arXiv subjects

Yanhong Chang

Publications and source records attributed to Yanhong Chang.

7 recordsLinked to original sources

arXiv2021

A cracking oxygen story: a new view of stress corrosion cracking in titanium alloys

Titanium alloys can suffer from halide-associated stress corrosion cracking at elevated temperatures e.g., in jet engines, where chlorides and Ti-oxide promote the cracking of water vapour in the gas stream, depositing embrittling species at the crack tip. Here we report, using isotopically-labelled experiments, that crack tips in an industrial Ti-6Al-2Sn-4Zr-6Mo alloy are strongly enriched (>5 at.%) in oxygen from the water vapour, far greater than the amounts (0.25 at.%) required to embrittle the material. Surprisingly, relatively little hydrogen (deuterium) is measured, despite careful preparation and analysis. Therefore, we suggest that a combined effect of O and H leads to cracking, with O playing a vital role, since it is well-known to cause embrittlement of the alloy. In contrast it appears that in alpha+beta Ti alloys, it may be that H may drain away into the bulk owing to its high solubility in beta-Ti, rather than being retained in the stress field of the crack tip. Therefore, whilst hydrides may form on the fracture surface, hydrogen ingress might not be the only plausible mechanism of embrittlement of the underlying matrix. This possibility challenges decades of understanding of stress-corrosion cracking as being related solely to the hydrogen enhanced localised plasticity (HELP) mechanism, which explains why H-doped Ti alloys are embrittled. This would change the perspective on stress corrosion embrittlement away from a focus purely on hydrogen to also consider the ingress of O originating from the water vapour, insights critical for designing corrosion resistant materials.

cond-mat.mtrl-sci↗

arXiv2020

Hydride growth mechanism in Zircaloy-4: investigation of the partitioning of alloying elements

The long-term safety of water-based nuclear reactors relies in part on the reliability of zirconium-based nuclear fuel. Yet the progressive ingress of hydrogen during service makes zirconium alloys subject to delayed hydride cracking. Here, we use a combination of electron back-scattered diffraction and atom probe tomography to investigate specific microstructural features from the as-received sample and in the blocky-alpha microstructure, before and after electrochemical charging with hydrogen or deuterium followed by a low temperature heat treatment at 400C for 5 hours followed by furnace cooling at a rate of 0. 5C per min. Specimens for atom probe were prepared at cryogenic temperature to avoid the formation of spurious hydrides. We report on the compositional evolution of grains and grain boundaries over the course of the sample's thermal history, as well as the ways the growth of the hydrides modifies locally the composition and the structure of the alloy. We observe a significant amount of deuterium left in the matrix, even after the slow cooling and growth of the hydrides. Stacking faults form ahead of the growth front and Sn segregates at the hydride-matrix interface and on these faults. We propose that this segregation may facilitate further growth of the hydride. Our systematic investigation enables us discuss how the solute distribution affects the evolution of the alloy's properties during its service lifetime.

cond-mat.mtrl-sci↗

arXiv2019

Could face-centered cubic titanium in cold-rolled commercially-pure titanium only be a Ti-hydride?

A face-centered cubic (FCC) phase in electro-polished specimens for transmission electron microscopy of commercially pure titanium has sometimes been reported. Here, a combination of atom-probe tomography, scanning transmission electron microscopy and low-loss electron energy loss spectroscopy is employed to study both the crystal structural and chemical composition of this FCC phase. Our results prove that the FCC phase is actually a TiHx (x>1) hydride, and not a new allotrope of Ti, in agreement with previous reports. The formation of the hydride is discussed.

cond-mat.mtrl-sci↗

arXiv2019

Quantification challenges for atom probe tomography of hydrogen and deuterium in Zircaloy-4

Analysis and understanding of the role of hydrogen in metals is a significant challenge for the future of materials science, and this is a clear objective of recent work in the atom probe tomography (APT) community. Isotopic marking by deuteration has often been proposed as the preferred route to enable quantification of hydrogen by APT. Zircaloy-4 was charged electrochemically with hydrogen and deuterium under the same conditions to form large hydrides and deuterides. Our results from a Zr hydride and a Zr deuteride highlight challenges associated to accurate quantification of hydrogen and deuterium, in particular associated to the overlap of peaks at low mass-to-charge ratio and of hydrogen/deuterium containing molecular ions. We discuss possible ways to ensure that appropriate information is extracted from APT analysis of hydrogen in a zirconium alloy systems that is important for nuclear power.

cond-mat.mtrl-sci↗

arXiv2018

Atomic-scale investigation of hydrogen distribution in a Ti-Mo alloy

Ingress of hydrogen is often linked to catastrophic failure of Ti-alloys. Here, we quantify the hydrogen distribution in fully \b{eta} and α+\b{eta} Ti-Mo alloys by using atom probe tomography. Hydrogen does not segregate at grain boundaries in the fully \b{eta} sample but segregates at some α/\b{eta} phase boundaries with a composition exceeding 20 at.% in the α+\b{eta} sample. No stable hydrides were observed in either sample. The hydrogen concentration in \b{eta} phases linearly decreases from ~13 at. % to ~4 at. % with increasing Mo-content, which is ascribed to the suppression of hydrogen uptake by Mo addition.

cond-mat.mtrl-sci↗

arXiv2018

The Laplace project: an integrated suite for correlative atom probe tomography and electron microscopy under cryogenic and UHV conditions

We present sample transfer instrumentation and integrated protocols for the preparation and correlative characterization of environmentally-sensitive materials by both atom probe tomography and electron microscopy. Ultra-high vacuum cryogenic suitcases allow specimen transfer between preparation, processing and several imaging platforms without exposure to atmospheric contamination. For expedient transfers, we installed a fast-docking station equipped with a cryogenic pump upon three systems; two atom probes, a scanning electron microscope / Xe-plasma focused ion beam and a N$_2$-atmosphere glovebox. We also installed a plasma FIB with a solid-state cooling stage to reduce beam damage and contamination, through reducing chemical activity and with the cryogenic components as passive cryogenic traps. We demonstrate the efficacy of the new laboratory protocols by the successful preparation and transfer of two highly contamination- and temperature-sensitive samples - water and ice. Analysing pure magnesium atom probe data, we show that surface oxidation can be effectively suppressed using an entirely cryogenic protocol (during specimen preparation and during transfer). Starting with the cryogenically-cooled plasma FIB, we also prepared and transferred frozen ice samples while avoiding significant melting or sublimation, suggesting that we may be able to measure the nanostructure of other normally-liquid or soft materials. Isolated cryogenic protocols within the N$_2$ glove box demonstrate the absence of ice condensation suggesting that environmental control can commence from fabrication until atom probe analysis.

cond-mat.mtrl-sci↗

arXiv2018

Characterizing solute hydrogen and hydrides in pure and alloyed titanium at the atomic scale

Ti has a high affinity for hydrogen and is a typical hydride formers. Ti-hydride are brittle phases which probably cause premature failure of Ti-alloys. Here, we used atom probe tomography and electron microscopy to investigate the hydrogen distribution in a set of specimens of commercially pure Ti, model and commercial Ti-alloys. Although likely partly introduced during specimen preparation with the focused-ion beam, we show formation of Ti-hydrides along α grain boundaries and α/\b{eta} phase boundaries in commercial pure Ti and α+\b{eta} binary model alloys. No hydrides are observed in the α phase in alloys with Al addition or quenched-in Mo supersaturation.

cond-mat.mtrl-sci↗