Our safety, comfort and peace of mind are heavily dependent upon our capability to prevent, predict or postpone the failure of components and structures on the basis of sound physical principles While the external loadings acting on a material or component are clearly important, There are other contributory factors including unfavourable materials microstructure, pre-existing defects and residual stresses Residual stresses can add to, or subtract from, the applied stresses and so when unexpected failure occurs it is often because residual stresses have combined critically with the applied stresses, or because together with the presence of undetected defects they have dangerously lowered the applied stress at which failure will occur Consequently it is important that the origins of residual stress are understood, opportunities for removing harmful or introducing beneficial residual stresses recognized, their evolution in-service predicted, their influence on failure processes understood and safe structural integrity assessments made, so as to either remove the part prior to failure, or to take corrective action to extend life This paper reviews the progress in these aspects in the light of the basic failure mechanisms

/pdf/residual-stress-and-its-role-in-failure-2ld6sd4vw6.pdf

Residual stress and its role in failure

X-ray absorption and dichroism of transition metals and their compounds

Wire and arc additive manufacturing (WAAM) is a novel manufacturing technique in which large metal components can be fabricated layer by layer. In this study, the macrostructure, microstructure, and mechanical properties of a Ti-6Al-4V alloy after WAAM deposition have been investigated. The macrostructure of the arc-deposited Ti-6Al-4V was characterized by epitaxial growth of large columnar prior-β grains up through the deposited layers, while the microstructure consisted of fine Widmanstatten α in the upper deposited layers and a banded coarsened Widmanstatten lamella α in the lower layers. This structure developed due to the repeated rapid heating and cooling thermal cycling that occurs during the WAAM process. The average yield and ultimate tensile strengths of the as-deposited material were found to be slightly lower than those for a forged Ti-6Al-4V bar (MIL-T 9047); however, the ductility was similar and, importantly, the mean fatigue life was significantly higher. A small number of WAAM specimens exhibited early fatigue failure, which can be attributed to the rare occurrence of gas pores formed during deposition.

Microstructure and Mechanical Properties of Wire and Arc Additive Manufactured Ti-6Al-4V

We employ the high-speed synchrotron hard X-ray imaging and diffraction techniques to monitor the laser powder bed fusion (LPBF) process of Ti-6Al-4V in situ and in real time. We demonstrate that many scientifically and technologically significant phenomena in LPBF, including melt pool dynamics, powder ejection, rapid solidification, and phase transformation, can be probed with unprecedented spatial and temporal resolutions. In particular, the keyhole pore formation is experimentally revealed with high spatial and temporal resolutions. The solidification rate is quantitatively measured, and the slowly decrease in solidification rate during the relatively steady state could be a manifestation of the recalescence phenomenon. The high-speed diffraction enables a reasonable estimation of the cooling rate and phase transformation rate, and the diffusionless transformation from β to α
 ’
 phase is evident. The data present here will facilitate the understanding of dynamics and kinetics in metal LPBF process, and the experiment platform established will undoubtedly become a new paradigm for future research and development of metal additive manufacturing.

Real-time monitoring of laser powder bed fusion process using high-speed X-ray imaging and diffraction

Actinide elements produce a plethora of interesting physical behaviors due to the $5f$ states. This review compiles and analyzes progress in the understanding of the electronic and magnetic structure of the $5f$ states in actinide metals. Particular interest is given to electron energy-loss spectroscopy and many-electron atomic spectral calculations, since there is now an appreciable library of core $d\ensuremath{\rightarrow}\text{valence}$ $f$ transitions for Th, U, Np, Pu, Am, and Cm. These results are interwoven and discussed against published experimental data, such as x-ray photoemission and absorption spectroscopy, transport measurements, and electron, x-ray, and neutron diffraction, as well as theoretical results, such as density-functional theory and dynamical mean-field theory.

Nature of the 5f states in actinide metals

The morphology and microstructure induced in high quality fused silica by UV (355 nm) laser pulses at high fluence (10–45 J/cm 2 ) have been investigated using a suite of microscopic and spectroscopic tools. The laser beam has a near-Gaussian profile with a 1/e 2 diameter of� 0.98 mm at the sample plane and a pulse length FWHM (full width at half maximum) of 7.5 ns. The damage craters consist of a molten core region (thermal explosion), surrounded by a near concentric region of fractured material. The latter arises from propagation of lateral cracks induced by the laser-generated shock waves, which also compact the crater wall,� 10 lm thick and� 20% higher in density. The size of the damage crater varies with laser fluence, number of pulses, and laser irradiation history. In the compaction layer, there is no detectable change in the Si/O stoichiometry to within ±1.6% and no crystalline nano-particles of Si were observed. Micro(1–10 lm) and nano- (20–200 nm) cracks are found, however. A lower valence Si 3+ species on the top 2–3 nm of the compaction layer is evident from the Si 2p XPS. The results are used to construct a physical model of the damage crater and to gain critical insight into laser damage process. � 2005 Elsevier B.V. All rights reserved. PACS: 61.80.Ba; 61.72.Ji; 78.60.Hk; 76.30.Mi

/pdf/morphology-and-microstructure-in-fused-silica-induced-by-2kpmguc090.pdf

Morphology and microstructure in fused silica induced by high fluence ultraviolet 3ω (355 nm) laser pulses

X-ray absorption spectroscopy at the Mn K edge was employed to elucidate the structure of colloidal tetraalkylammonium (TAA) manganese oxides in sols and gels obtained by different preparation and heat treatment procedures. Two series of colloidal TAA MnOx prepared with tetrapropylammonium (TPA) and tetraethylammonium (TEA) cations were studied. Several manganese oxides, birnessite, and feitknechtite were also measured and served as model compounds for structural refinements. Near edge structure (XANES) analysis revealed different average valences of the colloidal systems. As synthesized and heat-treated, TAA colloids exhibited an average valence of 3.6−3.7, whereas gelled TAA colloids showed a lower average valence of ∼3.5. Extended absorption fine structure (EXAFS) analysis was carried out to distances of ∼6.0 A around the central Mn atom using theoretical backscattering phases and amplitudes calculated from the ab initio FEFF code. All multiple-scattering (MS) paths with a weight of 2% and more with re...

Multiple-Scattering EXAFS Analysis of Tetraalkylammonium Manganese Oxide Colloids

In-situ observations of phase transformations during solidification and cooling of austenitic stainless steel welds using time-resolved x-ray diffraction

The ${\mathit{L}}_{1,2,3}$ edges of Pr, Gd, and Ho in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ were compared with a variety of reference compounds in an attempt to determine their valence and electronic configuration. A double-differential data analysis technique was used to resolve the multiplet structure of the ${\mathit{L}}_{1,2,3}$ edge resonances. Features were observed in the spectra that were specific for each valence. The results showed that Gd and Ho were trivalent while the spectra of Pr in Y-Ba-Cu-O were unique. The most intense ${\mathit{L}}_{2,3}$ edge resonances were at the same energy position and nearly the same in appearance as the trivalent Pr reference compounds but with unique, extra features attributable to ${\mathrm{Pr}}^{4+}$. Their intensity was inversely proportional to Pr concentration. We tentatively identified this structure as substitution of ${\mathrm{Pr}}^{4+}$ into the ${\mathrm{Cu}}_{2}$ site. These resonances may have important implications for understanding the electronic structure of Pr in Y-Ba-Cu-O and the effect on superconductivity.

Determination of the valence of Pr, Gd, and Ho in YBa2Cu3O7 by x-ray absorption spectroscopy.

Spatially resolved X-ray diffraction (SRXRD) is used to map the α → β → α phase transformation in the heat-affected zone (HAZ) of commercially pure titanium gas tungsten arc welds. In situ SRXRD experiments were conducted using a 180-µm-diameter X-ray beam at the Stanford Synchrotron Radiation Laboratory (SSRL) (Stanford, CA) to probe the phases present in the HAZ of a 1.9 kW weld moving at 1.1 mm/s. Results of sequential linear X-ray diffraction scans made perpendicular to the weld direction were combined to construct a phase transformation map around the liquid weld pool. This map identifies six HAZ microstructural regions between the liquid weld pool and the base metal: (1) α-Ti that is undergoing annealing and recrystallization; (2) completely recrystallized α-Ti; (3) partially transformed α-Ti, where α-Ti and β-Ti coexist; (4) single-phase β-Ti; (5) back-transformed α-Ti; and (6) recrystallized α-Ti plus back-transformed α-Ti. Although the microstructure consisted predominantly of α-Ti, both prior to and after the weld, the crystallographically textured starting material was altered during welding to produce different α-Ti textures within the resulting HAZ. Based on the travel speed of the weld, the α → β transformation was measured to take 1.83 seconds during heating, while the β → α transformation was measured to take 0.91 seconds during cooling. The α → β transformation was characterized to be dominated by long-range diffusional growth on the leading (heating) side of the weld, while the β → α transformation was characterized to be predominantly massive on the trailing (cooling) side of the weld, with a massive growth rate on the order of 100 µm/s.

Spatially resolved X-ray diffraction phase mapping and α → β → α transformation kinetics in the heat-affected zone of commercially pure titanium arc welds

Joe Wong

Papers

Morphology and microstructure in fused silica induced by high fluence ultraviolet 3ω (355 nm) laser pulses

Multiple-Scattering EXAFS Analysis of Tetraalkylammonium Manganese Oxide Colloids

In-situ observations of phase transformations during solidification and cooling of austenitic stainless steel welds using time-resolved x-ray diffraction

Determination of the valence of Pr, Gd, and Ho in YBa2Cu3O7 by x-ray absorption spectroscopy.

Spatially resolved X-ray diffraction phase mapping and α → β → α transformation kinetics in the heat-affected zone of commercially pure titanium arc welds