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The whisker morphology after being exposed in different environment SD condition for 22 days. [A-P. Xian and M. Liu: Effect of humidity on tin whisker growth from Sn3Nd intermetallic compound. p. 1652.]
The indentation size effect (ISE) and the bilinear behavior for pure face centered cubic (FCC) metals including aluminum, nickel, silver, and 70/30 copper–zinc (α-brass) alloy using a single Berkovich indenter tip in a single test machine were investigated. The results confirmed that this behavior is mechanistic in nature and were consistent with those reported by A.A. Elmustafa and D.S. Stone [J. Mech. Phys. Solids51, 357–381 (2003)] of the ISE and the bilinear behavior using two separate indenter tips (Berkovich and Vickers) from two separate machines. Therefore, the bilinear behavior is present regardless of tip geometry or machine used. We also investigated the cause for a deviation in the continuous stiffness measurement (CSM) data from discrete data points obtained using the load control protocol at shallow depth. We conducted experiments near grain boundaries to determine if the CSM deviation at shallow depths were caused by a hardening effect due to dislocation interaction with the interface.
In this article, the sizes of the volumes sampled by nanoindentation tests for hardness and modulus measurements are studied using finite element simulations. The zones of influence for hardness and modulus in single-phase systems are determined by modeling a hemispherical particle in a matrix, with properties close to those of each other, and monitoring the deviation of the measured values from those of the particle. It is found that, for hardness testing of elastic-perfectly plastic materials, the intrinsic hardness of the particle is measured as long as the plastic region is still within the particle, i.e., the contact radius is one half or less of the particle radius. Thus, in a hardness test of a single-phase material, all of the plastically deforming material, and only the plastically deforming material, contributes to the hardness measured. In contrast, the zone influencing the modulus is not restricted to a specific volume near the indenter. The modulus measured from the elastic response at the indentation point is dependent upon the entire specimen. A relationship is developed to describe the observed behavior of the measured modulus, that holds true for both sink-in and pile-up material behavior and for different indenter cone angles.
Lee and Radok [J. Appl. Mech.27, 438 (1960)] derived the solution for the indentation of a smooth rigid indenter on a linear viscoelastic half-space. They had pointed out that their solution was valid only for regimes where contact area did not decrease with time. In this article, a large number of finite element simulations and one typical experiment demonstrate that Lee-Radok solution is approximately valid for the case of reducing contact area. Based on this finding, three semiempirical methods, i.e., Step-Ramp method, Ramp-Ramp method and Sine-Sine method, are proposed for determination of shear creep compliance using the data of both loading and unloading segments. The reliability of these methods is acceptable within certain tolerance.
The local mechanical properties of ferritic and austenitic domains in a duplex stainless steel are locally studied by nanoindentation. The elastic and plastic properties of the two phases are determined. Without any specific surface treatment (chemical or electrochemical), the austenitic and ferritic domains present in the duplex stainless steel are distinguished using magnetic force microscopy. The magnetic scans allow nanoindentation results to be assigned to the respective phase, yielding the local mechanical properties of the duplex steel. The magnetic scans also show a sharp transition between the phases that is maintained even inside indentations. The ferrite phase is found to supersede austenite in the elastic modulus, hardness, and strain-hardening exponent, while both phases possess similar yield strength. Interface properties are a weighted average of the phase properties.
Electrical and electrochemical properties of the passive layer formed on 304L austenitic stainless steel are investigated by means of both conductive atomic force microscopy in air and electrochemical atomic force microscopy in chloride-containing media. The maps of local electrical conductivity of the oxide overlayer exhibit different patterns depending on the surface conditions after mechanical or electrochemical polishing. In particular, the passive film covering strain-hardened regions reveals a higher electrical conductivity. The local enhancement of the electrical conduction is explained by local changes of the stoichiometry of the passive film. Moreover, the highly conductive regions lead to a local breakdown of the native oxide in chloride-containing media and favor the initiation of localized pits.
First-principles fully relaxed tensile and shear test simulations were performed on tilt Fe grain-boundaries (GBs) with and without hydrogen (H) segregation, to investigate the mechanisms of GB embrittlement enhanced by H segregation. Premature fracture was found in the H-segregated GB, compared with the clean GB, in the tensile test simulations. The Fe–H bond showed covalent-like and ion-like characteristics. The covalent-like characteristics reinforced the Fe–Fe bonds, but the ion-like characteristics weakened the Fe–Fe bonds as a result of charge transfer. The effect of the latter increased with increasing strain, and prevailed over the former, resulting in GB embrittlement. In the shear test simulations, variation in the GB energy for the H-segregated GB was almost the same as that for the clean GB. This is because bond-breaking and rebonding occur concurrently in GB shearing and the variations in charge transfer during shear straining are less than those during tensile straining.
A dislocation–density-based model for slip transmission at variant boundaries and a microstructural failure criterion accounting for variant cleavage planes have been developed to determine optimal variant distributions for significantly improved ductility, through increased slip transmission, and fracture toughness, through increased resistance to crack propagation, in martensitic steels with refined blocks and packets. A crystal plasticity framework, accounting for variant morphologies and orientation relationships that are uniquely inherent to lath martensite, and specialized finite-element methodologies using overlapping elements to represent evolving fracture surfaces are used for a detailed analysis of fracture nucleation and intergranular and transgranular crack growth. The results indicate that the block sizes, variant orientations, and distributions are the key microstructural characteristics for toughening mechanisms, such as crack arrest and deflection, and for desired ductility, delayed crack nucleation, and greater fracture toughness. This approach can be the basis for validated design guidelines for the desired optimal behavior of high-strength and toughness steels.
In this work, the ultraﬁne nanoporous Ag ribbons were achieved through addition of 2 at.%–6 at.% Ce into the melt-spun Cu-Ag alloys and applying different electrochemical dealloying potentials. The dendritic morphology of the ligaments in the dealloyed Cu80Ag20 alloy varied to be equiaxial due to the addition of Ce, and the pore size reduced from 200 nm to less than 60 nm. The nanoporous Ag with an average pore size of ∼15 nm was obtained from the Cu74Ag20Ce6 alloy. The pore and ligament sizes of the nanoporous Ag prepared from the Cu76Ag20Ce4 alloy exhibited an increasing tendency with the increase of applied potentials, while the dealloyed Cu78Ag20Ce2 had an opposite variation. Moreover, the addition of Ce into the Cu-Ag alloys also promoted the dealloying. Nanoporous Ag exhibited the stronger enhancement of the surface enhanced Raman scattering effects with the increase of Ce contents in the precursory alloys.
Forced chemical mixing during severe plastic deformation was investigated at Cu-Nb face-centered-cubic (fcc)/body-centered-cubic (bcc) interfaces using molecular-dynamics simulations. Three Cu-Nb interfaces were considered, with either Kurdjumov-Sachs or Nishiyama-Wassermann orientation relationship (OR) between fcc and bcc phases. Forced mixing of a spherical bcc-Nb precipitate in fcc-Cu was also studied for comparison. Deformation was imposed by shape-preserving cycles using two different modes, biaxial compression and biplanar shearing to investigate the effects of strain path. For biplanar shear, the chemical mixing rate is strongly dependent on structure of the interface, with the Kurdjumov-Sachs OR and a (111)Cu‖(110)Nb habit plane being particularly resistant to mixing. During compression, no such dependence was found. Influences of interface diffuseness and roughness on stability were also investigated. The simulations show the interface mixing is inversely related to interface shear strength during shear deformation, but dominated by dislocation-glide through the Cu phase and subsequent absorption at Cu-Nb interfaces during compression deformation.
The use of Mg-Al-Ca alloys is limited mainly due to the hot crack defect. The exact mechanism of hot crack formation is not yet clearly understood. In this article, the hot crack mechanism is established from the present first-principles calculations based on the density functional theory and density functional perturbation theory . The thermal expansion behavior of Mg and the critical compounds Mg2Ca and Al2Ca in Mg-Al-Ca alloys is calculated. According to the present calculations, Mg2Ca is almost equal to Mg in thermal expansion, whereas the same in Al2Ca is much too lower. Al2Ca improves the creep resistance of Mg-Al-Ca alloys due to its high thermal stability, but it also accounts for the hot crack defect due to very small thermal expansion.
Nanostructured Al–Mn alloys are proposed as high-strength low-density materials, which can be electroformed (i.e., produced electrolytically and removed from the substrate) from ionic liquid. A variety of current waveforms, including direct current (DC) and pulsed current (PC), are used to electrodeposit nanostructured Al–Mn alloys, with some PC methods producing significant improvements in film ductility. Transmission electron microscopy observations point to a number of structural advantages induced by PC that apparently ductilize the Al–Mn alloys: (i) grain refinement to the nanocrystalline range without the introduction of a competing amorphous phase, (ii) unimodal nanocrystalline grain size distribution, and (iii) more homogeneous structure. The significant increase in apparent ductility in the PC alloys is also apparently related to stress- or deformation-induced grain growth, which leads to alloys with unique combinations of specific hardness and film ductility.
The spontaneous whisker growth phenomenon was investigated by exposing Sn3Nd intermetallic compound (IMC) to different environments. In a humid environment, tin whiskers grew rapidly; the incubation time for whisker formation was only 0.75 h. However, no whiskers were formed when Sn3Nd IMC was exposed to dry argon for 33 days or dry oxygen (DO) for 7 days. In situ observation of whisker growth during room ambient (RA) exposure gave an average whisker growth rate of the Sn3Nd IMC of about 11 Å/s, which are 2–3 orders of magnitude faster than that previously reported for tin plating. Following whisker growth, a new hydroxide compound, Nd(OH)3, was found to have formed on the Sn3Nd. The results show that the presence of humidity in the exposure environment is necessary for whisker growth from Sn3Nd. Finally, the driving force for whisker growth is also discussed.