 530 °C and 950 °C. Quenching transformations from the β-phase field include the formation ofO phase in oil quenching and β (disordered) +O phase in water quenching. The metastable β phase decomposes intoO + “Ω”,O, or α₂ + β_o/B2 phase when the as-quenched alloy is aged at various temperatures. By comparing the selection area diffraction patterns, it has been found that the ordered w phase in the alloy studied in this article is distinct in structure to the “Ω type” (P3m1) and B8₂ phase which are formed in the parent matrix of the ordered β(B2,D0₃) phases. It has also been shown by X-ray diffraction (XRD) analyses that the lattice parameters of the as-agedO phase do not remain constant in the alloy at various temperatures.