Crack-like indications were identified in a gas transmission pipeline during a routine integrity assessment. Visual examination and CT scanning confirmed the presence of cracks on the pipe internal diameter and throughout the wall thickness near the weld seam. Fractographic and metallographic analysis indicated that the cracking occurred in numerous repair welds that were made prior to the final pipe seam weld. The presence of copper on crack faces and within internal cracks, as well as the intergranular path of those cracks, indicated the cracking occurred by copper contamination cracking associated with repair welding.

Engineering materials exhibit an undesirable tradeoff between strength and resistance to crack propagation (fracture toughness). Here we demonstrate how this tradeoff can be circumvented by thermo-mechanical processing that produces a partially recrystallized, heterogeneous microstructure. An equimolar CrCoNi alloy was forged at room temperature (298 K) to produce high densities of three-dimensional crystallographic defect networks. Post-deformation heat treatments caused localized recrystallization that resulted in a bimodal microstructure with hard, non-recrystallized grains and soft, recrystallized grains. In this condition, the yield strength at 298 K is 2.75x the values previously obtained for the same alloy in the fully recrystallized state while the fracture toughness remains the same. The yield strength is further enhanced at 77 K without compromising the fracture toughness. This outstanding strength-toughness combination at 77 K exceeds those reported for other metallic materials and appears to result from the composite nature of the microstructure with non-recrystallized grains providing strength and recrystallized grains enabling plasticity that dissipates stresses during crack propagation. Our findings indicate that by tuning the degree of recrystallization through thermomechanical processing techniques, it will be possible to further expand the envelope bounding the strength and toughness of a range of structural metals at engineering component scales.

The desire to industrialize and expand the envelope of L-PBF additive manufacturing has driven development of Multi-Laser Powder Bed Fusion (ML-PBF) technology. Stitching of large parts by multiple lasers has become a reality, but the material effects have yet to be well understood. This study examines the differences in mechanical properties and microstructures for stitched and nominal, single laser exposed zones in Nickel-base alloy 718 produced via L-PBF. Multiple industry-relevant heat treatments were examined including stress relief, hot isostatic pressing (HIP), solution heat treatment, and aging. A range of responses between the stitched and nominal zones with each heat treatment have been determined. The addition of an 1120 degrees C HIP cycle eliminated most signs of heterogeneity between the two zones and provides a promising avenue for industrial adoption.

Heterogeneous, partially recrystallized (PRX) microstructures have recently been used to improve strengthductility combinations in high-entropy alloys. However, these microstructures are incompatible with conventional joining processes that require melting or prolonged exposure to elevated temperatures. This work presents an initial exploration of solid state joining in this challenging condition using vaporizing foil actuator welding (VFAW) applied to PRX equiatomic alloy CrCoNi.

A CrCoNi based medium entropy alloy with small additions of Ti, Al and Nb (denoted as (CrCoNi)93Al4Ti2Nb) in the as-quenched condition, exhibits tensile properties comparable to those of the equiatomic CrCoNi alloy at room temperature. Dark field transmission electron microscopy (TEM), atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) together with atom probe tomography (APT) show that spatially-localized long range ordering (LRO) L12 domains exist in this alloy. The evolution of deformation substructure with plastic deformation in this alloy was characterized using electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI) and STEM based techniques including the recently developed weak beam dark field STEM imaging. Plastic deformation occurs by the slip of a/2<110>dislocations, which are narrowly dissociated into Shockley partial dislocations on {111} slip planes. Their dissociation distances in the (CrCoNi)93Al4Ti2Nb alloy are much smaller than the widths of the corresponding partials in the equiatomic CrCoNi alloy due to one or more of the minor alloying elements (Al, Ti, Nb). Dislocation slip in this alloy has a pronounced planar character. The leading dislocations in slip bands glide as pairs due to the existence of LRO domains. Multipoles were formed through the slip of dislocations with opposite signs on adjacent {111} slip planes. Those multipoles serve as building blocks for the formation of subgrain structures consisting of fine slip bands. The distances between slip bands were continuously refined during plastic deformation and dynamic refinement of slip bands plays a crucial role in strain hardening. The effects of LRO domains on planar dislocation slip, the deactivation of deformation twinning and strain hardening of this alloy are discussed.

Cylindrical specimens of CrCoNi alloy with electropolished surfaces were subjected to constant total strain amplitude low cycle fatigue. The alloy exhibited an initial period of cyclic hardening followed by cyclic softening until failure occurred. At the end of hardening stage at the peak of cyclic stress, well-developed persistent slip markings (PSMs) consisting of extrusions and intrusions were associated with thin deformation twins. A sophisticated experimental workflow was designed to extract information from the surface and the bulk of tested material. A combination of SEM, EBSD, ECCI, FIB and HR-STEM was used to study the internal structure and the surface profiles around the deformation twins, which were produced during the initial period of cyclic loading. Furthermore, localized cyclic plastic strain and stress concentrations near deformation twins led not only to early, well-developed PSMs, but also to the activation of TWIP and TRIP plasticity even at low macroscopic stress amplitudes.

A new medium-entropy superalloy was produced based on the compositions of equiatomic CrCoNi and Ni-base superalloy Inconel 740H. Initial alloy design was performed using Thermo-Calc. The aging response and microstructural stability were assessed following heat treatment at temperatures between 600 and 900 degrees C and durations up to 100 h. Aging from a fully recrystallized state resulted in negligible grain growth and produced gamma' and sigma phases. The same phases were present after aging from a cold-rolled state, but partially recrystallized microstructures resulted in multi-modal size distributions and heterogeneous spatial arrangements. Room temperature hardness measurements were used to correlate aging conditions with quantitative precipitate measurements and mechanical properties. (C) 2019 Elsevier B.V. All rights reserved.

High strain-hardening rates in equiatomic CrCoNi and other multi-principal element alloys have been attributed to deformation twinning. This work shows that small additions of Al and Ti to a CrCoNi alloy deactivate deformation twinning with only minor changes to uniform elongation and ultimate tensile strength. The initial microstructure is free of chemically ordered (Al,Ti)-rich precipitates after solutionizing and quenching. Tensile properties for the alloy are reported and compared to equiatomic CrCoNi, and the post-deformation microstructure is assessed. Density functional theory calculations indicate that energetically unfavorable Al-Al bonds may discourage shearing via partial dislocations, which are necessary for twinning to occur.

High-entropy alloys (HEA)/multi-principal element alloys (MPEA) became a topic of very high interest among research groups all over the world.However, despite the effort which has been ongoing for several years, understanding and application of these new materials in real service conditions is presently lacking.Research in the field of MPEAs has been devoted to investigation of monotonic mechanical properties.Nevertheless, components in real service are usually exposed to cyclic external forces and loads, and thus understanding of fatigue damage and behavior is of significant interest.

Despite having otherwise outstanding mechanical properties, many single-phase medium and high entropy alloys are limited by modest yield strengths. Although grain refinement offers one opportunity for additional strengthening, it requires significant and undesirable compromises to ductility. This work therefore explores an alternative, simple processing route to achieve strength by cold-rolling and annealing an equiatomic CrCoNi alloy to produce heterogeneous, partially recrystallized microstructures. Tensile tests reveal that our approach dramatically increases the yield strength (to similar to 1100 MPa) while retaining good ductility (total elongation similar to 23%) in the single-phase CrCoNi alloy. Scanning and transmission electron microscopy indicate that the strengthening is due to the non-recrystallized grains retaining their deformation-induced twins and very high dislocation densities. Load-unload-reload tests and grain-scale digital image correlation are also used to study the accumulation of plastic deformation in our highly heterogeneous microstructures. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Texture evolution during room-temperature tensile testing of recrystallized equimolar CrCoNi was studied using electron backscatter diffraction and electron channeling contrast imaging on specimens from interrupted tests. Dominant deformation mechanisms included slip at low strains and deformation twinning at larger strains, which were accompanied by the development of a strong <111> texture parallel to the tensile axis. Highly deformed material also contained nanotwin/hcp lamellae, which have previously been hypothesized to act as potent barriers for non-coplanar dislocations. To examine this hypothesis, mean-field modeling was performed using the viscoplastic self-consistent framework with varying ratios for hardening by slip and twinning. In the optimal model, twinning produced approximately three times as much non-coplanar hardening as slip, which is larger than previous observations in other twinning-induced plasticity materials that do not form twin/hcp lamellae. Additional full-field elasto-viscoplastic simulations were performed using the fast Fourier transform (EVP-FFT) method to examine intragranular rotation and the effect of initial grain orientation on the deformation mode. Grains with initial orientations near <111> had the greatest propensity for deformation twinning while grains near <100> were more likely to deform by slip even at large strains. Excellent quantitative agreement was obtained between the experiments and EVP-FFT model.

A new processing route is demonstrated for producing a nickel-based alloy with 1099 MPa tensile yield strength and 30% elongation. A chromium- and cobalt-rich commercial alloy was solution-annealed, cold-rolled, and aged at different temperatures and times to develop partially-recrystallized microstructures with ordered Ni-3(ALTi)gamma' precipitates. These were tested in uniaxial tension and compared to alloys given the same heat treatment without rolling to assess the contributions of different strengthening mechanisms. Orientation mapping showed the development of a modest & LANG;111 & RANG; and & LANG;100 & RANG; double-fiber texture. Scanning transmission electron microscopy also revealed the development of exceptionally large dislocation densities including wall structures. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

High entropy alloys have attracted increasing research interest due to their exceptional mechanical properties such as high tensile strength, ductility, and fracture toughness [1,2]. These alloys normally consist of five or more components to achieve high configurational entropy. Recent study shows that ternary NiCoCr equiatomic solid solution alloy with “medium entropy” actually has superior properties, especially at cryogenic temperature, as compared with 5 component Cantor alloy [3,4]. Previous study shows that deformation twinning plays an important role in the deformation process of Cantor alloy [5]. However, the origin of the exceptional mechanical properties of ternary NiCoCr solid solution alloy is still unclear.

The equiatomic NiCoCr alloy exhibits an excellent combination of strength and ductility, even greater than the FeNiCrCoMn high entropy alloy, and also displays a simultaneous increase in strength and ductility with decreasing the testing temperature. To systemically investigate the origin of the exceptional properties of NiCoCr alloy, which are related to the evolution of the deformation substructure with strain, interrupted tensile testing was conducted on the equiatomic NiCoCr single-phase solid solution alloy at both cryogenic and room temperatures at five different plastic strain levels of 1.5%, 6.5%, 29%, 50% and 76%. The evolution of deformation substructure was examined using electron backscatter diffraction (EBSD), transmission Kikuchi diffraction (TKD), conventional transmission electron microscopy (CTEM), diffraction contrast imaging using STEM (DCI-STEM) and atomic resolution scanning transmission electron microscopy. While the deformation substructure mainly consisted of planar dislocation slip and the dissociation of dislocations into stacking faults at small strain levels (<= 6.5%), at larger strain levels, additional substructures including nanotwins and a new phase with hexagonal close packed (HCP) lamellae also appeared. The volume fraction of the HCP lamellae increases with increasing deformation, especially at cryogenic temperature. First principles calculations at 0 K indicate that the HCP phase is indeed energetically favorable relative to FCC for this composition. The effects of the nanotwin and HCP lamellar structures on hardening rate and ductility at both cryogenic and room temperature are qualitatively discussed. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

High-temperature deformation of superalloys is a complicated process involving many possible mechanisms that depend on temperature, applied load, and microstructure [1]. Observation and quantification of microstructural features are critical to understanding and predicting macroscopic behavior. Notably, there is a growing emphasis on coupling multi-scale material characterization to local mechanical behavior; this type of work is essential for understanding the deformation of materials exhibiting complex, heterogeneous microstructures [2].

Autonomic materials are those that automatically respond to a change in environmental conditions, such as temperature or chemical composition. While such materials hold incredible potential for a wide range of uses, their implementation is limited by the small number of fully-developed material systems. To broaden the number of available systems, we have developed a post-functionalization technique where a reactive Ru catalyst ink is printed onto a non-responsive polymer substrate. Using a succinimideamine coupling reaction, patterns are printed onto co-polymer or biomacromolecular films containing primary amine functionality, such as polyacrylamide (PAAm) or poly-N-isopropyl acrylamide (PNIPAAm) copolymerized with poly-N-(3-Aminopropyl)methacrylamide (PAPMAAm). When the films are placed in the Belousov-Zhabotinsky (BZ) solution medium, the reaction takes place only inside the printed nodes. In comparison to alternative BZ systems, where Ru-containing monomers are copolymerized with base monomers, reactive printing provides facile tuning of a range of hydrogel compositions, as well as enabling the formation of mechanically robust composite monoliths. The autonomic response of the printed nodes is similar for all matrices in the BZ solution concentrations examined, where the period of oscillation decreases in response to increasing sodium bromate or nitric acid concentration. A temperature increase reduces the period of oscillations and temperature gradients are shown to function as pace-makers, dictating the direction of the autonomic response (chemical waves).

On page 2835 Richard A. Vaia and co-workers discuss a novel design-fabrication concept for autonomic materials, where the size and arrangement of active-nodes enables the natural hydrogel to feed off of ambient chemical energy and convert it to mechanical motion in a biomimetic fashion. The image shows oscillating chemomechanical nodes and a model of the coupled oscillations producing an actuator. Lauren Aprill is acknowledged for the design of the inside cover.

Critical technologies from medicine to defense are highly dependent on advanced composite materials. Increasingly there is a greater demand for materials with expanded functionality. The state of the art includes a wide range of responsive composites capable of impressive structural feats such as externally triggered shape morphing. Here a different composite concept is presented, one in which a portion of the constituent materials feed off of ambient energy and dynamically couple to convert it to mechanical motion in a cooperative, biomimetic fashion. Using a recently developed self-oscillating gel based on gelatin and the oscillating Belousov-Zhabotinsky (BZ) reaction, a technique is demonstrated for producing continuous patterned heterogeneous BZ hydrogel composites capable of sustained autonomic function. The coupling between two adjacent reactive patches is demonstrated in an autonomic cantilever actuator which converts chemical energy into amplified mechanical motion. The design of heterogeneous BZ gels for motion using a basic finite element model is discussed. This work represents notable progress toward developing internally responsive, bio-inspired composite materials for constructing modular autonomic morphing structures and devices.

Autonomic systems in biology, such as the circulatory system, function independently without an external stimulus. Materials that mimic this behavior hold promise for energy and medical technologies in the form of sensors, energy conversion units, and versatile micrometer scale machinery. Self-oscillating hydrogels driven by the Belousov-Zhabotinsky (BZ) reaction are one class of autonomic materials that convert chemical energy to mechanical swell-deswell motion. Effective feedback control techniques, as well as materials and processing options, are key challenges in making these materials technologically relevant. To address these challenges we have expanded BZ hydrogel materials options, by utilizing a flexible water-based succinimide amine coupling reaction for functionalizing bio- and synthetic polymers containing primary amines for use in BZ devices. Herein we characterize the chemomechanical behavior of homogeneous BZ gelatin gels and determine the range of periods and strains attainable. We highlight the two most distinctive, and technologically relevant, features of BZ-gelatin, specifically, its ability to be postfunctionalized with Ru(sbpy) after gel formation and its ability to be patterned into compositionally heterogeneous composites via physical solidification through the thermal reversibility of its melt-gel transition.

A small press operated by electromagnetic repulsion and driven by a pulse power supply was constructed at The Ohio State University. This design that applies kinetic energy rather than static force to do work on materials is much lighter and potentially much less expensive than traditional hydraulic, mechanical or servo presses. Performance of the kinetic press is compared to traditional presses in the applications of powder compaction and forming. The results tend to indicate that modest impact speeds of 3 to 18 m/s can improve performance in these manufacturing operations as compared to traditional low-speed machines.