Geping Li

Acicular α2 precipitation induced by capillarity at α/β phase boundaries in Ti–14Al–2Zr–3Sn–3Mo–0.5Si titanium alloy

Abstract The precipitation of α2 phase based on Ti3Al from Ti–14Al–2Zr–3Sn–3Mo–0.5Si titanium alloy homogenized at different temperatures in the (α+β) two-phase field has been characterized. Minimal lattice mismatch between the α and α2 phase leads to a massive ordering transformation upon cooling and results in morphologies distinct from the ellipsoidal shape of the α2 particles. Enrichment of Al in front of growing β grains due to the Gibbs–Thomson effect, together with preferential diffusion of Al along 1 1 2 0 produces a layer of acicular α2 particles apparently perpendicular to the α/β phase boundaries. Removal of the capillarity-induced composition inhomogeneity by subsequent aging at 750 °C results in disappearance of the acicular shape of the α2 particles.

Considerable knock-on displacement of metal atoms under a low energy electron beam

Under electron beam irradiation, knock-on atomic displacement is commonly thought to occur only when the incident electron energy is above the incident-energy threshold of the material in question. However, we report that when exposed to intense electrons at room temperature at a low incident energy of 30 keV, which is far below the theoretically predicted incident-energy threshold of zirconium, Zircaloy-4 (Zr-1.50Sn-0.25Fe-0.15Cr (wt.%)) surfaces can undergo considerable displacement damage. We demonstrate that electron beam irradiation of the bulk Zircaloy-4 surface resulted in a striking radiation effect that nanoscale precipitates within the surface layer gradually emerged and became clearly visible with increasing the irradiation time. Our transmission electron microscope (TEM) observations further reveal that electron beam irradiation of the thin-film Zircaly-4 surface caused the sputtering of surface α-Zr atoms, the nanoscale atomic restructuring in the α-Zr matrix, and the amorphization of precipitates. These results are the first direct evidences suggesting that displacement of metal atoms can be induced by a low incident electron energy below threshold. The presented way to irradiate may be extended to other materials aiming at producing appealing properties for applications in fields of nanotechnology, surface technology, and others.

Martensitic Twinning in Alpha

The twinning structure of the orthorhombic 𝛼   martensite phase in alpha + beta Ti-3.5Al-4.5Mo (wt%) titanium alloy was studied using X-ray diffraction and transmission electron microscopy by water quenching from below transus temperatures. While water quenching from 910 ∘ C induced the formation of { 1 1 0 } ∘ twins, quenching from 840 ∘ C formed the 𝛼   martensite with { 1 1 1 } ∘ type I twins. The effect of the principle strains on the twinning structure was discussed. As compared to the previous studies, the principle strains play an important role in the formation of the twinning type.

Characterization of the products obtained from the reactions of Zircaloy-4 with an acid mixture of concentrated HNO3 and dilute HF with the aim of understanding pickle salts of zirconium alloys

Pickling of zirconium alloys in nitric-hydrofluoric acid has been widely used in the nuclear materials industry. However, the pickle salts still remain a curiosity and a thorough knowledge of the products of the system Zr–(HNO3–HF) still needs to be well established. The present study investigated the products obtained from the reactions of Zircaloy-4 with an acid mixture of HNO3 (65 wt%) and HF (40 wt%) in a volume ratio of 94 : 6, similar to the one used under nuclear industry conditions. It shows that after the reactions, a gel-like solution was present. When drying it at a high relative humidity (at 25 °C and 70 ± 5% RH), an amorphous solid product formed, while in the case of at a low relative humidity (at 15/25 °C and 25 ± 5% RH), besides the amorphous solid, crystalline zirconium hydroxide nitrates Zr(OH)2(NO3)2·4.71H2O and Zr(OH)2(NO3)2·1.65H2O (possible) with low degree of crystallinity formed. These compounds were identified with the help of successfully synthesizing completely crystalline Zr(OH)2(NO3)2·4.71H2O from the gel-like solution by ultrasonication, which in itself was a breakthrough. Such a study can be expected to provide a deep understanding of pickle salts of zirconium alloys in practical use.

A study of the microstructure of melt-quenched high-temperature Ti-5Al-4Sn-2Zr-1Mo-0.25Si-1Nd alloy

Abstract The microstructural morphology of the melt-quenched high-temperature Ti-55 alloy consists of a uniform distribution of bands of ultrafine Nd-rich phase particles. The calculated diameter of the particles is 6.648 nm, which is within the 6–15 nm range observed experimentally. The matrix first solidifies and engulfs the Nd-rich liquid droplets, and then the liquid droplets crystallize. Finally, bands of ultrafine particles are formed parallel to the advancing solidification plane-front.