Origin of the elastic limit of body-centered cubic metals at low temperature
Origin of low-temperature macroscopic elastic limit of body-centered cubic metals
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Low-temperature softening in body-centered cubic alloys
In the low-temperature range, bcc alloys exhibit a lower stress-temperature dependence than the pure base metals. This effect often leads to a phenomenon that is called 'alloy softening': at low temperatures, the yield stress of an alloy may be lower than that of the base metal. Various theories are reviewed; the most promising are based either on extrinsic or intrinsic models of low-temperature deformation. Some other aspects of alloy softening are discussed, among them the effects on the ductile-brittle transition temperature.
Pink, E.↗
Improved Mechanical Performance in FDM Cellular Frame Structures through Partial Incorporation of Faces
The utilization of lattice-type cellular architectures has seen a significant increase, owing to their predictable shape and the ability to fabricate templated porous materials through low-cost 3D-printing methods. Frames based on atomic lattice structures such as face-centered cubic (FCC), body-centered cubic (BCC), or simple cubic (SC) have been utilized. In FDM, the mechanical performance has been impeded by stress concentration at the nodes and melt-solidification interfaces arising from layer-by-layer deposition. Adding plates to the frames has resulted in improvements with a concurrent increase in weight and hot-pocket-induced dimensional impact in the closed cells formed. In this paper, we explore compressive performance from the partial addition of plates to the frames of a SC-BCC lattice. Compression testing of both single unit cells and 4 × 4 × 4 lattices in all three axial directions is conducted to examine stress transfer to the nearest neighbor and assess scale-up stress transfer. Our findings reveal that hybrid lattice structure unit cells exhibit significantly improved modulus in the range of 125% to 393%, specific modulus in the range of 13% to 120%, and energy absorption in the range of 17% to 395% over the open lattice. The scaled-up lattice modulus increased by 8% to 400%, specific modulus by 2% to 107%, and energy absorption by 37% to 553% over the lattice frame. Parameters that emerged as key to improved lightweighting.
FDM↗
Hydrogen trapping and the interaction of hydrogen with metals
A method has been developed for the determination of trapped hydrogen in metal alloys, involving the determination of mobile hydrogen using the electrochemical method and the determination of total hydrogen with the fusion method, the difference in hydrogen concentrations being due to trapped hydrogen. It has been found that hydrogen enters body-centered cubic structures through the grain bodies rather than through the grain boundaries. Hydrogen also diffuses much more rapidly in body-centered cubic structures on charging than in face-centered cubic structures, the hydrogen distribution being more uniform in nature. The energy necessary to cause hydrogen embrittlement is postulated to arise from the changes in crystal lattice energies brought about through interaction of hydrogen with atoms in the metal lattice. The total energy change is more negative for body-centered cubic structures, believed to be the cause of a greater tendency toward hydrogen embrittlement. Finally, the agreement of hydrogen concentrations obtained at 25 C by the electrochemical method with those obtained by the fusion method are taken as a strong indication of the power and validity of the electrochemical method.
Danford, Merlin D.↗
Vacancy Relaxation in Cubic Crystals
The configuration of the atoms surrounding a vacancy in four face-centered cubic and three body-centered cubic metals has been computed, using a pairwise, central-force model in which the energy of interaction between two atoms was taken to have the form of a Morse function. Only radial relaxations were considered. The first and second nearest-neighbor relaxations for the face-centered systems were found to be: Pb (1.42,−0.43), Ni (2.14,−0.39), Cu(2.24,−0.40) and Ca (2.73,−0.41, expressed in percentages of normal distances. For the body-centered systems the relaxations out to the fourth nearest neighbors to the vacancy were: Fe (6.07,−2.12, −0.25, −), Ba (7.85, −2.70, 0.70, −0.33) and Na (10.80, −3.14, 3.43, −0.20). The positive signs indicate relaxation toward the vacancy and the negative signs indicate relaxation away from the vacancy. The energies of relaxation (eV) are: Pb (0.162), Ni (0.626), Cu (0.560), Ca (0.400), Fe (1.410), Ba (0.950) and Na (0.172).
Girifalco, L. A.↗
The role of grain boundaries in hydrogen diffusion in metals at 25 C
The effect of grain size on hydrogen diffusion at 25 C was examined for 4340 steel (body-centered cubic) and for Inconel 718 (face-centered cubic). It was found that the effect of grain size is important for body-centered cubic structures, but plays a much less important role in face centered cubic structures. Accurate measurements of hydrogen desorption coefficients during hydrogen desorption show that these are not greatly different for both types of structures.
Danford, M. D.↗
Phase relationships in Fe-Ni alloys at high pressures and temperatures
Diffusionless phase transformations in Fe-Ni alloys with up to 35 wt pct Ni were studied using resistance-heated diamond anvil cells and synchrotron radiation. At temperatures up to 600 C and pressures up to 25 GPa, Fe-35 percent Ni showed only the face-centered cubic (fcc) phase while Fe-25 percent Ni and Fe-10 percent Ni showed the body-centered cubic (bcc) and hexagonal close-packed (hcp) phases in addition to the fcc phase. The phase relationships of these solid phases are topologically similar to those of pure iron. The triple point becomes lower in temperature, and the slope (dT/dP) of the fcc/hcp boundary decreased as Ni-content increases in the alloys. From these results it is inferred that Fe-Ni alloys in the earth's inner core might exist as two phases, fcc and hcp. The existence of these two phases could have played an important role in the separation of the inner and outer core.
Huang, Eugene↗
Magnetic phases in lunar fines - Metallic Fe or ferric oxides.
The ferromagnetic resonance observed for the Apollo 11 and 12 lunar fines is characterized by an asymmetric lineshape with a narrower appearance on the high field side. This asymmetry together with an anisotropy energy which varies from +640 to +500 G over the temperature range of 80 to 298 K indicate that the ferromagnetic resonance arises from metallic Fe having the body-centered cubic structure and not from hematite, magnetite or other Fe(3+) ions in magnetite-like phases. The g-value, the lineshape asymmetry, and the temperature dependence of the linewidth for the Apollo 14 and 15 fines as reported by other workers are found to be essentially similar to those observed for the Apollo 11 and 12 fines.
Tsay, F.-D.↗
Metallic Fe phases in Apollo 16 fines - Their origin and characteristics as revealed by electron spin resonance studies
The intense electron spin resonance (ESR) signals detected in the Apollo 16 fines from three sites (61141,4, Station 1; 64501,22, South Ray Crater; 67601,20, North Ray Crater) are found to be essentially similar in g-value, in lineshape asymmetry and in temperature dependence to those previously observed for the Apollo 11-15 fines. On the basis of these similarities, it is concluded that these ESR signals like those detected in the Apollo 11-15 fines are principally ferromagnetic in nature arising from metallic Fe phases having the body-centered cubic structure, and not from hematite, magnetite, or any other ferric oxides. It is shown that a quantitative correlation exists between the ESR linewidth observed for the Apollo 11-16 fines and their average Ni contents in the metallic Fe phases as determined by other means. A common source of meteoritic origin is indicated for the metallic Fe phases of these samples.
Tsay, F.-D.↗
The effects of surface films on mechanical behavior of B2 ordered intermetallic alloys
Surface films deposited on body-centered cubic metals can greatly reduce the yield and flow stresses and increase the ductility at low homologous temperatures. This softening process is associated with the ability of the film-substrate interface to generate large populations of mobile edge dislocations. This phenomenon is extended to B2 ordered intermetallic alloys, which display many deformation characteristics of bcc metals. Single crystal NiAl coated with thermally formed oxide films and polycrystalline FeAl with electrochemically formed Fe-Al-O films display film softening at and below room temperature. The extent of film softening is shown to depend on many factors, including crystal orientation, deformation temperature, operative slip systems, film thickness, film adherence, and film and substrate properties.
Noebe, R. D.↗
Scalability in SLA lattice through lattice orientation and hybrid frame and plate architectures
Lightweighting has been a key goal for engineers and designers, with lattice structures widely explored as the building blocks for structural components. Cellular structure-inspired lattice truss frame designs made of struts, have been extensively studied. All plate-based lattice designs have superior mechanical performance. Limits in scalability occur from formation of closed pockets limiting uv curing in processes like stereolithography (SLA). We examine hybrid frame and plate body-centered cubic-simply cubic (BCC-SC) lattices under compression. Unit cells and scaled up lattice exhibit an increase in yield stress and modulus with the addition of plates. The loading direction on the hybrid frame and plate unit cells affected the magnitude of improvement. Simulations and measurements indicated that the optimal lightweight lattice was determined to be when two plates were placed opposite each other with plates buttressing the struts, inhibiting buckling of the struts aligned with the loading direction. This lattice resulted in 63% improvement in specific modulus, a 137% improvement in specific yield point, and a 360% improvement in specific energy absorption (SEA) and the scaled up 4 × 4 × 4 scaled-up structures, showed a 107%, 148%, and 297% in specific modulus, specific yield point, and SEA, respectively A combined stretching-bending behavior was identified in optimal orientations reflecting the delayed buckling mechanism paired to a rising stress-strain curve past the elastic yield indicating bending resistance. The mass moment of inertia was found to be a key parameter correlating optimum orientation for the same number of plates added to the BCC-SC frame.
Mahan Ghosh↗
A structural analysis of small vapor-deposited 'multiply twinned' gold particles
High resolution selected zone dark field, Bragg reflection imaging and weak beam dark field techniques of transmission electron microscopy were used to determine the structure of small gold particles vapor deposited on NaCl substrates. Attention was focused on the analysis of those particles in the 50-150 A range that have pentagonal or hexagonal bright field profiles. These particles have been previously described as multiply twinned crystallites composed of face-centered cubic tetrahedra. The experimental evidence of the present studies can be interpreted on the assumption that the particle structure is a regular icosahedron or decahedron for the hexagonal or the pentagonal particles respectively. The icosahedron is a multiply twinned rhombohedral crystal and the decahedron is a multiply twinned body-centered orthorhombic crystal, each of which constitutes a slight distortion from the face-centered cubic structure.
Yang, C. Y.↗