Mechanical alloying (MA) is a solid-state powder processng technique involving repeated welding, fracturing, and rewelding of powder particles in a high-energy ball mill. Originally developed to produce oxide-dispersion strengthened (ODS) nickel- and iron-base superalloys for applications in the aerospace industry, MA has now been shown to be capable of synthesizing a variety of equilibrium and non-equilibrium alloy phases starting from blended elemental or prealloyed powders. The non-equilibrium phases synthesized include supersaturated solid solutions, metastable crystalline and quasicrystalline phases, nanostructures, and amorphous alloys. Recent advances in these areas and also on disordering of ordered intermetallics and mechanochemical synthesis of materials have been critically reviewed after discussing the process and process variables involved in MA. The often vexing problem of powder contamination has been analyzed and methods have been suggested to avoid/minimize it. The present understanding of the modeling of the MA process has also been discussed. The present and potential applications of MA are described. Wherever possible, comparisons have been made on the product phases obtained by MA with those of rapid solidification processing, another non-equilibrium processing technique.

Mechanical Alloying And Milling

This paper reviews the crystal chemistry, synthesis, densification, microstructure, mechanical properties, and oxidation behavior of zirconium diboride (ZrB2) and hafnium diboride (HfB2) ceramics. The refractory diborides exhibit partial or complete solid solution with other transition metal diborides, which allows compositional tailoring of properties such as thermal expansion coefficient and hardness. Carbothermal reduction is the typical synthesis route, but reactive processes, solution methods, and pre-ceramic polymers can also be used. Typically, diborides are densified by hot pressing, but recently solid state and liquid phase sintering routes have been developed. Fine-grained ZrB2 and HfB2 have strengths of a few hundred MPa, which can increase to over 1 GPa with the addition of SiC. Pure diborides exhibit parabolic oxidation kinetics at temperatures below 1100°C, but B2O3 volatility leads to rapid, linear oxidation kinetics above that temperature. The addition of silica scale formers such as SiC or MoSi2 improves the oxidation behavior above 1100°C. Based on their unique combination of properties, ZrB2 and HfB2 ceramics are candidates for use in the extreme environments associated with hypersonic flight, atmospheric re-entry, and rocket propulsion.

Refractory Diborides of Zirconium and Hafnium

In the laser treatment of a workpiece (9), e.g. for surface hardening, melting, alloying, cladding, welding or cutting, the adverse effect of reflectivity of the surface, when a pure, monochromatic laser (6) is used, is remedied by the simultaneous application of a relatively shorter wavelength beam (1). The two beams (1)(5) may be combined by a beam coupler (4) or may reach the workpiece (9) by separate optical paths (not shown). The shorter wavelength beam (1) improves the coupling efficiency of the higher- powered laser beam (5).

Laser Material Processing

Progress in material selection for solid oxide fuel cell technology: A review

This review article aims to provide an updated and comprehensive description of the development of the Electric Current Activated/assisted Sintering technique (ECAS) for the obtainment of dense materials including nanostructured ones. The use of ECAS for pure sintering purposes, when starting from already synthesized powders promoters, and to obtain the desired material by simultaneously performing synthesis and consolidation in one-step is reviewed. Specifically, more than a thousand papers published on this subject during the past decades are taken into account. The experimental procedures, formation mechanisms, characteristics, and functionality of a wide spectrum of dense materials fabricated by ECAS are presented. The influence of the most important operating parameters (i.e. current intensity, temperature, processing time, etc.) on product characteristics and process dynamics is reviewed for a large family of materials including ceramics, intermetallics, metal–ceramic and ceramic–ceramic composites. In this review, systems where synthesis and densification stages occur simultaneously, i.e. a fully dense product is formed immediately after reaction completion, as well as those ones for which a satisfactory densification degree is reached only by maintaining the application of the electric current once the full reaction conversion is obtained, are identified. In addition, emphasis is given to the obtainment of nanostructured dense materials due to their rapid progress and wide applications. Specifically, the effect of mechanical activation by ball milling of starting powders on ECAS process dynamics and product characteristics (i.e. density and microstructure) is analysed. The emerging theme from the large majority of the reviewed investigations is the comparison of ECAS over conventional methods including pressureless sintering, hot pressing, and others. Theoretical analysis pertaining to such technique is also proposed following the last results obtained on this topic.

Consolidation/synthesis of materials by electric current activated/assisted sintering

The use of silicon nitride as a sintering aid (5 vol.%) greatly improves the powder sinterability of zirconium diboride, in comparison to additive free ZrB2. Nearly full dense monolithic material is obtained by hot pressing at 1700 °C. The microstructure consists of fine regular ZrB2 grains and of various secondary grain boundary phases (e.g. BN, t-ZrO2, BN-rich glassy phase), mainly located at triple points. The addition of 20 vol.% of silicon carbide as a reinforcing particulate phase to the ZrB2+5vol.%Si3N4 powder mixture slows down the densification rate of ZrB2, therefore a higher hot pressing temperature (i.e. 1870 °C) is necessary to achieve nearly full density. Further addition of oxide additives (1vol.%Al2O3+0.5vol.%Y2O3) to the ZrB2–20vol.%SiC–5vol.%Si3N4 system enables the production of near fully dense composites at lower hot pressing temperature (1760 °C). The presence of SiC particles in both the ZrB2-based composites effectively improves strength, hardness and toughness, compared to monolithic zirconium diboride. Some mechanical properties are very interesting: flexural strength up to 700 and 600 MPa are measured at room temperature and 1000 °C, respectively. The properties are discussed in terms of the microstructural features.

Advances in microstructure and mechanical properties of zirconium diboride based ceramics

Two zirconium diboride-base composites were produced and characterised The chosen starting compositions were: 55 wt% ZrB2+41 wt%TiB2+4 wt% Ni and 83 wt% ZrB2+13 wt% B4C+4 wt% Ni The microstructure and properties of these composites were compared to those of a monolithic ZrB2+4 wt% Ni material In all cases, metallic Ni as the sintering aid promoted the formation of the liquid phase which improved mass transfer mechanisms during sintering From the powder mixture ZrB2+TiB2, two solid solutions of Zr–Ti–B were obtained In the case of the other mixture, B4C particles were dispersed in the ZrB2 matrix The composite materials have better mechanical properties than those of the monolithic ZrB2 ceramic; in particular the fracture toughness and the flexural strength were almost doubled at room temperature Long term oxidation tests indicated that the ZrB2-based composites, particularly the composite containing B4C as the second phase, were more resistant to oxidation than the monolithic ZrB2 due to the formation of surface oxide products which were protective against the complete degradation by oxidation observed for the ZrB2 matrix material

Processing and properties of zirconium diboride-based composites

The oxidation behavior of three different ZrB 2 -based ceramics, a monolithic ZrB 2 and two ZrB 2 -SiC composites, was studied up to 1350°C and under isothermal conditions at 1120°C for 20 h. Oxidation kinetics and microstructural changes in the oxidized specimens indicate that the monolithic zrB 2 has low thermal stability. On the contrary, the introduction of SiC particles markedly improves oxidation resistance due to the formation of an adherent and protective borosilicate glass layer that coats the sample surface, effectively limiting the inward diffusion of oxygen toward the reaction interface. The influence of the secondary grain boundary phases on oxidation is discussed.

Oxidation of ZrB2-Based Ceramics in Dry Air

The processing and the properties of two ultra-high temperature ceramics (UHTCs) designed for the manufacturing of aerospace sharp-shaped hot-structures are presented, along with the results obtained in the electrical discharge machining (EDM) of these UHTCs into sharp hot-structure components. The powder mixtures in the (ZrB2–SiC)-based systems were brought to full density by hot-pressing. The hot-pressed bodies were characterized by fine and uniform microstructures (typical grain size  Beyond basic mechanical properties, the machinability of UHTC blocks into a more complex shape by means of the EDM was also assessed. Sharp-shaped hot-structures in the form of a nose-cone were produced from 12 cm × 10 cm hot-pressed cylindrical blocks. The machined surfaces showed limited roughness Ra

Processing and properties of ultra-high temperature ceramics for space applications

This work investigated suitability and efficacy of the sintering technique known as spark plasma sintering to produce ultra-high-temperature-based Hf and Zr borides. Ceramic–matrix composites in the systems HfB2–SiC, ZrB2–MoSi2, and ZrB2–ZrC–SiC were processed by spark plasma sintering and hot pressing. The effects of processing were evaluated comparing the materials microstructure and properties. Compared with hot-pressing technique, spark plasma sintering offers the great advantage to fabricate successfully in short time (i.e., cuts in costs) poorly sinterable powder compositions without the help of any sintering activators.

Alida Bellosi

Papers

Advances in microstructure and mechanical properties of zirconium diboride based ceramics

Processing and properties of zirconium diboride-based composites

Oxidation of ZrB2-Based Ceramics in Dry Air

Processing and properties of ultra-high temperature ceramics for space applications

Fast Densification of Ultra‐High‐Temperature Ceramics by Spark Plasma Sintering