Modeling of Metal Powder Densification under Hot Isostatic Pressing.

The consolidation of metal powders is a complex thermomechanical process, and the temperature has a significant effect on the density distribution in the compact. The consolidation process of metal powders with an average particle size of 10 µm, 25 µm, and 50 µm under hot isostatic pressure was simulated by finite element modeling. The distribution and evolution of the relative density after being hot isostatic pressing (HIP) under 1050 °C/130 MPa/4 h, 1150 °C/130 MPa/4 h, and 1250 °C/130 MPa/4 h conditions were simulated, respectively. The experimental data of HIP at 1050 °C/130 MPa/4 h were used to verify the modeling results via the geometric change in the container. The relative density difference between the simulated results and the experimental results at different positions was less than 2%. This methodology called "modeling prediction, experimental validation" can accelerate experimental discovery in an economic manner.

Prediction of Grain Size in a High Cobalt Nickel-Based Superalloy.

With the advancement in computational approaches and experimental, simulation, and modeling tools in recent decades, a trial-and-validation method is attracting more attention in the materials community. The development of powder metallurgy Ni-based superalloys is a vivid example that relies on simulation and experiments to produce desired microstructure and properties in a tightly controlled manner. In this research, we show an integrated approach to predicting the grain size of industrial forgings starting from lab-scale cylindrical compression by employing modeling and experimental validation. (a) Cylindrical compression tests to obtain accurate flow stress data and the hot working processing window; (b) double-cone tests of laboratory scale validation; (c) sub-scale forgings for further validation under production conditions; and (d) application and validation on full-scale industrial forgings. The procedure uses modeling and simulation to predict metal flow, strain, strain rate, temperature, and the resulting grain size as a function of thermo-mechanical processing conditions. The models are calibrated with experimental data until the accuracy of the modeling predictions is at an acceptable level, which is defined as the accuracy at which the results can be used to design and evaluate industrial forgings.

Electrochemical Behaviors of Ni-Base Superalloy CMSX-4 in 3.5 wt.% NaCl: Effect of Temperature and Preoxidation.

The electrochemical behaviors of the Ni-base superalloy CMSX-4 were carried out in 3.5 wt.% NaCl solution using electrochemical technique. The electrochemical corrosion process was divided into four stages, and reactions at the alloy surface and corrosion morphology at each stage were analyzed. The passivity mechanism at the stable passivation stage and the occurrence of pitting corrosion at the transpassivation state were discussed especially. The corrosion parameters including Ecorr, Epass, ipass and Epit were compared at different temperatures to reveal the relationship between the temperature and the corrosion resistance properties. The corrosion products were investigated by the aid of X-Ray Diffraction (XRD) and Energy Dispersive Spectroscopy (EDS). By designing different preoxidation procedures, the corrosion mechanism of oxide scales was analyzed for the preoxidized samples.

Strain Amount Dependent Grain Size and Orientation Developments during Hot Compression of a Polycrystalline Nickel Based Superalloy.

Controlling grain size in polycrystalline nickel base superalloy is vital for obtaining required mechanical properties. Typically, a uniform and fine grain size is required throughout forging process to realize the superplastic deformation. Strain amount occupied a dominant position in manipulating the dynamic recrystallization (DRX) process and regulating the grain size of the alloy during hot forging. In this article, the high-throughput double cone specimen was introduced to yield wide-range strain in a single sample. Continuous variations of effective strain ranging from 0.23 to 1.65 across the whole sample were achieved after reaching a height reduction of 70%. Grain size is measured to be decreased from the edge to the center of specimen with increase of effective strain. Small misorientation tended to generate near the grain boundaries, which was manifested as piled-up dislocation in micromechanics. After the dislocation density reached a critical value, DRX progress would be initiated at higher deformation region, leading to the refinement of grain size. During this process, the transformations from low angle grain boundaries (LAGBs) to high angle grain boundaries (HAGBs) and from subgrains to DRX grains are found to occur. After the accomplishment of DRX progress, the neonatal grains are presented as having similar orientation inside the grain boundary.

Atomistic Origin of Deformation Twinning in Biomineral Aragonite.

Deformation twinning rarely occurs in mineral materials which typically show brittle fracture. Surprisingly, it has recently been observed in the biomineral aragonite phase in nacre under high rate impact loading. In this Letter, the twinning tendency and the competition between fracture and deformation twinning were revealed by first principles calculations. The ratio of the unstable stacking fault energy and the stacking fault energy in orthorhombic aragonite is hitherto the highest in a broad range of metallic and oxide materials. The underlining physics for this high ratio is the multineighbor shared ionic bonds and the unique relaxation process during sliding in the aragonite structure. Overall, the unique deformation twining along with other highly coordinated deformation mechanisms synergistically work in the hierarchical structure of nacre, leading to the remarkable strengthening and toughening of nacre upon dynamic loading, and thus protecting the mother-of-pearl from predatory attacks.

Origin of flaw-tolerance in nacre.

Over the past decades, our understanding of nacre's toughening origin has long stayed at the level of crack deflection along the biopolymer interface between aragonite platelets. It has been widely thought that the ceramic aragonite platelets in nacre invariably remain shielded from the propagating crack. Here we report an unexpected experimental observation that the propagating crack, surprisingly, invades the aragonite platelet following a zigzag crack propagation trajectory. The toughening origin of previously-thought brittle aragonite platelet is ascribed to its unique nanoparticle-architecture, which tunes crack propagation inside the aragonite platelet in an intergranular manner. For comparison, we also investigated the crack behavior in geologic aragonite mineral (pure monocrystal) and found that the crack propagates in a cleavage fashion, in sharp contrast with the intergranular cracking in the aragonite platelet of nacre. These two fundamentally different cracking mechanisms uncover a new toughening strategy in nacre's hierarchical flaw-tolerance design.

Order-disorder transition of aragonite nanoparticles in nacre.

Understanding nacre's bottom-up biomineralization mechanism, particularly, how individual aragonite platelets are formed, has long remained elusive due to its crystallographic peculiarity and structural complexity. Here we report that crystallographic order-disorder transition can be triggered within individual aragonite platelets in pristine nacre by means of heat treatment and/or inelastic deformation, offering a unique opportunity to discriminate mysterious aragonite nanoparticles in transmission electron microscopy. Our findings unambiguously uncover why aragonite nanoparticles in pristine nacre have long been inaccessible under TEM observation, which is attributed to the monocrystal-polycrystal duality of the aragonite platelet. The underlying physical mechanism for why an individual aragonite platelet adopts a highly oriented attachment of aragonite nanoparticles as its crystallization pathway is, for the first time, explained in terms of the thermodynamics. The finding of an order-disorder transition in nacre provides a new perspective for understanding the formation for other biominerals.

Uncovering high-strain rate protection mechanism in nacre.

Under high-strain-rate compression (strain rate approximately 10(3) s(-1)), nacre (mother-of-pearl) exhibits surprisingly high fracture strength vis-à-vis under quasi-static loading (strain rate 10(-3) s(-1)). Nevertheless, the underlying mechanism responsible for such sharply different behaviors in these two loading modes remains completely unknown. Here we report a new deformation mechanism, adopted by nacre, the best-ever natural armor material, to protect itself against predatory penetrating impacts. It involves the emission of partial dislocations and the onset of deformation twinning that operate in a well-concerted manner to contribute to the increased high-strain-rate fracture strength of nacre. Our findings unveil that Mother Nature delicately uses an ingenious strain-rate-dependent stiffening mechanism with a purpose to fight against foreign attacks. These findings should serve as critical design guidelines for developing engineered body armor materials.

Unveiling the formation mechanism of pseudo-single-crystal aragonite platelets in nacre.

We demonstrate direct evidence that a single-crystal-like aragonite platelet is essentially assembled with aragonite nanoparticles. The aragonite nanoparticles are readily oriented and assembled into pseudo-single-crystal aragonite platelets via screw dislocation and amorphous aggregation, which are two dominant mediating mechanisms between nanoparticles during biomineralization. These findings will advance our understanding of nacre's biomineralization process and provide additional design guidelines for developing biomimetic materials.