A. Mocellin

Some recent developments in mechanical activation and mechanosynthesis

After outlining the general characteristics of high-energy ball-milling, mechanochemical synthesis is argued to be an attractive method for the synthesis and transformation of materials. Phase transformations induced by milling, annealing of mechanically activated iron silicides, mechanically activated self-heat sustaining reactions (MASHS), for instance of FeAl, are first discussed. The route, which starts from mechanosynthesized powders to reach consolidated alumina-(Fe,Ti) composites, yields materials whose original morphologies and some mechanical properties are then described.

Polymorphic transformations of titania induced by ball milling

Abstract The high-pressure modification of TiO2 with an α-PbO2-type structure is formed transiently during room-temperature grinding of anatase. Rutile is the only phase present after prolonged ball milling. These experimental results are at variance with reports of a direct transformation of anatase into rutile by room-temperature grinding.

Synthesis of nanostructured materials by mechanical alloying

Materials can be processed by grinding not only for comminution, but also to obtain a variety of structures (amorphous, nanophased), fine mixing of phases, alloys and to directly synthesize compounds such as carbides. Nanocomposites can also be synthesized by reduction, exchange and mixing reactions driven by grinding, as well as by combining the above processes. Large scale economical production of such materials is feasible by grinding in mills designed for that purpose. The mechanical properties of materials are enhanced by the nanophase structure.

The Laguerre model of grain growth in two dimensions I. Cellular structures viewed as dynamical Laguerre tessellations

The basic concepts of a deterministic model for simulating grain growth or foam coarsening in two dimensions are presented, and the fundamental tools necessary for its implementation into computer codes are provided. It is assumed that the actual cell structure can be satisfactorily represented by a Laguerre (or weighted Voronoi) diagram, entirely determined by a collection of weighted sites or circles. The cellular motion (including the induced elementary topological transformations) is subjected to the motion of this collection, allowing for the design of robust and efficient algorithms. An example of a simple motion equation, previously used in vertex models of grain growth, is reformulated to be operational for the collection of the circles generating Laguerre diagrams. The model as a whole can be extended in three dimensions without principle difficulty. Part II of the paper will report and discuss simulation results obtained in two dimensions.

Review : High temperature deformation of Al2O3-based ceramic particle or whisker composites

Abstract The major theoretical models for creep and the creep rate equations of ceramic materials and their dispersed phase composites are briefly reviewed. Then the literature on high temperature deformation behaviours of Al 2 O 3 -based oxide ceramic particle composites (Al 2 O 3 -ZrO 2 , Al 2 O 3 -Y 3 Al 5 O 12 , Al 2 O 3 -Tio 2 ) and Al 2 O 3 -based non-oxide ceramic particle or whisker composites (Al 2 O 3 -SiC (w) , Al 2 O 3 -SiC (p) , Al 2 O 3 -TiC x N 1- x ) since the mid 1980s is reviewed. Most studies have been concerned with the Al 2 O 3 -ZrO 2 and Al 2 O 3 -SiC systems. The influences of various factors on the creep behaviours, the changes of the microstructure in the deformed specimens and the creep mechanisms of these composites are summarised and analysed.

Alumina-chromium cermets by hot-pressing of nanocomposite powders

Abstract Alumina-chromium cermets with up to 36 vol% metal have been prepared from powders obtained by high energy ball-milling. Firstly, the powder is prepared from initially blending Cr2O3 or CrO3 and Al for reactive milling and from Al2O3 and Cr for a direct milling. The materials as-pressed at 1400 °C under 30 MPa consist of an alumina based matrix with metallic Cr inclusions. The difference of reactivities between Cr2O3 and CrO3 during milling on the final material is discussed and the advantages of reactive compared to direct milling are emphasised. In a second part, the more efficient technique is used to obtain cermets with different compositions. Nanostructured powders were synthesized from Cr2O3, Al and Al2O3. The influence of chromium metal content is studied by microstructural observations, physical, mechanical and electrical properties evaluations. The expected improvement of mechanical properties compared to pure alumina is indeed observed. The importance of the milling time and the temperature-pressure cycle during the forming and sintering process are reflected on the final material structure and properties.

The Laguerre model for grain growth in three dimensions

We propose a novel and poweful methodology for three- dimensional (3D) grain growth modelling in both the anisotropic and the locally inhomogeneous cases, thus significantly generalizing previous related two-dimensional work. The fundamental modelling structures for the polycrystals and their behaviour are dynamically evolving Laguerre diagrams in the flat 3D torus. The weighted generating sites of such diagrams obey motion equations ensuing from system interface energy minimization with consequent evolution of the associated structures and resulting in elementary topological transformations thereof. We have implemented the models in state-of-the-art computer simulation codes and made large-scale runs assuming either constant (isotropic) or variable (anisotropic) interface specific energies. In both cases, the simulated evolution reproduced the main features of the normal grain growth process in polycrystalline materials, that is the grain growth power law, typical distributions of grain sizes and shapes, and the scaling behaviour in long-term regime. The simulated distribution of grain-to- grain misorientation and its evolution in a simple hypothetical case have also been obtained for the first time.

Preparation and characterization of alumina–iron cermets by hot-pressing of nanocomposite powders

Alumina composites with 0–36 vol% iron metal dispersed inclusions have been prepared by a new processing route. Nanostructured powders were synthesized by a high energy dry ball-milling process and then consolidated by sintering at 1700 K under a pressure of 30 MPa. The materials consist of micrometre-size Al2O3 with metallic Fe in isolated regions from 10 μm down to nanometre size. From low to high metallic contents, the same type of interwoven microstructure is observed. Hardness, elastic and shear moduli, fracture toughness and the size of the inclusions vary regularly with the metal content. Addition of metal increased the fracture toughness from 3.0 to 8 MPa m1/2.

The Laguerre model of grain growth in two dimensions II. Examples of coarsening simulations

Abstract The basic concepts of a deterministic scheme for simulating grain growth or foam coarsening in two dimensions have been presented in Part I. Implementation of the corresponding Laguerre (or weighted Voronoi) diagram in computer manageable practical terms is now presented. The internal constituents of the model are discussed first and include the initial configurations of the cellular network, obtained from different schemes. An internal clock is designed and an arbitrary dimensionless parameter is introduced to provide a relative weight to the rates of change of the generating circle positions with those of their sizes. A criterion for evaluating robustness and consistency of simulations is also presented. Actual simulation experiments are subsequently described as conducted from a range of initial structures built from different numbers of cells-up to 105. The essential theoretical attributes of normal grain growth were found to be recoverable in all simulations, notably statistical self-similar...

Kinetics of formation of nanocrystalline TiO2 II by high energy ball-milling of anatase TiO2

Abstract Ball-milling is a way of inducing phase transformations, chemical reactions and changes in the reactivity of solids. The high pressure and high temperature phase of TiO2 : TiO2 II is formed transiently during room-temperature grinding of anatase TiO2-Rutile is the only phase present after prolonged ball milling. The influences of physical and of chemical processing conditions on the kinetics of phase transformations are considered.