Large Flow Stress Research Articles

Grain boundaries dependence of plastic deformation in nanocrystalline Cu film investigated by phase field and molecular dynamics methods

The phase filed model was used to create the nanocrystalline (NC) Cu film with more natural grain boundaries (GBs) for molecular dynamics (MD) simulation. The results obtained by using these methods are in good agreement with previous experimental results. In the NC Cu film, the plastic deformation is governed by the stacking faults (SFs) and deformation twinnings (DTs). DTs in NC Cu film have a great influence on the deformation behaviors of grains, which can make two adjacent grains merge into one larger grain, generate a new grain, and twist a grain. The model with average grain size (d) of 17.84 nm exhibits large flow stress and ductility because of the large fraction of hexagonal-close-packed (HCP) structures including SFs and twinning boundaries (TBs). Besides, the deformation of GBs in the 17.84 nm model is large, which can reduce the stress-concentration on the GBs and then results in delaying the generation of DTs. The deformation behavior of curved GBs in the initial tensile stage is that atoms migrate from face-centered cubic (FCC) structures to curved GBs along (111) surface, which is different from the shuffling motion of the flat GBs.

Seam-Joining between High-Strength Steel Sheet and Aluminum Alloy Sheet by Magnetic Pulse Welding

In this paper, we describe the process parameters and the joining property of seam-welded sheets obtained by magnetic pulse welding. In this method, it is possible to weld an aluminum alloy sheet to a high-strength steel sheet (SPFH), for example, A5052-H34/SPFH or A6061-T6/SPFH. The aluminum sheet is deformed to a convex, because the electromagnetic force generated in the sheet is symmetrical with respect to the central axis. Therefore, the seam-welded sheet is joined by two lines, not a plane. Collision times of the three kinds of the aluminum sheets are examined for various gap lengths at 2.0 kJ discharge energy in experiments, and collision velocities are calculated using the collision times. The collision times of the three sheets are approximately the same at gap lengths less than or equal to 2.48 mm, because magnetic pulse welding is a phenomenon with a high strain rate. It is difficult to weld an aluminum sheet with a large flow stress to a high-strength steel sheet. Therefore the choice of process parameters is important, the influential parameters are discharge energy and gap length. We should choose the values of these two parameters such that the first collision occurs before the maximum discharge current.

Obstacles and sources in dislocation dynamics: Strengthening and statistics of abrupt plastic events in nanopillar compression

Mechanical deformation of nanopillars displays features that are distinctly different from the bulk behavior of single crystals: Yield strength increases with decreasing size and plastic deformation comes together with strain bursts or/and stress drops (depending on loading conditions) with a very strong sensitivity of the stochasticity character on material preparation and conditions. The character of the phenomenon is standing as a paradox: While these bursts resemble the universal, widely independent of material conditions, noise heard in bulk crystals using acoustic emission techniques, they emerge primarily with decreasing size and increasing strength in nanopillars. In this paper, we present a realistic but minimal discrete dislocation plasticity model for the elasto-plastic deformation of nanopillars that is consistent with the main experimental observations of nano pillar compression experiments and provides a clear insight to this paradox. With increasing sample size, the model naturally transitions between the typical progressive behavior of nanopillars to a behavior that resembles evidence for bulk mesoscale plasticity. The combination of consistent strengthening, large flow stress fluctuations and critical avalanches is only observed in the depinning regime where obstacles are much stronger than dislocation sources; in contrast, when dislocation source strength becomes comparable to obstacle barriers, then yield strength size effects are absent but plasticity avalanche dynamics is strongly universal, across sample width and aspect-ratio scales. Finally, we elucidate the mechanism that leads to the connection between depinning and size effects in our model dislocation dynamics. In this way, our model builds a way towards unifying statistical aspects of mechanical deformation across scales.

アルミニウム合金薄板と低炭素鋼板の電磁圧接における接合特性に及ぼす加工条件と材料特性

In this paper, we describe the joining property of welded sheets obtained by magnetic pulse welding. In this method, it is possible to weld aluminum alloy sheets to a low-carbon sheet, for example, A6061-T6 or A2017-T3. An electromagnetic force deforms the aluminum sheet, and the height of the sheet is governed by both the force and the properties of the sheet. Collision times are examined in various gap lengths for the four kinds of aluminum sheets in 2.0 kJ discharge energy experiments, and collision velocities are calculated using the collision times. The relationship between shearing load of the welded sheet and the gap length is also examined. The collision times of the four sheets are approximately equal at gap lengths less than or equal to 2.48 mm and increase in accordance with the materials property values of quasi-static process at gap lengths more than 2.48 mm. Magnetic pulse welding is phenomenon in a high strain rate and the strain rate of the sheet exceeds 10000 s-1 as a rough estimate. It is difficult to weld the sheets with a large flow stress to the low-carbon sheet, and the welding depends on the flow stress. Therefore, the flow stress of aluminum sheet also is a factor influencing the joining property of welded sheets.

Superplastic Forming and Foaming Process for Porous Aluminum Plates

A new application of superplasticity has been developed in the field of porous metals. Porous metals manufactured through a liquid-state foaming process often include large pores and inhomogeneous pore distribution which decrease in the mechanical properties. A solid-state foaming process enables to suppress the collapse and coalescence of pores, but high porosity is not achieved due to the large flow stress. Superplastic flow during the solid-state foaming is effective to accelerate the foaming rate and to increase the porosity. Superplastic 5083 aluminum alloy sheets are used as a starting material for manufacturing of porous aluminum. A preform plate containing titanium hydride particles as a foaming agent is produced through roll-bonding processing. Porous aluminum with small pores and relatively high porosity is manufactured through superplastic foaming process. A thin sandwich panel with a porous aluminum core and porous bulge structures can be manufactured by utilizing the superplastic flow. This technique of superplastic forming and foaming (SPFF) processing has a potential for near net-shape forming as well as evaluation of the solid-state foaming of porous metals.

Effects of induction heat treatment on mechanical properties of TiAl-based alloy

The effects of rapid heating cyclic heat treatment on mechanical properties of a TiAl-based alloy (Ti-33Al-3Cr) were studied by means of an induction heating machine. The results show that: 1) fine fully-lamellar microstructure with colony size of about 50 µm and lamellar spacing of about 0.12 µm can be obtained; 2) the compression mechanical properties can be improved to a large extent and the best comprehensive compression mechanical properties can reach the yield stress 745 MPa, the large flow stress 1 672 MPa and the compression ratio 19.4%; and 3) the compression fracture at room temperature after induction heat treatment and aging is still typical cleavage fracture.

Steady-state creep of a composite

The termination of composite creep, when no sliding or diffusion can occur on matrix/inclusion interfaces and plastic volume strain constancy is maintained at every point, is addressed. A Fourier analysis is presented which shows that for a non-uniform distribution of plastic strain the elastic energy increases with increasing macroscopic plastic strain. This indicates that steady-state creep is not possible in such a material. Hutchinson's analysis of polycrystalline plasticity is also adapted to reach the same conclusion, by giving some grains an infinitely large flow stress; namely, those grains equivalent to elastic inclusions, on whose interfaces with the matrix neither sliding nor diffusion occurs. The creep strain, at which creep in a composite terminates, is determined. If the above conditions are abandoned, creep can proceed. This is discussed with various examples. Structural changes such as interface debonding and inclusion fracture are discussed as possible causes of continued creep in a composite. It is also pointed out that sliding and diffusion on matrix inclusion interfaces is also a necessary condition for thermal cycle ratcheting.

Experimental analysis of the flattening of the cross-section, the springback and the bending moment of clad tubes in uniform bending

In order to clarify the fundamental bending-deformation characteristics and the load property of clad tubes, the flattening behavior, the springback and the bending moment of adhesive-bonded two-layer clad tubes are investigated experimentally. The experiments were carried out on clad tubes consisting of soft aluminum, hard aluminum and copper by means of a 4-point bending test, a new measuring instrument having been developed for the continuous measurement of the flattening and the bending curvature during deformation. A unique negative flattening deformation behavior in the bending direction was observed in the case of the clad tubes (Al/Al) with hard aluminum on the outside. It is found that the springback on the bending curvature and the bending moment of a clad tube having a material with large flow stress on the outside are greater than those of a clad tube having the reverse configuration.

Effects of high magnetic fields on the flow stress of 18-8 stainless steels

The plastic deformation of two 18-8 stainless steels was studied in magnetic fields as large as 18 T at temperatures of 4, 77 and 290 K. Changes in stress-strain behavior during magnetic exposure at cryogenic temperatures are reported, and are associated with an enhanced f.c.c.→ b.c.c. martensitic transformation. In the magnetic field a reduced flow stress was observed at small strains, but at larger strains an increased rate of work hardening and a larger flow stress were observed. Additionally, a reduced elongation was found for the specimens tested in the magnetic field. Although these effects of magnetic fields are probably too small to be of engineering importance, they provide a unique means of determining how the f.c.c.→ b.c.c. martensitic transformation affects the plastic deformation of 18-8 stainless steels.

Characterization of aluminum–aluminum nitride coatings sputter deposited using the pulsed gas process

A dc triode magnetron has been used to produce free‐standing Al/Al+AlN lamellar foils by sputter deposition. The 5‐μm‐thick foils produced on both flat substrates as well as curved substrates exhibited good specularity as well as excellent mechanical properties. The pulse spacing was varied from zero to 100‐nm spacing. The yield strength of the material was found to obey the Hall–Petch relation σys = 230+0.07/d1/2, where σys is in MPa. Auger electron spectroscopy and secondary ion mass spectroscopy indicate that the large flow stress of 230 MPa must be due to grain refinement of the extended source and not an impurity effect. The result is that limitations of masking found in uniaxial flux sources for curved surfaces can be removed allowing the high quality coating of more general shapes.

DYNAMIC STRAIN AGEING IN LiF SINGLE CRYSTALS

Dislocation-point defect iiiterac~ions have been investigated in high purity LiF single crystals containing less than 5 ppni of Mg?-, utilising dynamic strain ageing techniques. Two mechanisms of strain ageing have been found. Below 200 the mechanism is one of weak pinning of dislocations by tetragonal strain fields around impurity-vacancy dipoles giving rise to a slight flow stress increase after ageing under stress. Above 200 C. the strain ageing is due to the strong pinning of dislocations by ordered atmospheres containing impurity-vacancy dipoles and othcr point defects. This results in the presence of serrated yielding, and a large flow stress increase after ageing under stress. Thcse erects may also be important in previo~~sly reported work hardening data in LiF single crystals.

Elastic Deformation, Plastic Flow, and Dislocations in Single Crystals of Titanium Carbide

Single crystals of the brittle refractory-hard-metal TiC have been plastically deformed at temperatures from 800° to 2200°C. Dislocation etch-pit studies on {100} cleavage surfaces after bending showed that the slip plane is {111} in the rocksalt structure. The slip direction is taken to be that of the shortest Burgers vector, 〈110〉. The slip system is then the same as that of fcc metals and diamond cubic semiconductors. The resolved shear stress for slip in TiC at 1100°C was found to be large: ∼10 kg/mm2. TiC crystals loaded at room temperature did not deform plastically, but did accommodate elastic strain up to 1.2% if the surfaces were electropolished. A shear stress of 0.4 of the theoretical strength was obtained. These results are consistent with the long-held hypothesis that the binding in hard metal carbides is partially covalent and not exclusively metallic. The Peierls stress resisting dislocation motion should then be large, as in the covalent crystals Ge and Si, and thermal activation would be required for plastic flow. The observed formation of debris by jogs in moving dislocations might alternatively be responsible for the large flow stress. The effects of impurities, precipitates, and composition differences are being investigated.