I.M. Robertson

Hydrogen embrittlement of α titanium: In situ tem studies

The effect of hydrogen on fracture in the h.c.p. α Ti-4 wt % Al alloy and the role of titanium hydride in the fracture process have been studied by deforming samples in situ in a high-voltage electron microscope equipped with an environmental cell. Two fracture mechanisms have been observed in a gaseous hydrogen environment at room temperature; one is fracture by localized plastic deformation enhanced by the presence of hydrogen, the other is a brittle fracture of the stress-induced titanium hydride which precipitates at elastic singularities. The local stress intensity determines which mechanism predominates. At high stress intensities the crack propagates by the process of hydrogen enhanced localized plasticity, while at low stress intensities titanium hydrides form in the vicinity of crack tips and crack propagation proceeds though the hydrides.

The effect of hydrogen on dislocation dynamics

Abstract Deformation studies in numerous materials have been performed in situ in a transmission electron microscope equipped with an environmental cell to elucidate the mechanisms of hydrogen embrittlement. The primary results from these studies are that solute hydrogen can increase the velocity of dislocations, increase the crack propagation rate, decrease the stacking-fault energy of 310s stainless steel and increase the propensity for edge character dislocations. Evidence from bulk mechanical property tests to support these results is also discussed.

An HVEM study of hydrogen effects on the deformation and fracture of nickel

Abstract The mechanisms of hydrogen “embrittlement” of nickel have been investigated by performing in situ straining experiments in a high-voltage electron microscope equipped with an environmental cell. The generation rate and velocity of dislocations and the crack propagation rate were markedly increased by the presence of hydrogen. The advance of transgranular cracks, in both vacuum and hydrogen, occurs by either the direct emission of dislocations from the crack tip or by a complex process of hole nucleation and growth ahead of the crack. Intergranular cracks also propagate by these mechanisms; these cracks advance along the deformation zone that follows the contour of the boundary rather than along the boundary interface. The effect of hydrogen is to decrease the stress required for crack advance and to localize the deformation. When viewed macroscopically, this confinement of the plastic deformation to a narrow zone gives the impression of a brittle type fracture. Hydrogen “embrittlement” of nickel therefore occurs by a mechanism whereby hydrogen locally enhances plastic processes rather than by a decohesion mechanism.

TEM in situ deformation study of the interaction of lattice dislocations with grain boundaries in metals

Abstract The passage of dislocations across grain boundaries in metals has been studied by using the in situ TEM deformation technique. A detailed analysis of the interaction of glissile matrix dislocations with grain-boundary dislocations has been performed. The results show that the dislocations piled-up at the grain boundary can: (1) be transferred directly through the grain boundary into the adjoining grain; (2) be absorbed and transformed into extrinsic grain-boundary dislocations; (3) be accommodated in the grain boundary, followed by the emission from the grain boundary of a matrix dislocation; and (4) be ejected back into their original grain. To predict which slip system is favourable for slip transfer, three criteria have been considered, namely: (1) the angle between the lines of intersection of the incoming and outgoing slip planes with the grain boundary, this should be as small as possible; (2) the resolved shear stress acting on the possible slip systems in the adjoining grain, this should ...

An In Situ Transmission Electron Microscope Deformation Study of the Slip Transfer Mechanisms in Metals

The slip transfer mechanisms across grain boundaries in 310 stainless steel, high-purity aluminum, and a Ni-S alloy have been studied by using thein situ transmission electron microscope (TEM) deformation technique. Several interactions between mobile lattice dislocations and grain boundaries have been observed, including the transfer and generation of dislocations at grain boundaries and the nucleation and propagation of a grain boundary crack. Quantitative conditions have been established to correctly predict the slip transfer mechanism.

Silver Cluster Formation, Dynamics, and Chemistry in Metal−Organic Frameworks

Synthetic methods used to produce metal nanoparticles typically lead to a distribution of particle sizes. In addition, creation of the smallest clusters, with sizes of a few to tens of atoms, remains very challenging. Nanoporous metal-organic frameworks (MOFs) are a promising solution to these problems, since their long-range crystalline order creates completely uniform pore sizes with the potential for both steric and chemical stabilization. We report a systematic investigation of silver nanocluster formation within MOFs using three representative MOF templates. The as-synthesized clusters are spectroscopically consistent with dimensions < or =1 nm, with a significant fraction existing as Ag(3) clusters, as shown by electron paramagnetic resonance. Importantly, we show conclusively that very rapid TEM-induced MOF degradation leads to agglomeration and stable, easily imaged particles, explaining prior reports of particles larger than MOF pores. These results solve an important riddle concerning MOF-based templates and suggest that heterostructures composed of highly uniform arrays of nanoparticles within MOFs are feasible.

Prediction of slip transfer mechanisms across grain boundaries

Etude des interactions entre les dislocations et les joints de grains dans un acier inoxydable deforme in situ dans un microscope electronique en transmission.Determination,a partir de ces observations des conditions de prevision des systemes de glissement.

Time-dependent, protein-directed growth of gold nanoparticles within a single crystal of lysozyme

Gold nanoparticles are useful in biomedical applications due to their distinct optical properties and high chemical stability. Reports of the biogenic formation of gold colloids from gold complexes has also led to an increased level of interest in the biomineralization of gold. However, the mechanism responsible for biomolecule-directed gold nanoparticle formation remains unclear due to the lack of structural information about biological systems and the fast kinetics of biomimetic chemical systems in solution. Here we show that intact single crystals of lysozyme can be used to study the time-dependent, protein-directed growth of gold nanoparticles. The protein crystals slow down the growth of the gold nanoparticles, allowing detailed kinetic studies to be carried out, and permit a three-dimensional structural characterization that would be difficult to achieve in solution. Furthermore, we show that additional chemical species can be used to fine-tune the growth rate of the gold nanoparticles.

HVEM STUDIES OF THE EFFECTS OF HYDROGEN ON THE DEFORMATION AND FRACTURE OF AISI TYPE 316 AUSTENITIC STAINLESS STEEL

Abstract The mechanisms of hydrogen embrittlement in AISI type 316 austenitic stainless steel have been investigated by in situ straining in a high-voltage electron microscope (HVEM) equipped with an environmental cell. Hydrogen effects on strain-induced phase transformations, the generation rate and velocity of dislocation, and crack propagation rates were studied. The salient features of the fracture were similar for cracks propagating in vacuum and in hydrogen gas. In each case, e and α′ martensite formed at the crack; the e phase extended ahead of the crack while the α′ phase was restricted to high stress regions near the crack tip. The principal effect of hydrogen was to decrease the stress required for dislocation motion, for phase transformation of the austenite, and for crack propagation.

Hydrogen Embrittlement Understood

The connection between hydrogen-enhanced plasticity and the hydrogen-induced fracture mechanism and pathway is established through examination of the evolved microstructural state immediately beneath fracture surfaces including voids, “quasi-cleavage,” and intergranular surfaces. This leads to a new understanding of hydrogen embrittlement in which hydrogen-enhanced plasticity processes accelerate the evolution of the microstructure, which establishes not only local high concentrations of hydrogen but also a local stress state. Together, these factors establish the fracture mechanism and pathway.