Nanoprecipitates In Alloys Research Articles

Effect of Zn content on the formability and aging precipitation of Al–Zn–Mg–Cu–Nb alloys prepared by LPBF

In this work, the effect of Zn content on the formability and aging precipitation of Al–Zn–Mg–Cu–Nb alloys prepared by laser powder bed fusion (LPBF) were investigated via experiments and in-depth characterization. Results show that the increase in Zn content narrows the LPBF processing window, owing to the significant evaporation of Zn element during LPBF. The increase of Zn content not only accelerates the aging kinetics of Al–Zn–Mg–Cu–Nb alloys, but also increases the number density of nanoprecipitates and reduces the PFZ width, resulting in an enhancement in the peak microhardness. The microstructure evolution and aging behavior of alloys during direct aging (DA) and traditional solution treatment and aging (STA) were compared. DA can maintain fine equiaxed grains, while a bimodal grain structure with coarse and fine grains can be obtained after STA. The peak microhardness and the number density of nanoprecipitates in alloys treated with STA is higher than that of alloys treated with DA, which indicates that the traditional STA heat treatment is still applicable to Al–Zn–Mg–Cu–Nb alloys fabricated by LPBF.

Novel high-entropy alloys with high-density ε-D019 and abnormal phase transformation

Precipitation strengthening is considered to be one of the most effective ways to improve engineering structural materials. However, introducing high-density nanoprecipitates in alloys is a big challenge. In this study, novel CoFeNiVTa(Nb) high-entropy alloys (HEAs) with high-density ε-D019 nanoprecipitates featuring Widmanstätten patterns and coherent interfaces with the matrix are developed using the lattice misfit design concept. The highly ordered hexagonal ε-D019 phase is identified as (Co, Ni, Fe)3(V, Nb, Ta, Fe) by atomic resolution energy dispersive spectrometry and this phase is metastable and can be abnormally transformed into δ-D0a phase instead of η-D024. The superior thermodynamic stability of the δ phase compared with the ε phase is confirmed by first-principle calculations. High-density ε-D019 precipitates with Widmanstätten patterns contribute to the improved strengthens and friction properties of CoFeNiVTa(Nb) HEAs.

High throughput crystal structure and composition mapping of crystalline nanoprecipitates in alloys by transmission Kikuchi diffraction and analytical electron microscopy

Statistically significant crystal structure and composition identification of nanocrystalline features such as nanoparticles/nanoprecipitates in materials chemistry and alloy designing using electron microscopy remains a grand challenge. In this paper, we reveal that differing crystallographic phases of nanoprecipitates in alloys can be mapped with unprecedented statistics using transmission Kikuchi diffraction (TKD), on typical carbon-based electron-transparent samples. Using a case of multiphase, multicomponent nanoprecipitates extracted from an improved version of 9% chromium Eurofer-97 reduced-activation ferritic-martensitic steel we show that TKD successfully identified more than thousand M23C6, MX, M7C3, and M2X (M=Fe, Cr, W, V, Ta; X = C, N) nanoprecipitates in a single scan, something that is currently unachievable using a transmission electron microscope (TEM) without incorporating a precision electron diffraction (PED) system. Precipitates as small as ∼20–25 nm were successfully phase identified by TKD. We verified the TKD phase identification using high-resolution transmission electron microscopy (HRTEM) and convergent beam electron diffraction (CBED) pattern analysis of a few precipitates that were identified by TKD on same sample. TKD study was combined with state-of-art analytical scanning transmission electron microscopy (STEM)-energy dispersive X-ray (EDX) spectroscopy and multivariate statistical analysis (MVSA) which provided the complete crystal structure and distinct chemistries of the precipitates in the steel in a high throughput automated way. This technique should be applicable to characterizing any multiphase crystalline nanoparticles or nanomaterials. The results highlight that combining phase identification by TKD with analytical STEM and modern data analytics may open new pathways in big data material characterization at nanoscale that may be highly beneficial for characterizing existing materials and in designing new materials.

Improvements in the mechanical properties of the 18R↔6R high-hysteresis martensitic transformation by nanoprecipitates in CuZnAl alloys

Abstract The 18R ↔ 6R martensite–martensite transformation in Cu-based alloys exhibits large hysteresis, large pseudoelastic strain and weak transformation stress dependence on temperature. However, concomitant plastic deformation taking place in the 6R phase inhibits the use of these properties for applications. A novel approach to minimizing or even suppressing 6R plastic deformation during the 18R–6R transformation in CuZnAl shape-memory alloy single crystals with electronic concentration e/a = 1.48 is presented. The method is based on a thermal treatment that introduces nanoprecipitates in the alloy. Results suggest that the role of CuZnAl shape-memory alloys in engineering should be reconsidered, as many energy damping applications could benefit from the huge hysteresis associated with the 18R–6R transformation, once the 6R plastic deformation is suppressed.