Hardness Anisotropy Research Articles

Solvent‐Assisted Anisotropic Cleavage of Transition Metal Carbide into 2D Nanoflakes

2D Transition Metal Carbides Nanoflakes In article number 2100039, Yu-Long Tang, Qiang Wang, Hao-Li Zhang, and co-workers fabricate 2D nanoflakes from nonlayered transition metal carbides by a solvent-assisted anisotropic cleavage method utilizing the intrinsic anisotropic hardness properties. The nonlayered 2D nanoflakes are successfully applied in ultrafast photonic as saturable absorbers. This method paves a new way for the fabrication of novel 2D nanomaterials.

Microhardness Indentation Studies of 2‐4‐6 Trinitrotoluene

AbstractThe microhardness of the {001} faces of 2‐4‐6 trinitrotoluene crystals has been investigated using both Vickers and Knoop indentation methods. The Vickers hardness number was found to be 22.5 kg mm−2 independent of crystal orientation and perfection. At ambient temperatures (∼20 °C) the Knoop hardness number varied between 20.5 kg mm−2 and 24.0 kg mm−2 with crystal orientation. At higher temperature (50 °C) the Knoop hardness anisotropy curve retained its shape, although the overall hardness decreased by 10 %. We interpret this change as reflecting a simple temperature dependant loosening of the crystal lattice rather than any change in deformation mechanism. No variation of Knoop hardness was evident with changing load. The hard direction was [010] and the soft [100]. The dominant operative slip system was defined to be {001}[010].

Reduction in anisotropy of mechanical properties of Ti-6Al-4V through martensite decomposition

Orientation induced anisotropic hardness and modulus of Ti-6Al-4V alloy has been minimized through microstructural engineering. This involves the formation of martensite upon quenching and further its decomposition facilitated by cold deformation and low temperature heat treatment. The martensite (α′) formed from the β phase has nearly equal distribution of multiple variants in the form of fine laths. After decomposition, this transforms to fine equiaxed β along the lath boundaries of α, instead of a thin lamellar β. The variations in the nano-indentation derived hardness and modulus are much smaller compared to the reported literature and is attributed to the fine nature and morphology of the constituent phases.

Microstructure and mechanical properties of Titanium grade 23 produced by selective laser melting

Selective laser melted Titanium grade 23 was characterized by low porosity, relatively large surface roughness and pronounced surface texture (i.e. surface grooves orientation). The band/layer microstructure was built of mixed α and β phases. The as printed structure exhibited very high compressive residual stresses with strong anisotropy (i.e., − 512 ± 17 MPa and − 282 ± 14 MPa along the laser scanning direction and along the transverse direction, respectively) and strong fiber crystallographic texture. The latter one is responsible for the anisotropy of hardness in the material. Annealing at 600 °C during four hours significantly removed residual stresses (i.e. to − 14 ± 2.8 MPa) and slightly weakened the texture. Yield strength, 1120 ± 50 MPa, and ultimate tensile strength, 1210 ± 50 MPa, of the annealed material are significantly higher and tensile elongation, 3.9%, lower than for commercial Titanium grade 23. Final mechanical polishing to obtain flat and relatively smooth surface induced desired compression residual stress in the subsurface (i.e., equal to about − 90 MPa). Low absorbed gas contents (oxygen, nitrogen, hydrogen) and low porosity of the printed material indicates the correctness of the technology and allows the printed material to be classified as meeting the requirements of ASTM standards for Titanium grade 23. Besides traditional testing techniques, the optical profilometry, X-ray analysis (texture and residual stresses measurement) and infrared absorption method were applied for the product characterization and some potential of these testing methods and usefulness in technological practice was discussed, what can be particularly interesting both to practitioners from industry and researches from scientific laboratories.

Mechanical characterization of boron carbide single crystals

AbstractOut‐of‐plane anisotropy in the mechanical response of boron carbide was studied by performing nanoindentation experiments on four specific crystallographic orientations of single crystals, that is, , , , and . For each orientation of the single crystals, in‐plane variations of indentation modulus and hardness were also studied by monitoring the relative rotation between the crystal surface and a Berkovich indenter tip. A significant out‐of‐plane anisotropy in indentation modulus was observed with ~80 GPa difference between the highest and lowest values. A smaller but measurable out‐of‐plane anisotropy in indentation hardness was also observed. In‐plane anisotropy, on the other hand, was found to be significantly influenced by the scatter in the data and geometrical imperfections of the indenter tip. Investigations of indentation pop‐in events suggested that deformation is entirely elastic prior to the first pop‐in. Furthermore, quasi‐plastic flow along the orientation of the single crystals was found to be more homogeneous than the other tested orientations. For select indents, cross‐sectional transmission electron microscopy (TEM) of the indented regions showed formation of a quasi‐plastic zone in the form of lattice rotation and various microstructural defects. The quasi‐plastic zone grew in size with increasing the indentation depth. The TEM observations also suggested the crystal slip to be a potential mechanism of quasi‐plasticity and a precursor for formation of amorphous bands that could eventually lead to cracking and fragmentation. The proposed failure mechanism provides valuable insights for calibrating constitutive computational models of failure in boron carbide.

Effect of hot rolling on microstructure, crystallographic texture, and hardness of AZ31 alloy

In this study, relationships between microstructure, texture, and hardness of hot-rolled AZ31 alloy were investigated. It was found that the grain size decreased into the ultra-fine domain after 60% hot deformation, reaching a grain size of 1 μm. Several dynamic recrystallization (DRX) mechanisms activated after 40% and 60% deformations including discontinuous dynamic recrystallization (DDRX), twin dynamic recrystallization (TDRX), continuous dynamic recrystallization (CDRX), and particle stimulated nucleation (PSN). With increasing the thickness reduction, the intensity of basal texture first increased, then slightly decreased, and finally significantly increased to 15.7 multiple of random density. The average misorientation angle decreased when the thickness reduction increased owing to the formation of a very strong {0001} basal texture. With increasing the strains, the average microhardness increased because of decreasing grain size and increasing twin fraction and shear band density. There were no differences between the hardness values of RD–TD and RD–ND planes for AZ31 samples after 10% and 40% hot rolling and these samples were isotropic. After 60% deformation, the hardness of the RD–TD plane (73.5 HB) was higher than that of the RD–ND one (68.4 HB) that resulted in hardness anisotropy due to the generation of very strong basal texture.

Solvent‐Assisted Anisotropic Cleavage of Transition Metal Carbide into 2D Nanoflakes

Mechanical exfoliation of bulk layered materials into atomic‐layer thin nanosheets has become a general strategy to obtain 2D nanomaterials, which seems intuitively unsuitable for non‐van der Waals crystals. However, a new method is proposed, namely, solvent‐assisted anisotropic cleavage (SAAC), for producing thin nanostructures from transition metal carbides (TMCs), including titanium carbide (TiC) and tungsten carbide (WC). The SAAC method harnesses the intrinsic anisotropic hardness properties of the TMC and enables efficient cleavage of bulk TMC into 2D layers. Under ultrasonication in appropriated solvent, TiC crystals show preferable cleavage along [110] direction and produce 2D nanoflakes. The obtained 2D nanoflakes are successfully applied for ultrafast photonics as saturable absorbers (SAs) in mode‐locked thulium fiber lasers, where stable third‐order harmonically mode‐locked dissipative solitons are realized at 2 μm. The success of the SAAC process opens a new pace toward preparation of 2D nanomaterials.

Hardness anisotropy and active slip systems in a (Hf-Ta-Zr-Nb)C high-entropy carbide during nanoindentation

Hardness anisotropy and deformation-induced dislocations in (Hf-Ta-Zr-Nb)C high-entropy carbide grains of low-index facets were investigated using nanoindentation and transmission electron microscopy (TEM). The (Hf-Ta-Zr-Nb)C sample was prepared by a two-step spark plasma sintering (SPS) and nanoindentation was performed on grains mapped previously by electron backscatter diffraction. To investigate the deformation zones under the indents, thin lamellas were extracted from grains of {001}, {101} and {111} orientations using a focused ion beam (FIB) technique and were studied by TEM. Nanoindentation revealed that grains of {100} orientation exhibit about 7% lower hardness (~35 GPa) compared to the {110} and {111} facets (~38 GPa) while there was no anisotropy found for the indentation modulus. TEM analysis of the lamellas revealed heavily deformed zones under the indents with dislocations of a/211¯0 type Burgers vector. TEM-based trace analysis of dislocations revealed the activation of the 1101¯11 and 1101¯10 slip systems in grains indented on the {001} and {101} facets, respectively. The hardness anisotropy of (Hf-Ta-Zr-Nb)C was attributed to the different operating slip systems, similar to that was reported for the component monocarbides in the literature.

Simulating Knoop hardness anisotropy of aluminum and β-HMX with a crystal plasticity finite element model

We report a crystal plasticity finite element model for simulating the variation of the Knoop hardness number with indenter orientation, i.e., the hardness anisotropy. We propose a modification to the Voce hardening law to include the effects of slip on multiple systems on the hardening rate. The model is validated using single crystal aluminum in a two-step procedure: (a) calibration of the material model with single crystal tension experiments, and (b) prediction of the hardness anisotropy of the {001}, {011} and {021} planes. The predicted hardness anisotropy follows experimental trends very well for the {001} and {021} planes, while for the {011} plane there is a deviation of the predictions from experiments at higher angles of the indenter. The model is then used to simulate the hardness anisotropy of the (010), (011) and (110) planes of β-HMX measured by Gallagher et al. (2015). Due to the lack of single crystal data, we were unable to calibrate the model prior to simulating the hardness anisotropy. Hence, we assumed unknown variables to make predictions for comparison to experiments, including the set of active slip systems and the hardening and twinning behavior. From simulations of the indentation of the (010) plane with the set of slip systems from Barton et al (2009) we found that the hardness anisotropy is inversely proportional to the activity of the slip system (101)[01¯0], and to a lesser extent the twin system. Suppressing the (101)[01¯0] slip system improved the predictions, suggesting that the viable set of slip systems for β-HMX need not include it. The predicted twinning activity did not match well with the observations from Gallagher et al. (2015). Suppressing the twinning resulted in a better prediction of the hardness anisotropy. Due to the simplicity of the twinning part of the model we do not make definite conclusions related to twinning. We report predictions for the hardness anisotropy of the (011) and (110) planes of β-HMX using parameters that maximized agreement between simulations and experimental indentation of (010) plane. These predictions are in reasonable agreement with experiments. In addition, we report the predictions of the hardness anisotropy using the sets of slip systems proposed by Pal and Picu (2018) and Zhou et al. (2012), which both gave inferior agreement with experiments.

Effect of extrusion ratios on hardness, microstructure, and crystal texture anisotropy in pure niobium tubes subjected to hydrostatic extrusion

Nb tubes were fabricated through hydrostatic extrusion at extrusion ratios of 3.1 and 6.1 at ambient temperature, and then their microstructure, texture, and Vickers hardness were investigated based on electron back-scattered diffraction (EBSD) data. The fraction of low-angle boundaries (LABs) largely decreased with a sharp decrease in mean grain sizes after hydrostatic extrusion and was not proportional to extrusion ratios, assuming that mixed-asymmetrical junctions forming LABs dissociate at high extrusion ratios from the external stress (>981 MPa) with thermal activation by the generated heat. The correlation between grain size and Vickers hardness followed the Hall−Petch relationship despite the texture gradient of the 〈111〉 cyclic fiber textural microstructure at low extrusion ratios and the 〈100〉 true fiber textural microstructure at high extrusion ratios. The increase in hydrostatic pressure on the Nb tubes contributed to texture evolution in terms of extrusion ratios due to the difference between {110}〈111〉 and {112}〈111〉 components based on EBSD data.

Effect of grain boundary on polycrystalline diamond polishing by high-speed dynamic friction

In-situ micro-area analysis of precisely polished polycrystalline diamond (PCD) film by high-speed three-dimensional dynamic friction polishing (3DM-DFP) is carried out in this work, continuing from the previous works focusing on the subsurface cleavage and polishing mechanism. Multi-orientational grains with variable hardness anisotropy are generally considered the reason for the surface irregularity of PCD after smoothing. The unreached surface ultra-smoothness in local region after polishing is mainly resulted from the height difference at grain boundary (GB) area (e.g., a selected GB micro-region is of around Ra roughness of 5.7 nm and altitude intercept of 6.6 nm). However, different individual grains nearby showing (100), (110), (111) orientations are all of roughness <0.5 nm, demonstrating these places are contiguously polished. The observed local roughening at GBs region after mechanical sliding accompanying with friction heat is the major factor to cause the surface irregularity, which is affected by the inhomogeneity of crystal thermal expansion and contraction associated with the defects in multi-orientational crystals generated by mechanical process. Among multi-crystals the low-position (110) region has a Ra roughness of 0.1 nm but with great tensile stress and relatively weak distortion compared with other orientational areas. An insight of the inhomogeneous volume change associated with thermal expansion and contraction affected by defects condition was proposed to straightforwardly illuminate the rationale of GB effects on the ultra-smoothing of PCD by 3DM-DFP.

The effects of quaternary alloying additions on the γTiAl alloy: Preferential site occupancy, interfacial energetics to physical parameters

The effect of Nb, Sn, Mn, and Si at a concentration of (0.3-2 at. %) on γ-TiAl based alloys’ preferential occupancy, interfacial energetics and mechanical properties were investigated by density functional theory (DFT) encompassed by the typical gradient estimation configuration and substantiated by experimental work. The site occupancies, elastic constants, shear and bulk modulus, B/G ratio, Poisson's ratio, hardness and universal anisotropy are systematically elucidated. The results indicated that Mn adamantly occupies Ti sites, while the Nb, Sn and Si occupy Al sites, with no significant influence on the alloy composition in-terms of their site preference. Interfacial energy of γ/α2-M system is the prerequisite energy to generate an interface from bulk materials. The stability criteria were satisfied attributable to Cij’s values greater than zero. The overall mechanical properties of the substitutional solid solution-TiAl alloy exhibited improved ductility. The alloy was produced by vacuum arc melting, with subsequent annealing to achieve homogeneity. The TEM results demonstrated that the α2/γ interface boundaries yielded the γ-TiAl {111} <110>||α2-Ti3Al (0001)<112¯0> orientation relationship.

A novel strategy for c-axis textured silicon nitride ceramics by hot extrusion

Highly c-axis textured β-silicon nitride (β-Si3N4) ceramic with fine grains was prepared by a new method of hot extrusion for the first time. The (002) pole figure on the section plane vertical to extruding direction showed a characteristic of center rotational symmetry. The average cline angle between elongated β-Si3N4 grains and hot extruding direction was about 14.4°. The degree of c-axis texturing by hot extrusion was comparable to that achieved by rotating magnetic field. The hardness and toughness anisotropy in different direction was apparent and relatively higher hardness was achieved in the present work mainly due to the finer grain size. Therefore, many different compositions of c-axis aligned Si3N4-based ceramics with tailored mechanical properties could be achieved by the strategy of hot extrusion.

Microstructure Refinement by Low-Temperature Ausforming in an Fe-Based Metastable High-Entropy Alloy

The effects of ausforming in an Fe30Mn10Cr10Co high-entropy alloy on the microstructure, hardness, and plastic anisotropy were investigated. The alloy showed a dual-phase microstructure consisting of face-centered cubic (FCC) austenite and hexagonal close-packed (HCP) martensite in the as-solution-treated condition, and the finish temperature for the reverse transformation was below 200 °C. Therefore, low-temperature ausforming at 200 °C was achieved, which resulted in microstructure refinement and significantly increased the hardness. Furthermore, plasticity anisotropy, a common problem in HCP structures, was suppressed by the ausforming treatment. This, in turn, reduced the scatter of the hardness.

Microhardness and SHG efficiency of pure and picric acid-added KDP crystals

Pure and picric acid-added KDP (potassium dihydrogen phosphate) crystals are grown by slow evaporation technique from aqueous solution. Unit cell parameters for pure and doped KDP crystals were measured from single-crystal X-ray diffraction studies. Vickers and Knoop microhardness studies were carried out on the grown crystals over a load range of 0.1 to 0.5 N. The microhardness decreases with load P, i.e., normal indentation size effect (ISE), and from Meyers index it is shown that grown crystals falls in to hard material category. Mechanical properties, such as Young’s modulus, yield strength, elastic stiffness constant, were calculated for pure and doped KDP crystals. Hardness anisotropy has been observed in accordance with the orientation of the crystal. Etching studies were carried out using aqueous solution as an etchant. Improved second-harmonic generation efficiency is observed for doped KDP crystals in comparison to pure KDP crystals.

Influence of grain orientation on hardness anisotropy and dislocation behavior of AlN ceramic in nanoindentation

The anisotropic hardness, elastic modulus, and dislocation behavior of AlN grains were investigated by Berkovich nanoindentation. Grain orientation had a strong correlation with hardness and elastic modulus. The hardness of the basal plane (0001) was higher than those on the two prismatic planes (101‾0) and (1‾21‾0), while the modulus was showing the opposite trend. Further investigation on the deformation behavior of AlN revealed that pop-in, pile-up and slip lines only occurred on the two prismatic planes (101‾0) and (1‾21‾0). Through molecular dynamics (MD) simulation, the mechanisms of anisotropic pop-in events and dislocations were disclosed. The activation of the slip system for different grain orientations was presented by Schmid factor. The method of using Schmid factor to determine the activated slip system was verified by the observation of pile-up and slip lines around the indent, and proven to be accurate in identifying the grain orientation of AlN ceramic. Lower hardness and higher modulus were found to be related to more plastic events like pop-in, pile-up and slip lines, and the crystalline orientation could be roughly determined by the observation of plastic activities from nanoindentation.

Structural anisotropy effect on the nanoscratching of monocrystalline 6H-silicon carbide

Monocrystalline 6H-silicon carbide is a promising material for advanced components and devices; but is also a difficult-to-machine material due to its hardness, brittleness and structural anisotropy. With the aid of large-scale molecular dynamics simulations, this paper comprehensively studied the structural anisotropy effect on the nanoscratching of 6H–SiC. Six typical combinations of scratching plane and direction were selected, namely, (0001)<112‾0>, (0001)<11‾00>, (112‾0)<11‾00>, (112‾0)<0001>, (11‾00)<0001> and (11‾00)<112‾0>. It was found that the scratching-induced deformation morphology, activated dislocations and scratching forces varied significantly under different combinations of scratching conditions due to the strong anisotropy effect of the material. By evaluating the actual depth of cut, elastic recovery, surface roughness and maximum subsurface damage depth, basal plane (0001) along <11‾00> direction was identified as the best combination for conducting nanoscratching on 6H–SiC. Corresponding high-resolution TEM results show that the mechanism revealed by the MD analysis reflects the true deformation of the material.

Improving mechanical anisotropy and corrosion resistance of extruded AA7075 alloy by warm cross rolling and annealing

The anisotropy of mechanical properties and poor corrosion resistance are important problems for high strength aluminum plate. In this study, the combined process of hot extrusion and warm cross rolling was conducted on AA7075 alloy, and both the extruded and rolled plates were annealed at various temperatures. The effects of warm cross rolling and annealing on the microstructure, mechanical properties, and corrosion behavior were studied. The results indicated that both cross rolling and annealing weakened the intensity of main textures of brass, Dillamore, and S. Compared to the extruded plate, the cross rolled ones exhibited higher hardness, higher conductivity, and lower anisotropy of tensile strength. Both cross rolling and annealing made the tensile strength of a plate along different directions more uniform. Moreover, annealing greatly enhanced the corrosion resistance by consuming the distortion energy, reducing the number of crystalline defects and increasing the fraction of low-Σ coincident site lattice grain boundaries. After annealing, the rolled AA7075 plates exhibited better corrosion resistance than the extruded ones, since they possessed lower fractions of brass texture, finer grains, and higher fraction of low-Σ coincident site lattice grain boundaries.

Effect of particle size distribution on the structure and mechanical properties in the process of laser powder bed fusion

The paper studies effects of particle size distribution on the structure and mechanical properties of monolithic samples obtained by L-PBF. A powder of 321 austenitic stainless steel of one batch was divided into three fractions 0–20, 20–40, and 0–40 μm. It was established that for narrow fractional powder composition, hardness anisotropy is observed that depends on the building direction, whereas for wide fractional powder composition, hardness anisotropy is practically absent. It was found that the particle size composition of AISI 321 steel powder does not fundamentally affect the morphology of the grain structure. Despite the general preferred orientation of the {101} planes, a weak effect of the powder composition on the crystallites orientation is observed.

Hardening behavior and deformation microstructure beneath indentation in heavy ion irradiated 12Cr-ODS steel at elevated temperature

Hardness behavior and deformation of the microstructure beneath the indentation in 12Cr-ODS steel after exposure to heavy ion irradiation at 573 K were investigated with combined applications of nano-indentation tests, EBSD analysis, and TEM observation. The inhomogeneous nano-indentation hardness behavior was firstly investigated in both non-irradiated and irradiated specimens and compared to those at room temperature (RT) irradiation. Subsequently, TEM observations were performed to understand the deformation behaviors beneath the indentation tested at typical low-indexed orientations. Obvious irradiation hardening was confirmed in both the normal direction (ND) and rolling direction (RD) specimens, and the hardening effect intensified with an increase in irradiation dose. Meanwhile, various anisotropic hardness behaviors were observed such as a lower hardness value and a weaker hardening effect in the RD specimen compared to the ND specimen, and a lower hardness value but a greater hardening effect in [001] orientation relative to [111] orientation. Furthermore, results from 573 K irradiation showed similar anisotropic hardness behaviors with those obtained from RT irradiation, despite the fact that irradiation at 573 K exhibited a weakened irradiation hardening effect in this steel. TEM observation of the deformation microstructure beneath the indentation reveals that (i) the depth of the deformation affected zone varied with orientation, ranging from ~6-9 times that of the indent depth, and (ii) distinct dislocation-grain boundary (GB) interactions beneath the indentations was confirmed to exhibit dependence on the geometric relation between GB and indenter impression direction. Correlation between the anisotropic hardness behavior and microstructural features is discussed based on Schmid factor analysis, comparison of geometric favorability between indenter and possible slip planes, and TEM observation of deformation microstructure. The results conclude that the anisotropic hardness behaviors in the present 12Cr-ODS steel are ascribed to the combined effect of crystal orientation and the non-uniform grain morphology with the existence of the elongated grains having low aspect ratio in the ND plane and nano-layered grains in the RD plane.