Variation in pore distribution along sample length in sintered 7xxx aluminum alloy

An experimental and computational fluid dynamics (CFD) numerical study of the sintering of an Al?7Zn?2.5Mg?1Cu alloy in flowing nitrogen was presented. Three rectangular bars with dimensions of 56 mm × 10 mm × 4.5 mm each, equally spaced 2 or 10 mm apart, were sintered in one batch at 620 °C for 40 min in a tube furnace. The pore distribution in the selected cross section of sintered samples was found to be dependent on the sample separation distance and the distance from the cross section examined to the sample end. A three-dimensional (3D) CFD model was developed to investigate the nitrogen gas behavior near each sintering surface of the three samples during isothermal sintering. The variation in porosity in the cross section of each sintered sample along sample length was found to be closely related to the nitrogen gas flow field near the sintering surfaces.

Variant selection in additively manufactured alpha-beta titanium alloys

Alpha-beta(α-β)titanium alloys such as Ti-6Al-4V(wt.%,here-after the same)and Ti-6Al-2Sn-4Zr-2Mo fabricated by fusion-based additive manufacturing(AM)typically exhibit a strong columnar prior-βgrain structure.These columnar prior-βgrains with their001along the build direction not only lead to solidification tex-ture but also cause subsequentα-phase textures[1].The forma-tion of theseα-phase textures is a consequence of theβ→αtransformation obeying the Burgers orientation relationship(BOR)[1-5],which results in[0001]of theα-phase being orientated at about either 45°or 0°relative to the horizontal.They affect the deformation behaviour and mechanical properties[1,6].Defined by the BOR,a singleβ-phase grain can bring forth 12α-phase vari-ants[2,7],leading to significant microstructural intricacy.Theseα-phase variants do not form randomly in eachβ-phase grain.Rather,their formation displays specific crystallographic features,known asα-variant selection,which is common in Ti alloys.In an extreme case,α-variant selection can lead to the formation of a singleα-phase crystal through anα→β→αtransformation cy-cle[8,9].

Ultralight,ductile metal mechanical metamaterials with super elastic admissible strain(0.1)

Mechanical metamaterials are architectured cellular materials with unusual properties.Herein we report another type of metal mechanical metamaterials-their elastic admissible strain(EAS)is on the order of 0.1,compared to about 0.01 for common metallic materials.Four conditions are required for a metal mechanical metamaterial to achieve this super EAS:(i)bending-dominated deformation;(ii)low density;(iii)an appropriate lattice topology,and(iv)an intrinsically high EAS for the lattice strut constituent material.The findings of this work extend perspectives on metal mechanical metamaterials.

Alloy solidification: Assessment and improvement of an easy-to-apply model

It has been a central task of solidification research to predict solute microsegregation. Apart from the Lever rule and the Scheil-Gulliver equation, which concern two extreme cases, a long list of microsegregation models has been proposed. However, the use of these models often requires essential experimental input information, e.g., the secondary dendrite arm spacing(λ), cooling rate( ˙T) or actual solidification range(△T). This requirement disables these models for alloy solidification with no measured values for λ,˙T and △T. Furthermore, not all of these required experimental data are easily obtainable. It is therefore highly desirable to have an easy-to-apply predictive model that is independent of experimental input,akin to the Lever rule or Scheil-Gulliver model. Gong, Chen, and co-workers have recently proposed such a model, referred to as the Gong-Chen model, by averaging the solid fractions(f_(s)) of the Lever rule and Scheil-Gulliver model as the actual solid fraction. We provide a systematic assessment of this model versus established solidification microsegregation models and address a latent deficiency of the model, i.e.,it allows the Lever rule solid fraction fsto be greater than one(f_(s)> 1). It is shown that the Gong-Chen model can serve as a generic model for alloy solidification until fsreaches about 0.9, beyond which(f_(s)> 0.9) its applicability is dictated by both the equilibrium solute partition coefcient(k) and the solute diffusion coefcient in the solid(Ds), which has been tabulated in detail.