Microhardness and fracture properties of nanocrystalline NiP alloy

Rediscovering the intrinsic mechanical properties of bulk nanocrystalline indium arsenide

Is the inverse Hall-Petch relation in ceramic systems the same as that in metal systems? The premise to explore this subject is the synthesis of a dense bulk nanocrystalline material with clean grain boundaries. By using the reciprocating pressure-induced phase transition (RPPT) technique, compact bulk nanocrystalline indium arsenide (InAs) has been synthesized from a single crystal in a single step, while its grain size is controlled by thermal annealing. The influence of macroscopic stress or surface states on the mechanical characterization has been successfully excluded by combining first-principles calculations and experiments. Unexpectedly, nanoindentation tests show a potential inverse Hall-Petch relation in the bulk InAs with a critical grain size (D_cri) of 35.93 nm in the experimental scope. Further molecular dynamics investigation confirms the existence of the inverse Hall-Petch relation in the bulk nanocrystalline InAs with D_cri = 20.14 nm for the defective polycrystalline structure, with its D_cri significantly affected by the intragranular-defect density. The experimental and theoretical conclusions comprehensively reveal the great potential of RPPT in the synthesis and characterization of compact bulk nanocrystalline materials, which provides a novel window to rediscover their intrinsic mechanical properties, for instance, the inverse Hall-Petch relation of bulk nanocrystalline InAs.

Controlling Grain Boundaries by Magnetic Fields

The ability to use external magnetic fields to influence the microstructure in polycrystalline materials has potential applications in microstructural engineering. To explore this potential and to understand the complex interactions between electromagnetic fields and solid-state matter transport we consider a phase-field-crystal model. Together with efficient and scalable numerical algorithms this allows the examination of the role that external magnetic fields play on the evolution of defect structures and grain boundaries, on diffusive timescales. Examples for planar and circular grain boundaries explain the essential atomistic processes and large scale simulations in 2D are used to obtain statistical data on grain growth under the influence of external fields.

Bidirectional Transformation Enables Hierarchical Nanolaminate Dual-Phase High-Entropy Alloys

Microstructural length-scale refinement is among the most efficient approaches to strengthen metallic materials. Conventional methods for refining microstructures generally involve grain size reduction via heavy cold working, compromising the material's ductility. Here, a fundamentally new approach that allows load-driven formation and permanent refinement of a hierarchical nanolaminate structure in a novel high-entropy alloy containing multiple principal elements is reported. This is achieved by triggering both, dynamic forward transformation from a faced-centered-cubic γ matrix into a hexagonal-close-packed ε nanolaminate structure and the dynamic reverse transformation from ε into γ. This new mechanism is referred to as the "bidirectional transformation induced plasticity" (B-TRIP) effect, which is enabled through a near-zero yet positive stacking fault energy of γ. Modulation of directionality in the transformation is triggered by local dissipative heating and local micromechanical fields. The simple thermodynamic and kinetic foundations for the B-TRIP effect render this approach generally suited for designing metastable strong and ductile bulk materials with hierarchical nanolaminate substructures.

Structural effect of two-dimensional BNNS on grain growth suppressing behaviors in Al-matrix nanocomposites

While nanocrystalline (NC) metals exhibit superior strength to conventional microcrystalline metals, their thermal instability has hampered their application at high temperatures. Herein, two-dimensional (2D) boron nitride nanosheets (BNNS) are proposed as reinforcement to enhance the strength as well as the thermal stability of NC Al. The strength of pure Al was increased from 80 to 468 MPa by refining its grains from ~600 to ~40 nm, and it was further enhanced to 685 MPa by incorporating 2 vol% of BNNS. Moreover, the small amount of BNNS was found to effectively suppress grain growth of NC Al at 580 °C (~0.9 T_m, where T_m is the melting point of Al), which prevented a strength drop at high temperature. Finally, the Zener pinning model in conjunction with phase-field simulations was utilized to qualitatively analyze the effect of the BNNS on the grain boundary pinning as a function of volume, shape, and orientation of the reinforcement. The model demonstrated that the pinning force of 2D reinforcements is much higher than that of spherical particles. Hence, 2D BNNS offer the possibility of developing Al-matrix nanocomposites for high-temperature structural applications.

The Inverse Hall-Petch Effect—Fact or Artifact?

This paper critically reviews the data in the literature which gives softening—the inverse Hall-Petch effect—at the finest nanoscale grain sizes. The difficulties with obtaining artifactfree samples of nanocrystalline materials will be discussed along with the problems of measurement of the average grain size distribution. Computer simulations which predict the inverse Hall-Petch effect are also noted as well as the models which have been proposed for the effect. It is concluded that while only a few of the experiments which have reported the inverse Hall-Petch effect are free from obvious or possible artifacts, these few along with the predictions of computer simulations suggest it is real. However, it seems that it should only be observed for grain sizes less than about 10 nm.

Effect of Porosity on Properties of Nanocrystalline Materials

This paper discusses the effects residual porosity may have on some properties of nanocrystalline materials. The two examples considered here are Young's modulus and saturation magnetization. It will be shown that the large variations observed for these properties may be explained, at least in part, on the basis of the residual porosity in thematerials which may vary considerably depending on the production techniques used to synthesize nanostructured materials.

Mechanical Hardness as a Probe of Nanocrystalline Materials

The use of mechanical hardness as a probe of nanocrystalline materials is reviewed. The fact that the grain size dependence of hardness is very different for nanocrystalline materials compared to conventional (≥1 μm diameter) polycrystals suggests a different deformation mechanism may be operative in nanocrystalline materials. Hardness is useful for following the sintering, densification reactions of nanoparticles. Solid solution hardening in nanocrystalline alloys is found to be overwhelmed by the grain boundary hardening. If alloying decreases the grain boundary hardening, i.e. increases grain size, an apparent solid solution softening effect is observed.

Grain Size Dependent Mechanical Properties in Nanophase Materials

It has become possible in recent years to synthesize metals and ceramics under well controlled conditions with constituent grain structures on a nanometer size scale (below 100 nm). These new materials have mechanical properties that are strongly grain-size dependent and often significantly different than those of their coarser grained counterparts. Nanophase metals tend to become stronger and ceramics are more easily deformed as grain size is reduced. The observed mechanical property changes appear to be related primarily to grain size limitations and the large percentage of atoms in grain boundary environments. A brief overview of our present knowledge about the grain-size dependent mechanical properties of nanophase materials is presented.

Minimum Grain Size in Nanocrystalline Metal Powders

Nanocrystalline metal powders can be synthesized by mechanical attrition in a highenergy ball mill. A general relation determining the grain size of these materials is inferred. The ultimate grain size of nanocrystalline metals (typically 6 − 22 nm) is governed by the competition between the severe plastic deformation introduced during ball milling and the recovery behavior of the material. The lower bound grain size achievable by mechanical attrition is given by the minimum distance between two dislocations in a pile-up within a grain for all pure metals. Foar binary alloys the ultimate grain size depends on the composition of the material. Varying the composition changes the grain size reversibly. This can be explained by introducing solid solution hardening effects in the general relation for the lower bound grain size in pure metals. Thus, the proposed model for the ultimate grain size achievable by ball milling seems to be. applicable to all metals and alloys subjected to heavy mechanical deformation. However, reversible grain size changes are not restricted to mechanical attrition, but have also been observed for nanocrystalline Pd-H solid solutions produced by hydriding at constant pressure. Solid solutions prepared at different compositions, i.e. samples with different compositions, exhibit different grain sizes. Cycling between different temperatures/compositions changes the grain size reversibly. This cannot be explained by a model based on plastic deformation as in the case of ball-milled metal powders. The results are compared with data for ball-milled powders and samples prepared by inert gas condensation. The grain size changes are discussed with respect to the compositional changes and the grain boundary energy of the material.

Phase field crystal study of deformation and plasticity in nanocrystalline materials

We introduce a modified phase field crystal (MPFC) technique that self-consistently incorporates rapid strain relaxation alongside the usual plastic deformation and multiple crystal orientations featured by the traditional phase field crystal (PFC) technique. Our MPFC formalism can be used to study a host of important phase transformation phenomena in material processing that require rapid strain relaxation. We apply the MPFC model to study elastic and plastic deformations in nanocrystalline materials, focusing on the "reverse" Hall-Petch effect. Finally, we introduce a multigrid algorithm for efficient numerical simulations of the MPFC model.

Modeling the dependence of strength on grain sizes in nanocrystalline materials

A model was developed to describe the grain size dependence of hardness (or strength) in nanocrystalline materials by combining the Hall-Petch relationship for larger grains with a coherent polycrystal model for nanoscale grains and introducing a log-normal distribution of grain sizes. The transition from the Hall-Petch relationship to the coherent polycrystal mechanism was shown to be a gradual process. The hardness in the nanoscale regime was observed to increase with decreasing grain boundary affected zone (or effective grain boundary thickness, Δ) in the form of Δ-1/2. The critical grain size increased linearly with increasing Δ. The variation of the calculated hardness value with the grain size was observed to be in agreement with the experimental data reported in the literature.

Strength weakening by nanocrystals in ceramic materials

A key question in nanomechanics concerns the grain size effects on materials' strength. Correct solution to this question is critical to design and tailor the properties of materials for particular applications. The full map of grain sizes-hardness/yield stress relationship in metals has been built. However, for ceramic materials, the similar studies and understandings are really lacking. Here we employed a novel technique to comparatively study the mechanical features of titanium dioxide (TiO(2)) with different crystallite sizes. On the basis of peak profile analysis of the X-ray diffraction data, we determined yield strength for nanocrystalline and bulk TiO(2). Our results reveal a remarkable reduction in yield strength as the grain size decreases from 30-40 microm to approximately 10 nm, providing the only evidence of a strength weakening by nanocrystals relative to their bulk counterparts. This finding infers an inverse Hall-Petch effect, the first of its kind for ceramic materials, and a dramatic strength weakening after the breakdown of classic Hall-Petch relation below a characteristic grain size.

HighP-TNano-Mechanics of Polycrystalline Nickel

We have conducted highP-Tsynchrotron X-ray and time-of-flight neutron diffraction experiments as well as indentation measurements to study equation of state, constitutive properties, and hardness of nanocrystalline and bulk nickel. Our lattice volume-pressure data present a clear evidence of elastic softening in nanocrystalline Ni as compared with the bulk nickel. We show that the enhanced overall compressibility of nanocrystalline Ni is a consequence of the higher compressibility of the surface shell of Ni nanocrystals, which supports the results of molecular dynamics simulation and a generalized model of a nanocrystal with expanded surface layer. The analytical methods we developed based on the peak-profile of diffraction data allow us to identify "micro/local" yield due to high stress concentration at the grain-to-grain contacts and "macro/bulk" yield due to deviatoric stress over the entire sample. The graphic approach of our strain/stress analyses can also reveal the corresponding yield strength, grain crushing/growth, work hardening/softening, and thermal relaxation under highP-Tconditions, as well as the intrinsic residual/surface strains in the polycrystalline bulks. From micro-indentation measurements, we found that a low-temperature annealing (T < 0.4 Tm) hardens nanocrystalline Ni, leading to an inverse Hall-Petch relationship. We explain this abnormal Hall-Petch effect in terms of impurity segregation to the grain boundaries of the nanocrystalline Ni.

Modeling elastic and plastic deformations in nonequilibrium processing using phase field crystals

A continuum field theory approach is presented for modeling elastic and plastic deformation, free surfaces, and multiple crystal orientations in nonequilibrium processing phenomena. Many basic properties of the model are calculated analytically, and numerical simulations are presented for a number of important applications including, epitaxial growth, material hardness, grain growth, reconstructive phase transitions, and crack propagation.