查看更多>>摘要：Understanding the effect of crystallographic orientation on the twinnin/detwinning mechanisms in NiTi shape memory alloys at an atomistic scale can help to control and tune the mechanical properties and failure behavior of such materials. In this work, we employed classical molecular dynamics (MD) and density functional theory (DFT) computational methods to better understand how twinning and detwinning occurs through a combination of slip, twin, and shuffle on (010), (110), and (1 11) crystallographic orientations under uniaxial tensile test. Elastic constants including Young's Modulus (E), Bulk modulus (B), Poisson's ratio (nu), and Shear Modulus (G) are obtained and computed for resultant stress-induced martensite variants as a function of crystallographic orientation using DFT calculations. In addition, computational nanoindentation tests are carried out using MD simulations to evaluate the effect of crystallographic orientation on the twinning and detwinning characteristics in martensite in NiTi alloys under sphere indenter, both qualitatively and quantitatively. Based on a careful polyhedral template matching (PTM) and dislocation analysis (DXA) by taking into account the textures, it is determined that the microscopic stress-strain and load-displacement responses strongly depend on the crystallographic orientation. Our findings reveal that the size of twinned and detwinned zones in martensite increases in the order of (111) < (010) < (110). Based on DFT results, against (111) direction, abrupt changes in the free energy-strain curves occurs at 4% strain in (001), and 8% strain in (11 0) directions. The twinning and detwinning mechanisms are controlled by monoclinic martensite (B19') -* orthorhombic martensite (B19) phase transformation in (11 0) orientation and by body-centered orthorhombic martensite (BCO) -* an intermediate structure (B19'') -* monoclinic martensite (B19') phase transformation in (001) orientation. Finally, the predicted orientation-dependent critical energy release rate is analyzed to examine the effect of the twinning and detwinning process on the fracture toughness of the material. Our results show that reducing the density of twins results in increasing the critical energy release rate. Therefore, the fracture stress intensity increases in the order of (0 01) < (11 0) < (111).
查看更多>>摘要：Titanium and its alloys undergo temperature-driven martensitic phase transformation leading to the development of complex microstructures at mesoscale. Optimizing the mechanical properties of these materials requires an understanding of the correlations between the processing parameters and the mechanisms involved in the microstructure formation and evolution. In this work, we study the temperature-induced phase transition from BCC to HCP in pure titanium by atomistic modeling and investigate the influence of local stress conditions on the final martensite morphology. We simulate the transition under different stress conditions and carry a detailed analysis of the microstructural evolution during transition using a deformation gradient map that characterizes the local lattice distortion. The analysis of final martensite morphologies shows how mechanical constraints influence the number of selected variants and the number/type of defects in the final microstructure. We give insight on the origin and structure of different interfaces experimentally observed, such as inter-variant boundaries and antiphase defects. In particular, we show how antiphase defects originate from the two-fold degeneracy shuffling displacement arriving during the transition and how the triple junction formation drives the texture evolution when local stresses prevent a free shape change of the matrix surrounding the growing martensite nuclei.
查看更多>>摘要：Nanograined (NG) metals with nanotwinned (NT) regions can overcome the inferior ductility of NG metals and achieve high strength and modest ductility. Based on the strain gradient plasticity and Johnson-Cook failure criterion, we simulate the dependences of their strength and ductility on volume fraction, twin spacing, as well as shape and distribution of NT regions in NG Cu. It is found that these factors have significant impact on the overall ductility. In particular, the overall ductility abnormally decreases with the increase in the volume fraction of NT regions, which is directly related to the failure modes of this material system. Interface debonding can explain the above abnormal decrease in overall ductility. In addition, with the increase in twin spacing, fracture of NT regions can cause different reversals of overall ductility. We also found that, when the NT regions are of the oblique square type, the overall ductility is significantly lower than when they are of the circular and square types. In most cases, the array arrangement of NT regions is superior to the staggered arrangement for the improvement of the overall ductility. It is believed that these reported results can contribute to a deeper understanding of this novel material system.
查看更多>>摘要：The surface modification of Te element is feasible in experiment, based on this, we replaced a layer of Se atoms of WSe2 bilayer with Te atoms. By using first-principles calculations, we learned that the properties of WSe2 bilayer will be significantly changed after tellurization. Due to structural configuration and Coulomb ion-dipole interaction, the interlayer distance of WSeTe/WSe2 heterostructure (HS) is reduced compared with WSe2 bilayer, leading to the stronger interlayer coupling and a smaller bandgap (1.50 eV). The bandgap of WSeTe/WSe2 heterostructure will decrease by more than 50% under 4% tensile strain, and the bandgap is continuously adjustable under 1-4% tensile strain. After tellurization, the optical absorption will be enhanced by 43% in the 547-800 nm wavelength range. The absorption intensity will be further enhanced by about 20% in the 413-578 nm wavelength range under 4% tensile strain. The WSeTe/WSe2 heterostructure also possesses smaller work function compared with that of WSe2 bilayer, which will enable improved photodetection and photoresponse time. This work expounds the influence of telluride on the properties of WSe2 bilayer, including interlayer coupling, electronic and optoelectronic properties, providing a basis for the construction of related highperformance optoelectronic devices.
Tang, LuomengDin, Muhammad Aizaz UdHe, DafangDu, Xue...
9页
查看更多>>摘要：In this study, we propose a novel strain engineering strategy to tune the electronic structures and optical properties of InS single crystal. InS single crystal has the negative Poisson's ratio character in the zigzag and armchair directions, and owns a direct band-gap of 3.027 eV at the static state. There is a transition from direct to indirect band gap for different deformation modes (uniaxial strains epsilon(x), epsilon(y), epsilon(z), and biaxial strain epsilon(y)equivalent to epsilon(z)). For the compressive strain mode, the band gap of InS single crystal first increases and then decreases with the increasing strain, and it changes from a direct to an indirect band gap at a critical strain. However, for the tensile strain pattern, the band gap of InS single crystal monotonously decreases with the increasing strain. It also changes from a direct to an indirect band gap at a critical strain. The biaxial strain mode epsilon(y)equivalent to epsilon(z) among all the stretching modes is the most advantageous way to realize the strain-dependent band gap engineering of InS single crystal. Thus, for all the considered strain modes, with the increasing strain, InS single crystal exhibits the enhanced absorption coefficient accompanied by the red-shifted optical absorption edge.
Muchiri, P. W.Korir, K. K.Makau, N. W.Amolo, G. O....
8页
查看更多>>摘要：The development of super-hard materials has recently focused on systems containing heavy transition metal and light non-metallic elements. Niobium carbides and nitrides have previously been identified as potential candidates, however, the volatility of carbon and nitrogen during synthesis make them prone to formation of anionic vacancies, which have the ability of changing the electronic structure, dynamical stability and adversely affecting the mechanical properties. This study presents ab initio Density Functional Theory calculations that probe the occurrence of anionic vacancies as a function of concentration, thereafter, pertinent mechanical properties are investigated. Our results showed that the presence of anionic vacancies in NbC and NbN tend to deteriorate the mechanical properties and ultimately the mechanical hardness due to vacancy softening that can be attributed to defect induced covalent to metallic bond transition. Further, it was observed that anionic vacancies in NbC tend to modify its toughness; in particular, NbC in ZB structure becomes brittle while NbC in WZ phase becomes ductile in presences of C vacancies of up to 6%. The dynamical stability of NbC in RS, ZB, and WZ phases and NbN in WZ phase were found to be insensitive to the presence C and N vacancies, while NbN in RS and ZB becomes dynamically stable with the introduction of N vacancies. In addition, the toughness of NbN was found to be insensitive to defect concentration of even up to 8%. Consequently, stringent control of anionic defects during synthesis of NbC and NbN is critical for realization of the desired mechanical response that can make these materials ideal for super-hard and related applications.
查看更多>>摘要：In general, with the addition of solutes, the yield strength of alloys is expected to increase; this phenomenon is known as solid solution strengthening. However, this model of strengthening implicitly assumes that dislo-cations are available and it is their motion that causes plastic deformation. On the other hand, in dislocation starved regimes, it is the nucleation of dislocations that controls the deformation behaviour. In this regime, the solute atoms can act as sites of heterogeneous nucleation for dislocations and "anomalous"softening with alloying additions are not uncommon. In this study, using Molecular Dynamics (MD) simulations, we show anomalous softening in Cu-Al alloys deformed at 300 K. We show that there are two components to this softening; in addition to the heterogeneous nucleation at solute sites, the reduction in stacking fault energy with alloying addition promotes the (homogeneous) nucleation of partial dislocation loops. In order to be consistent, we interpret the MD simulation results of deformation behaviour using parameters and energies derived from the same potentials used in the simulations. Specifically, we carry out the thermodynamic integration to evaluate the free energies in crystals with and without the stacking faults and hence calculate the SFE as a function of Al content at 300 K. Using the SFE values thus obtained, and using a continuum model of homogeneous nucleation of partial dislocation loops, we rationalise the deformation behaviour seen in pure copper. On the other hand, in Cu-Al alloys, the drop in yield strength can only be explained using a combination of the continuum model (which assumes homogeneous nucleation), and the reduction in heterogeneous nucleation barrier (which is a function of the ratio of the unstable and stable stacking fault energies). Thus, our results indicate that deformation experiments of pure copper and copper-aluminium alloys in the dislocation starved regime could be interesting and might show qualitatively and quantitatively different behaviour.
查看更多>>摘要：An array of columnar dendrites is important to the microsegregation and permeability of interdendritic liquid flow. In this study, an array of tilted columnar dendrites growing during the directional solidification of a binary alloy was investigated via large-scale phase-field simulation. The main conclusion is that the hexagonal array is the dominant array for reasonably tilted dendrites as well as dendrites that grow along the temperature gradient. It is also concluded that the array ordering gradually improves with growth for all tilt angles, and the ordering rate is faster for a small tilt angle.
查看更多>>摘要：A density functional study is presented for HC4N3BN monolayer, which is triazine g-C4N3 tailored with H at its graphitic sites and B plus N at vacant sites, under uniaxial or biaxial in-plane strain. For moderate strains up to about +/- 8 percent, first-principles molecular dynamics simulations prove its thermal stability at room temperature and electronic structure calculations show the persistence of antiferromagnetic ground state of the system. The material undergoes magnetic transitions to ferro-and ferrimagnetic orders respectively at the critical biaxial strain of -8.3% and uniaxial strain of 7.8%. Interestingly, both transitions occur when the electronic structures of strained systems show the character of a spin gapless semiconductor. The combined effect of spin charge transfer and band shifting is proposed as a possible explanation for these transitions. Our finding on this monolayer signifies the importance of strain engineering in designing novel materials for antiferromagnetic spintronics.
查看更多>>摘要：Classical molecular dynamics simulations were performed for five model metallic glass-forming liquids to investigate the structure correlation with both short-time beta and long-time alpha relaxation dynamics. It is found that Debye-Waller factor characterizing the short-time beta relaxation decreases with increasing five-fold local symmetry in atomic clusters, following a power-law behavior. It is also found that the equation of Debye-Waller factor and alpha-relaxation time, which is widely studied in polymer systems, is applicable in metallic glass-forming systems as well. Furthermore, a new equation is derived for the five-fold local symmetry and alpha-relaxation time, which describes the simulated data very well. These findings indicate that there exist a common structural basis for both the short-time beta-relaxation and the long-time alpha-relaxation dynamics.
NSTL
Elsevier