Study of solidification pathway of a MoSiBTiC alloy by optical thermal analysis and in-situ observation with electromagnetic levitation

MoSiBTiC alloys are promising candidates for next-generation ultrahigh-temperature materials. However, the phase diagram of these alloys has been unknown. We have developed an ultrahigh-temperature thermal analyser based on blackbody radiation that can be used to analyse the melting and solidification of the alloy 67.5Mo–5Si–10B–8.75Ti–8.75 C (mol%). Furthermore, electromagnetic levitation (EML) was used for in-situ observation of solidification and microstructural study of the alloy. On the basis of the results, the following solidification pathway is proposed: Mo solid solution (Moss) begins to crystallize out as a primary phase at 1955 °C (2228 K) from a liquid state, which is followed by a (Moss+TiC) eutectic reaction starting at 1900 °C (2173 K). Molybdenum boride (Mo2B) phase precipitates from the liquid after the eutectic reaction; however, the Mo2B phase may react with the remaining liquid to form Moss and Mo5SiB2 (T2) as solidification proceeds. In addition, T2 also precipitates as a single phase from the liquid. The remaining liquid reaches the (Moss + T2 + TiC) ternary eutectic point at 1880 °C (2153 K), and the (Moss + T2 + Mo2C) eutectic reaction finally occurs at 1720 °C (1993 K). This completes the solidification of the MoSiBTiC alloy.

Growth Directions of Primary Phases in Directionally Solidified Ti-45Al-7Nb Alloys

Ti-45Al-7Nb alloys are produced by liquid-metal-cooling (LMC) directional solidification (DS) furnace with different temperature gradients (G) at a range of 40–80 K/cm. The growth directions of primary β phases and the deflection angles relative to the 〈100〉β direction were investigated. The solidification pathway of alloy is: Liq. → Liq. + β → β → β + α → α + γ → lamella (α2 + γ) + B2. Primary dendrite arm spacing λ reduced from 611.4 µm to 508.7 µm when G increased from 40 K cm−1 to 80 K cm−1. Grains with lamellar orientation aligned at the angles of 12–15° and 55° to growth direction were favored for various G. The orientations of the γ lamellas in steady-state regions were identified by the electron backscattered diffraction analysis (EBSD). The results indicated that the growth directions of primary bcc-β dendrites were 〈211〉β and 〈110〉β at 40 K/cm, 〈210〉β and 〈211〉β at 50 K/cm and 〈211〉β at 80 K/cm, the deflection angle of 〈211〉β relative to 〈100〉β direction is 35.3°.

Real Time Synchrotron X-Ray Imaging for Nucleation and Growth of Cu6Sn5 in Sn-7Cu-0.05Ni High Temperature Lead-Free Solder Alloys

This paper demonstrates how recent progress for real-time solidification observation at SPring-8 synchrotron has contributed to the development of Sn-7wt%Cu-0.05wt%Ni high temperature lead-free solder alloys. Lead-free solder alloys in the composition range Sn-0.7 to 7.6wt%Cu that consist of primary Cu6Sn5in a eutectic Sn-Cu6Sn5matrix have been proposed as solders for application at temperatures up to 400°C for the assembly high current semiconductors. It is shown that trace levels of Al have a marked effect on the solder microstructure and refine the size of the primary Cu6Sn5. The solidification pathway that leads to the refinement was observed in real-time using X-ray synchrotron observations.

Microstructure and Elevated Temperature Flow Properties of Rapidly Solidified TiAl-Base Alloys

ABSTRACTRapid solidification processing (RSP) of TiAl-base alloys often results in the formation of metastable crystalline products (e.g., Ti3Al) A key parameter controlling both the solidification pathway and the stability of the metastable structure is alloy chemistry. This study focuses upon the influence of minor alloy additions on the microstructure and properties of Ti52Al48 alloys produced by RSP. While alloys having minor Ta addi ions (<1 at.%) maintain near-equilibrium solidification structures similar to the binary alloy, the addition of Cr at a level of 5 at.% results in a nearly 100 vol.% metastable Ti3Al structure containing a fine dispersion of TiCr2 grain boundary precipitates. These precipitates limit grain growth during elevated temperature exposure, maintaining the fine (TiAl) grain size created during decomposition of the metastable product structure. Mechanical property evaluation of the consolidated RSP material indicates a greatly reduced flow stress for the fine grained Cr modified alloys. A preliminary analysis attributes this to the reduction in grain size and enhanced elevated temperature diffusion rates for the Cr containing TiAl-base alloys.