ACTA

Microstructural instability in traditional nanocrystalline metals limits the understanding of the fundamental effect of grain size on mechanical behavior under extreme environmental conditions such as high temperature and loading rates. In this work, the interplay between Ta concentrations and processing temperature on the resulting microstructure of a powder processed, fully dense Cu-Ta alloy along with their tensile and compressive behavior at different strain rates are investigated to probe the possibility of manipulating or tuning microstructurally dependent parameters to control the flow-stress upturn phenomenon. Consequently, the results reveal that there is a crucial length scale, i.e., small grain size and appropriate cluster spacing, below which such upturn is damped out. The observation of changes in flow stress upturn behavior is consistent with the observed changes in measured high-rate plasticity, which enhances below the critical length scale. Furthermore, tension-compression asymmetry also tends to be suppressed in nanocrystalline Cu-Ta alloys, while it becomes evident as the grain size increases to an ultrafine regime. Overall, this work presents a systematic approach to control or engineer reduced high strain rate flow stress upturn behavior in metallic alloys, for high-rate applications.

ACTA

Grain boundary cracking in non-weldable superalloy fabricated with selective electron beam melting is affected by the interaction of multiple factors including mechanical and compositional effects. In this study, we construct process maps in a wide range high-dimensional parameter space for the non-weldable superalloy Alloy713ELC through employing a machine learning approach, and we could fabricate many cracked and crack-free samples under the optimized conditions by excluding the extrinsic effect of process defects on cracking. Comparing between cracked and crack-free samples reveals that the samples with fine columnar grains can be cracked while those with coarse columnar grains can be crack-free, and that the cracking propensity in the optimized samples within a process window with scan speed ≤ 800 mm/s can be ranked by using a quasi-total plastic strain index (QTPSI), which is calculated via thermo-mechanical analysis. The total plastic strain level is a critical cracking factor and a larger scan speed tends to elevate the total plastic strain level, exhibiting a larger deviation beyond the QTPSI. Besides, the non-weldability in Alloy713ELC significantly attributes to its thermal expansion effect, which correlates to the large Al content. This thermal expansion effect combined with the liquation effect and the strain-age cracking effect reveals the intrinsic cause of non-weldability in Alloy713ELC.

ACTA

ACTA

ACTA

High-pressure torsion was applied to join two disks of single-phase equiatomic fcc CoCrFeMnNi and bcc HfNbTaTiZr high entropy alloys (HEAs). After 15 revolutions a bulk nanocomposite had developed with alternating nano-lamellae of elongated nanocrystalline CoCrFeMnNi and mixed amorphous-nanocrystalline HfNbTaTiZr, exhibiting complex microstructures with numerous vortex-like regions. While the fcc phase retains a rather homogeneous elemental distribution, the former bcc high entropy alloy experiences a chemical separation towards Ta-rich and Ta-poor phases. The joining of dissimilar HEAs under constrained conditions (cold shut) enables the design of novel HEA-based composites by non-equilibrium processing.

ACTA

向上滑动阅览英文摘要

The role of hydrogen in the formation behavior of various lattice defects induced by plastic strain in pure iron specimens has been investigated using low-temperature thermal desorption spectroscopy (L-TDS), which can start measurement from −200 °C. Specimens were subjected to various plastic strains within a uniform elongation range in the absence and presence of hydrogen. Strain-induced lattice defects were quantitatively evaluated using L-TDS for specimens saturated with hydrogen as a probe for detecting lattice defects, not only vacancies but also dislocation cores. The L-TDS spectra contained two hydrogen desorption peaks: a low-temperature peak associated with dislocations and a high-temperature peak associated with vacancies. The tracer hydrogen content corresponding to each peak increased with plastic strain. However, when compared at the same plastic strain of 25%, the low-temperature peak, i.e., dislocations, was not enhanced by hydrogen, whereas the high-temperature peak, i.e., vacancies, was enhanced by hydrogen, increasing by about six times at 25% plastic strain and by seven times at 40% plastic strain. Analyses conducted by electron channeling contrast imaging (ECCI) and electron backscattered diffraction (EBSD) revealed that strain was localized along the vicinity of the ferrite grain boundaries at 25% plastic strain in the presence of hydrogen. Hence, the role of hydrogen in the formation behavior of plastic strain-induced lattice defects is presumably to enhance vacancy formation and the localization of the dislocation configuration near the grain boundaries without affecting the amount of dislocations formed.

ACTA

向上滑动阅览英文摘要

Twin Boundaries (TBs) are fundamental interfaces in materials science which, despite over a century of research, continue to surprise us. A longstanding anomaly in the field is the experimental observation of a Type II TB in NiTi with two distinct indicial identities: (0.7205 1 -1)≈(3 4 -4) and (8 9 -9). The nanostructure of this interface is still unclear, with varying propositions put forth over the past 4 decades. We consider multi-scale energetics, employing Molecular Statics simulations and anisotropic elasticity formalisms, to establish a Terrace-Disconnection (TD) topology as the energy-minimal nanostructure. A theoretical framework is developed based on continuum strain-energy arguments to determine the influence of microstructural strain and local twin volume fraction on interface topology. It is shown that it is energetically favorable for the topology to evolve across a continuous spectrum of indicial identities under coupled influence of both parameters. Consequently, experimental observations that were thus far considered contrasting are proposed as distinct states within this spectrum, transposing as evidence of the proposed evolving capability in the Type II TB. This topological evolution fundamentally arises from a strain-mediated change of the dislocation-spacing (equivalently, a change in the interface dislocation-density). We further propose the prevalence of this evolving behavior in both Type I and Compound TBs (in NiTi) exhibiting a seamless transition between coherent and semi-coherent states, significantly changing dislocation-densities (upto 8-fold) and exhibiting irrational Miller-index identities under non-zero strain. An “Evolving Interface” theory is proposed as an extension to the Topological Modeling framework, allowing determination of equilibrium topologies at non-zero strain and unsymmetric twin volume fractions.

Vol. 208，15 Apr. 2021, 116719

Vol. 208，15 Apr. 2021, 116718

Vol. 208，15 Apr. 2021, 116707