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.     

Solidification Behavior and the Evolution of Phase in Laser Rapid Forming of Graded Ti6Al4V-Rene88DT Alloy

The graded material of Ti6Al4V-Rene88DT superalloy, which has shown the potential to be applied in aeroengines, was fabricated from prealloyed metal powders using the technique of laser rapid forming (LRF). A compositional gradient, from 100 pct Ti6Al4V to Ti6Al4V-38 pct Rene88DT, was achieved within a thickness of laser deposit of 54 mm. The solidification behavior and the phase evolution of the compositionally graded material were studied. The results of the X-ray diffraction (XRD) analysis together with the metallographic study showed that a series of phase evolutions, α + β → α + β + Ti₂Ni → β + Ti₂Ni, along the compositional gradient have occurred. The hardness value of the graded material was measured along the compositional gradient, and the results were explained in terms of the various phases present. The condition for columnar-to-equiaxed transition observed in the graded material was explained, based on Hunt’s criterion. Finally, the morphology of β + Ti₂Ni anomalous eutectic is discussed.

---

Structure Optimization of FePt Nanoparticles of Various Sizes for Magnetic Data Storage

Surfactant-coated, fcc disordered FePt nanoparticles of three different sizes between 3 and 7 nm were prepared and washed according to a modified reaction route based on Sun et al.[1] Hexane dispersions of nanoparticles were dried on transmission electron microscopy (TEM) grids, and the resulting monolayers were annealed by a sinter-free procedure at 600 °C and at 650 °C for 2 hours, respectively, developing the L1₀ phase while preserving their monodispersity. The order parameter of the L1₀ phase was measured using electron diffraction from statistically large ensembles of nanoparticles using an X-ray-like signal collection technique. The order parameter increases with particle size and with the temperature in the range 600 °C to 650 °C, being close to unity for the 7.2-nm particles annealed at 650 °C for 2 hours. Electronic supplementary material   The online version of this article (doi: ) contains supplementary material, which is available to authorised users. This article is based on a presentation made in the symposium entitled “Phase Transformations in Magnetic Materials”, which occurred during the TMS Annual Meeting, March 12–16, 2006, in SanAntonio, Texas, under the auspices of the Joint TMS-MPMD and ASMI-MSCTS Phase Transformations Committee.

---

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.     

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.,     

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     

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 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.