Diffusion Models for Solid-state Homogenization of Dendrite Segregation

New mathematical diffusion models for solid state homogenization of dendrite segregation were presented to calculate the concentration evolution of alloying elements during solid-state homogenization of dendrite segregation in any cross section of a three-dimensional dendrite system. With mathematical accumulation, all the individual concentration profiles of every dendrite can be accumulated to form a concentration equation, which may be required by sine model or not. Then diffusion homogenization processes for any cross sections of dendrites in solid state can be calculated from the equation. The only assumption of the new models was that any individual concentration profile in a dendrite system could be simulated by Gauss equation of Fick-2 law.

Effect of solidification rate on dendrite segregation and mechanical properties of Cu-15Ni-8Sn alloy prepared by directional solidification

The microstructures and mechanical properties of the directionally solidified Cu-15Ni-8Sn alloy were investigated at solidification rates ranging from 100 to 3000μm/s.The results showed that the solidification rate significantly affects the phase distribution of the as-cast Cu-15Ni-8Sn alloy.The primary and secondary dendritic spacing reduces and eventually becomes stable as the solidification rate increases.Meanwhile,the size of the primary phase decreases,and its distribution becomes more uniform.The most severe segregation problem of this alloy has been greatly improved.Upon solidification at 100μm/s,the as-cast Cu-15Ni-8Sn alloy consists of the α-Cu matrix,γ-CuNi_(2)Sn phase,discontinuous precipitation structure,modulated structure,and DO_(22) ordered phases.However,as the solidification rate increases,the discontinuous precipitation structure,modulated structures,and DO_(22) ordered phases decrease and even disappear,reducing hardness.As the solidification rate increases,after homogenization treatment,the composition and microhardness distributions of Cu-15Ni-8Sn alloy become more uniform.The time for homogenization is also shortened.It reduces production energy usage and facilitates further mechanical processing.

Microstructural evolution of Al-Zn-Mg-Zr alloy with trace amount of Sc during homogenization treatment

The microstructural evolution of Al-Zn-Mg-Zr alloy with trace amount of Sc during homogenization treatment was studied by means of metallographic analysis, scanning electron microscopy （SEM）, energy dispersive X-ray （EDX） and differential scanning calorimetry （DSC）. The results show that serious dendritic segregation exists in studied alloy ingot. There are many eutectic phases with low melting-point at grain boundary and the distribution of main elements along interdendritic region varies periodically. Elements Zn, Mg and Cu distribute unevenly from grain boundary to the inside of alloy. With increasing the homogenization temperature or prolonging the holding time, the residual phases are dissolved into matrix α（Al） gradually during homogenization treatment, all elements become more homogenized. The overburnt temperature of studied alloy is 476.7 °C. When homogenization temperature increases to 480 °C, some spherical phases and redissolved triangular constituents at grain boundaries can be easily observed. Combined with microstructural evolution and differential scanning calorimeter, the optimum homogenization parameter is at 470 °C for 24 h.

Microstructural evolution of 2026 aluminum alloy during homogenization

The microstructural evolution of 2026 aluminum alloy during homogenization treatment was investigated by optical microscopy(OM),scanning electron microscopy(SEM),energy dispersive X-ray spectrometry(EDS),differential scanning calorimetry(DSC)and X-ray diffraction(XRD).The results show that severe dendritic segregation exists in the as-cast 2026 alloy and the main secondary phases at grain boundary are S(Al2CuMg)andθ(Al2Cu)phases.Elements Cu,Mg and Mn distribute unevenly from grain boundary to the inside of as-cast alloy.With the increase of homogenization temperature or the prolongation of holding time,the residual phases gradually dissolve into the matrixα(Al)and all the elements become more homogenized.According to the results of microstructural evolution,differential scanning calorimetry and X-ray diffraction,the optimum homogenization parameter is at 490°C for 24 h,which is consistent with the result of homogenization kinetic analysis.

Effect of laser remelting on microstructure and mechanical properties of CrMnFeCoNi high entropy alloy

Equiatomic CrMnFeCoNi high entropy alloy prepared by powder metallurgy was remelted by laser.The relative density and microstructure of fusion zone are evaluated.The nanoindentation tests are conducted to reveal the hardness difference of dendrite arms and interdendritic areas.Tensile tests are conducted to assess the mechanical properties of remelted HEA.After laser remelting,the number and morphology of voids changed significantly.Dendritic structure with face-centered cubic phase form in the fusion zone.Fe,Cr and Co are enriched in dendrite arm,while Mn and Ni are enriched in interdendritic area.Elements segregation led to a nanohardness difference between dendrite arm and interdendritic area.Local deformation occurs in interdendritic area during tensile tests and results in a fracture with directionality.