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The solidification mechanism and segregation behavior of laser-melted Mn₃₅Fe₅Co₂₀Ni₂₀Cu₂₀was firstly investigated via in situ synchrotron x-ray diffraction at millisecond temporal resolution. The transient composition evolution of the random solid solution during sequential solidification of dendritic and interdendritic regions complicates the analysis of synchrotron diffraction data via any single conventional tool, such as Rietveld refinement. Therefore, a novel approach combining a hard-sphere approximation model, thermodynamic simulation, thermal expansion measurement and microstructural characterization was developed to assist in a fundamental understanding of the evolution of local composition, lattice parameter, and dendrite volume fraction corresponding to the diffraction data. This methodology yields self-consistent results across different methods. Via this approach, four distinct stages were identified, including: (I) FCC dendrite solidification, (II) solidification of FCC interdendritic region, (III) solid-state interdiffusion and (IV) final cooling with marginal diffusion. It was found out that in Stage I, Cu and Mn were rejected into liquid as Mn₃₅Fe₅Co₂₀Ni₂₀Cu₂₀solidified dendritically. During Stage II, the lattice parameter disparity between dendrite and interdendritic region escalated as Cu and Mn continued segregating into the interdendritic region. After complete solidification, during Stage III, the lattice parameter disparity gradually decreases, demonstrating a degree of composition homogenization. The volume fraction of dendrites slightly grew from 58.3 to 65.5%, based on the evolving composition profile across a dendrite/interdendritic interface in diffusion calculations. Postmortem metallography further confirmed that dendrites have a volume fraction of 64.7% ± 5.3% in the final microstructure.

 

When subjected to the lap shear testing, spot welds created by brazing, resistance welding, or other techniques may fail either by a plug failure mode (also called pull-out mode) or an interfacial shear failure mode. In the past, plug failure mode was thought to be depend- ent on base metal ultimate tensile strength, spot diameter and plate thickness, while interfacial failure can be determined by interface shear strength and spot area. No fracture mechanics model or failure process is invoked in such an approach, and its predictive capability is often doubted compared to realistic experiments. This work conducts a parametric study to assess the failure behavior as a function of dominant three-dimensional geometric parameters based on the Gurson-Tvergaard-Needleman (GTN) damage mechanics model and no-damage mod- el respectively. Different necking conditions are considered as precursors to the two failure modes in the no-damage model. It is found out that a small ratio of spot diameter to plate thickness promotes interfacial shear failure while a large ratio favors plug failure. Other geometric parameters such as the filler interlayer thickness, if used, play a secondary role. The calculated peak force Fwt is not much different between the GTN and no-damage analyses, and better agreement is shown in the small nugget region. Normalized peak force calculated from the GTN model with the porosity f0 set to 0.01 showed the best agreement with pervious tensile shear tests on spot-welded DP980 lap joints in comparison to that calculated from the GTN model with f0 at 0.02 and the no-damage model. Note that heterogeneous distribution of materi- al strength across the joint region was considered in the GTN model, which was estimated based on the hardness map measured across the joint cross section. 

The performance of a newly developed multiprincipal-element alloy (MPEA) filler metal for brazing of nickel-based superalloys was directly compared to a conventional boron- and silicon-suppressed filler (BSSF) metal. The comparison was demonstrated on an Alloy 600 substrate with a brazing temperature of 1200°C. Single-phase solidification behavior and the absence of boron and silicon in the MPEA led to a joint microstructure devoid of eutectic constituents or brittle phases in brazes employing this filler metal. In the brazes using the conventional BSSF metal, incomplete isothermal solidification and subsequent athermal solidification of the residual liquid resulted in large particles of a chromium-rich boride phase distributed throughout the microstructure. Tensile testing of brazed butt joints at both room temperature and 600°C testing conditions demonstrated that the MPEA joints exhibited total ductility values at least one order of magnitude greater than that of BSSF joints, but they showed comparable yield strengths in both testing conditions. Fractographic assessment confirmed that boride phases nucleated cracks and resulted in brittle failure in the BSSF joints, while the MPEA joints exhibited extensive ductile microvoid coalescence. Fine-scale porosity and oxide inclusions may be the dominant factors limiting the overall ductility observed in the MPEA brazes.