Polycrystalline materials with nanometer-sized grains, termed nanocrystalline materials, can be formed by crystallizing completely amorphous solids under proper heat treatment conditions. The crystallized nanocrystalline materials exhibit some unique structural characteristics and novel properties which are fundamentally different from those of the conventional coarse-grained polycrystalline materials. This article reviews the present state of the art in this field. The current status of research and developments on the nanocrystallization, microstructure and properties of the materials will be summarized. Comparisons of structural characteristics and properties are made between the crystallized nanocrystalline materials and those prepared by other methods. Further considerations of the development and applications of this new class of materials will also be presented.

Nanocrystalline metals crystallized from amorphous solids: nanocrystallization, structure, and properties

An account is given of the research that has been carried out on mechanical alloying/milling (MA/MM) during the past 25 years. Mechanical alloying, a high energy ball milling process, has established itself as a viable solid state processing route for the synthesis of a variety of equilibrium and non-equilibrium phases and phase mixtures. The process was initially invented for the production of oxide dispersion strengthened (ODS) Ni-base superalloys and later extended to other ODS alloys. The success of MA in producing ODS alloys with better high temperature capabilities in comparison with other processing routes is highlighted. Mechanical alloying has also been successfully used for extending terminal solid solubilities in many commercially important metallic systems. Many high melting intermetallics that are difficult to prepare by conventional processing techniques could be easily synthesised with homogeneous structure and composition by MA. It has also, over the years, proved itself to be superior to rapid solidification processing as a non-equilibrium processing tool. The considerable literature on the synthesis of amorphous, quasicrystalline, and nanocrystalline materials by MA is critically reviewed. The possibility of achieving solid solubility in liquid immiscible systems has made MA a unique process. Reactive milling has opened new avenues for the solid state metallothermic reduction and for the synthesis of nanocrystalline intermetallics and intermetallic matrix composites. Despite numerous efforts, understanding of the process of MA, being far from equilibrium, is far from complete, leaving large scope for further research in this exciting field.

Novel materials synthesis by mechanical alloying/milling

Hydrogen is considered a good energy carrier candidate for future automotive applications that could be part of a carbon-free cycle. Metal hydrides are often preferred over pressurized gas and other hydrogen storage methods because of their gravimetric and volumetric storage capacities and safe operating pressures. In addition to the hydrogen storage capacity, other properties that have often been disregarded must now be addressed before hydrogen storage in metal hydrides becomes feasible. The slow hydriding/dehydriding kinetics, high release temperature, low storage efficiency due to the high enthalpy of formation, and thermal management during the hydriding reaction remain important difficulties in meeting the objectives set by the Department of Energy (DOE) for hydrogen storage systems. Nanotechnology offers new ways of addressing those issues by taking advantage of the distinctive chemical and physical properties observed in nanostructures. Nanostructured materials significantly improve the reaction kinetics, reduce the enthalpy of formation, and lower the hydrogen absorption and release temperatures through destabilization of the metal hydride and multiple catalytic effects in the system. But nanostructures can also lead to poor cyclability, degradation of the sorption properties, and a significant reduction of the thermal conductivity that could make metal hydrides impractical for hydrogen storage. This review summarizes the effects that nanotechnology can have on the main properties of metal hydrides and highlights the main competing behaviours between the system requirements, the necessary trade-offs, and the research priorities necessary to obtain hydride storage materials for practical automotive applications. Copyright © 2007 John Wiley & Sons, Ltd.

Size effects on the hydrogen storage properties of nanostructured metal hydrides: A review

This is a first systematic report on the synthesis of completely nanocrystalline metals by high-energy deformation processes. Pure metals with body-centered cubic (bcc) and hexagonal close-packed (hcp) structures are subjected to ball milling, resulting in a decrease of the average grain size to ≈9 nm for metals with bcc and to ≈13 nm for metals with hcp crystal structures. This new class of metastable materials exhibits an increase of the specific heat up to 15 pct at room temperature and a mechanically stored energy determined as up to 30 pct of the heat of fusion after 24 hours of high-energy ball milling. The grain boundary energy as determined by calorimetry is higher than the energy for fully equilibrated high-angle grain boundaries.

Nanocrystalline metals prepared by high-energy ball milling

The current status of research and development in Fe-based bulk metallic glasses (BMGs) is reviewed. Bulk metallic glasses are relatively new materials possessing a glassy structure and large section thickness. These materials have an exciting combination of properties such as high mechanical strength, good thermal stability, large supercooled liquid region and potential for easy forming. Ever since the first synthesis of an Fe-based BMG in an Fe–Al–Ga–P–C–B system in 1995, there has been intense activity on the synthesis and characterisation of Fe-based BMGs. These BMGs exhibit some unique characteristics which have not been obtained in conventional Fe-based crystalline alloys. This uniqueness has led to practical uses of these bulk glassy alloys as soft magnetic and structural materials. This review presents the recent results on the glass-forming ability, structure, thermal stability, mechanical properties, corrosion behaviour, soft magnetic properties and applications of Fe-based bulk glassy a...

https://www.tandfonline.com/doi/pdf/10.1179/1743280412Y.0000000007

Iron-based bulk metallic glasses

A simple method is proposed to describe the thermodynamic properties of disordered grain boundaries in nanocrystalline metals and semiconductors. Based on a free-volume approach at negative pressure of the universal equation of state, the maximum free volume, thermal expansion coefficient, specific-heat capacity, and excess enthalpy, entropy, and free energy of grain boundaries are derived from well-known thermodynamic relationships. The results show good agreement with available experimental data and suggest an intrinsic mechanical instability of a nanograin boundary (Gibbs instability) accompanied by divergences of \ensuremath{\alpha} and ${\mathit{c}}_{\mathit{p}}$, and the possibility of an entropy stabilization of nanograin boundaries.

Intrinsic instability and entropy stabilization of grain boundaries.

We investigated the stability of the CsCl-type compounds AlRu, SiRu, NiTi, and CuEr under heavy deformation by high-energy ball milling. As a result, nanocrystalline intermetallic compounds have been synthesized with a microstructure showing a mixture of highly strained crystallites (5–12 nm) with random orientation. Thermal analysis of the samples indicates stored deformation energies of 5–10 kJ/mole stored by incorporation of lattice defects, chemical disorder and grain boundaries, and an increase of the heat capacity of up to 13%. Chemical disordering of the crystal is monitored by means of the long-range order parameter. The observed decrease in chemical order can be explained by the high grain boundary density of nanocrystalline metals.

Stability of CsCl-type intermetallic compounds under ball milling

Crystal to amorphous transformations are observed in metals and alloys when a crystalline phase is prepared in, or driven to, a non-equilibrium configuration and kinetic constraints are maintained which prevent the full reestablishment of equilibrium. Examples of this include amorphization by solid state interdiffusion reactions of crystalline reactants or a crystal with a gaseous reactant, radiation-induced amorphization and amorphization by mechanical alloying. The transformation to the amorphous phase bears a striking resemblance to ordinary melting. For example, it generally exhibits characteristics of a first-order phase transformation (e.g. latent heat, a sharp crystal-glass interface, etc.) . However, under certain conditions, the crystalline phase may become unstable and the transformation catastrophic. This paper discusses various aspects of the crystal to glass transformation. Particular attention is given to the limits of stability of metastable crystals and attempts to develop criteria for defining these limits. These concepts are related to experimental observation.

Mechanisms of instability in crystalline alloys with respect to vitrification

We investigated through X- ray diffraction and transmission electron microscopy the crystal refinement of the intermetallic compound AIRu by high- energy ball milling. The deformation process causes a decrease of crystal size to 5–7 rum and an increase of atomic level strain. This deformation is localized in shear bands with a thickness of 0.5 to 1 micron. Within these bands the crystal lattice breaks into small grains with a typical size of 8–14 rum. Further deformation leads to a final nanocrystalline structure with randomly oriented crystallite grains separated by high- angle grain boundaries.

Hans J. Fecht

Papers

Intrinsic instability and entropy stabilization of grain boundaries.

Stability of CsCl-type intermetallic compounds under ball milling

Mechanisms of instability in crystalline alloys with respect to vitrification

Mechanism of Achieving Nanocrystalline AIRu By Ball Milling

Nanostructure formation and properties of metals and composites processed mechanically in the solid state