We investigate the dependence of the hardness of materials on their elastic stiffness. This is possible by constructing a series of model potentials of Morse type; starting with modelling natural Cu, the model potentials exhibit an increased elastic modulus, while keeping all other potential parameters (lattice constant, bond energy) unchanged. Using molecular-dynamics simulation, we perform nanoindentation experiments on these model crystals. We find that the crystal hardness scales with the elastic stiffness. Also the load drop, which is experienced when plasticity sets in, increases in proportion to the elastic stiffness, while the yield point, i.e. the indentation at which plasticity sets in, is independent of the elastic stiffness. 相似文献
The deformation behavior of [001]T- and [011]T-cut single crystal solid solution of Pb(Zn1/3Nb2/3)O3–6% PbTiO3 (PZN–6%PT) in both unpoled and poled states has been investigated by nanoindentation. Nanoindentation experiments reveal that material pile-up and local damage around the indentation impressions are observed at ultra-low loads. These pile-ups and local damage cause a pop-in event (i.e. a sudden increase in displacement at an approximately constant load) in the nanoindentation load–displacement curve (P–h curve). Detailed studies of the relationships between indentation load (P), displacement (h) and harmonic contact stiffness (S) suggest that there is a surface layer, possibly due to crystal fabrication processes, which possesses different mechanical properties from the interior. The thickness of this surface layer is estimated to be approximately 300 nm. Furthermore, it is found that [011]T-cut crystal is stiffer than [001]T-cut crystal. On the other hand, both [001]T- and [011]T-cut crystals in unpoled state possess lower contact stiffness than poled crystals. This finding suggests that poling improved the mechanical property of the crystal. In summary, poled [001]T-cut crystals have an elastic modulus of (107 ± 6) GPa and a hardness of (5.1 ± 0.4) GPa. In contrast, the modulus for [011]T-cut crystals is not constant but increases with indentation depth. 相似文献
The generalized stacking fault (GSF) energy surfaces in the organic energetic molecular crystal, hexahydro-1,3,5-trinitro-s-triazine (RDX), were studied through atomistic simulations. Using a fully flexible molecular potential in highly damped molecular dynamics simulations, we determined quenched 0?K GSF energy surfaces and structures for a set of planes in the α-polymorph RDX crystal and subsequently compare predictions of slip or cleavage with available experimental observations. To account for the steric contributions and elastic shearing due to the presence of flexible molecules, a modified calculation procedure for the GSF energy surface is proposed that enables the distinction of elastic shear energy from the energy associated with the interfacial displacement discontinuity at the slip plane. Comparisons of the unstable stacking fault energy with the surface energy are used to differentiate cleavage planes from likely slip planes, and the calculations are found to be largely in agreement with available experimental data. 相似文献
The elastic–plastic deformation behavior of (001)- and (011)-oriented single crystal solid solutions of Pb(Zn1/3Nb2/3)O3–(6–7)% PbTiO3 (PZN–PT) have been studied using a nanoindentation technique. A procedure is presented here to isolate the elastic, elastic–plastic and plastic contributions to the deformation using the unloading data, and a parameter, referred to as relaxation, is defined to characterize the elastic–plastic deformation during nanoindentations. This relaxation parameter increases with the maximum indentation load due to the higher indentation stress induced, and it also causes less recovery of the material upon indentation unloading compared to predicted pure elastic recovery. For a (001) surface, the relaxation value remains virtually unchanged within the range of the maximum indentation load of 10–50 mN, possibly due to a complete localized depoling of the non-180° domain switching. It is also found that the unpoled surface is more prone to stress-induced depolarization compared to the poled surfaces. Furthermore, by applying the continuous stiffness measurement (CSM) technique, the effects of multiple loading/unloading are studied for both (001)- and (011)-oriented PZN–PTs using the maximum indentation loads of 20 and 50 mN. With more loading/unloading cycles at higher CSM frequencies, stress-induced depolarization becomes prevalent and the contribution of the domain reorientation towards elastic recovery is significantly reduced. As a consequence, the relaxation value is increased, indicating more elastic–plastic deformation. This CSM effect is especially pronounced for poled (011) surfaces. 相似文献
ABSTRACTA single-phase fcc high-entropy alloy (HEA) of 20%Cr–40%Fe–20%Mn–20%Ni composition and its strength with yttrium and zirconium oxides version was irradiated with 1.4?MeV Ar ions at room temperature and mid-range doses from 0.1 to 10 displacements per atom (dpa). Transmission electron microscopy (TEM), scanning transmission electron microscopy with energy dispersive X-ray spectrometry (STEM/EDS) and X-ray diffraction (XRD) were used to characterise the radiation defects and microstructural changes. Nanoindentation was used to measure the ion irradiation effect on hardening. In order to understand the irradiation effects in HEAs and to demonstrate their potential advantages, a comparison was performed with hardening behaviour of 316 austenitic stainless steel irradiated under an identical condition. It was shown that hardness increases with irradiation dose for all the materials studied, but this increase is lower in high-entropy alloys than in stainless steel. 相似文献
Plastic-deformation behaviors of gradient nanotwinned (GNT) metallic multilayers are investigated in nanoscale via molecular dynamics simulation. The evolution law of deformation behaviors of GNT metallic multilayers with different stacking fault energies (SFEs) during nanoindentation is revealed. The deformation behavior transforms from the dislocation dynamics to the twinning/detwinning in the GNT Ag, Cu, to Al with SFE increasing. In addition, it is found that the GNT Ag and GNT Cu strengthen in the case of a larger twin gradient based on more significant twin boundary (TB) strengthening and dislocation strengthening, while the GNT Al softens due to more TB migration and dislocation nucleation from TB at a larger twin gradient. The softening mechanism is further analyzed theoretically. These results not only provide an atomic insight into the plastic-deformation behaviors of certain GNT metallic multilayers with different SFEs, but also give a guideline to design the GNT metallic multilayers with required mechanical properties. 相似文献
Nitrogen incorporated diamond‐like carbon (N‐DLC) thin films were grown, at high base pressure of 5 × 10−3 Torr, under varied nitrogen gas pressure (NGP) (variation from 55 to 85 mTorr) and varied negative self‐bias (NSB) (variation from 35 to 190 V), using primary pump (rotary pump) processed radio frequency‐plasma enhanced chemical vapour deposition technique. These films then studied for their structural compositional and nanomechanical properties. Time of flight‐secondary ion mass spectroscopy was used to confirm the presence of nitrogen and to investigate the depth profile of these N‐DLC films, whereas energy dispersive X‐ray analysis measurement was carried to analyze the atomic concentration of constituents of film. Micro‐Raman analysis reveals change in D and G peak positions and ID/IG ratio with the change in NSB as well as NGP. It is worth noting that N‐DLC film grown at NGP and NSB of 55 mTorr and 100 V, respectively, exhibited very high hardness as 40.3 GPa. Hence, due to better nanomechanical properties by simpler, high deposition rate and cost effective process, which yielded very high throughput may leads these N‐DLC films to their wide industrial applications such as hard and protective coating on cutting tools, automobile parts, bio‐implants or wear resistant coating on magnetic hard disk.
We have shown in a recent study that substitution of Ho3+ ions (4f10; magnetic momenμB) in La0.7Ca0.3MnO3 causes significant reduction in electrical resistivity compared with Y3+ (4d0; non-magnetic) ion substitution. This reduction in resistivity was attributed to the reduced spin disorder scattering in La0.7Ca0.3MnO3 samples containing magnetic Ho3+. We have estimated the Mn-spin canting angles in Ho3+ - and Y3+-doped La0.7Ca0.3MnO3 compounds from the resistivity data using the magnetic localization model. We find that the canting angles of the Mn spins in the Ho3+ doped compounds are smaller than those obtained for the Y3+-doped compounds for all compositions and at all applied magnetic fields, showing clearly a reduction in the spin disorder in the former. The difference between the TC values for Ho3+ - and Y3+-doped compounds for all compositions may be attributed to the presence of an internal field due to Ho3+ doping. This internal field may be responsible for the decrease in spin disorder in the Ho3+-doped compounds. The increase in the canting angles with increase in Ho3+ and Y3+ content could be attributed to the decrease in the strength of the ferromagnetic exchange interactions. A strong ferromagnetic coupling (as discussed recently by the present authors and co-workers) of Ho3+ moments with the Mn moments is responsible for the observed behaviour. 相似文献
Nanoindentation was carried out on thin films of hydrogenated amorphous silicon (a-Si:H) prepared by plasma-enhanced chemical vapor deposition. The composite values of elastic (Young's) modulus, Ec, and hardness, Hc, of the film/substrate system were evaluated from the load–displacement curves using the Oliver–Pharr approach. The film-only parameters were obtained employing the extrapolation of the depth profiles of Ec and Hc. Scanning probe microscopy was employed to image the nanoindenter impressions and to estimate the effect of film roughness and material pile-up on the testing results. It was established that the elastic modulus of thin a-Si:H films is in the range 117–131 GPa, which is lower than for crystalline silicon. In contrast, the values of hardness are in the range 12.2–12.7 GPa, which is comparable to crystalline silicon and higher than for hydrogen-free amorphous silicon. It is suggested that the plastic deformation of a-Si:H proceeds through plastic flow and it is the presence of hydrogen in the amorphous matrix that leads to a higher hardness. 相似文献
This paper presents a physicochemical model that establishes a connection between the elastic strength of the surface layer (SL) of metal and its surface Gibbs energy. The elastic limit of SL along the low-index face of the metal single crystal under stress during the transition from elastic to plastic deformation was calculated. Calculation shows that the elastic limit of metal SL with fcc and bcc structures is approximately three orders of magnitude higher than the yield strength of these metals in bulk and close to nanohardness of the metals, in particular; for Cu(111) и Al(111), it is 5.3 and 2.8?GPa, respectively. In the light of the proposed model, the effect of lowering the elastic strength of metal SL due to adsorption of surfactant is formulated. 相似文献
