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

The MN+1AXN phases: A new class of solids

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

This paper presents a review of the principal developments in functionally graded materials (FGMs) with an emphasis on the recent work published since 2000. Diverse areas relevant to various aspects of theory and applications of FGM are reflected in this paper. They include homogenization of particulate FGM, heat transfer issues, stress, stability and dynamic analyses, testing, manufacturing and design, applications, and fracture. The critical areas where further research is needed for a successful implementation of FGM in design are outlined in the conclusions. DOI: 10.1115/1.2777164

https://web.mst.edu/~vbirman/papers/Modeling%20and%20Analysis%20of%20FGM_Review_2007.pdf

Modeling and Analysis of Functionally Graded Materials and Structures

Liquid phase sintering (LPS) is a process for forming high performance, multiple-phase components from powders. It involves sintering under conditions where solid grains coexist with a wetting liquid. Many variants of LPS are applied to a wide range of engineering materials. Example applications for this technology are found in automobile engine connecting rods and high-speed metal cutting inserts. Scientific advances in understanding LPS began in the 1950s. The resulting quantitative process models are now embedded in computer simulations to enable predictions of the sintered component dimensions, microstructure, and properties. However, there are remaining areas in need of research attention. This LPS review, based on over 2,500 publications, outlines what happens when mixed powders are heated to the LPS temperature, with a focus on the densification and microstructure evolution events.

/pdf/review-liquid-phase-sintering-19fo5ee4w3.pdf

Review: liquid phase sintering

Thermal fracture behavior of metal/ceramic functionally graded materials

Mechanical properties of polycrystalline Ti3SiC2 at ambient and elevated temperatures

Recently, titanium silicon carbide (Ti3SiC2) has received considerable attention because of the unusual combination of metallic and ceramic properties [1]. Like metals Ti3SiC2 has good electric and thermal conductivity, relatively low hardness and high fracture resistance. Like ceramics it shows good oxidation resistance and remarkable refractoriness. Synthesis processes of Ti3SiC2 can be divided into two categories: powder-based process and chemical vapor deposition (CVD). The CVD process was first used to synthesize single-phase Ti3SiC2 films, in which plastic deformation was observed at ambient temperature [2]. To date, most investigations were focused on the synthesis of Ti3SiC2 by way of powder-based processes, which are more attractive for the productivity in comparison with the CVD process. However, powderbased processes usually produced Ti3SiC2 with small amounts of TiC and/or SiC. Nevertheless, recently Barsoum and El-Raghy [1] successfully fabricated high-purity Ti3SiC2 polycrystals by reactively hotpressing the mixture of Ti, graphite and SiC powders. Their synthesis process was characteristic of the relatively coarse SiC powder, with a mean diameter of 99 μm, that was used as the source. Although TiC also was used as the starting powders in the other investigations [7–9], we considered that using the elemental Ti, Si and C powders as the source should be more effective for the formation of Ti3SiC2, as compared to chemically stable SiC or TiC. However, the validity of such starting materials has not been confirmed yet by the few previous investigations [3–6]. In the present study, the reaction between the elemental Ti, Si and C powders was investigated to determine the processing condition for the formation of Ti3SiC2. Then, singlephase Ti3SiC2 bulk samples with near-full density were fabricated successfully by hot-isostatic pressing of the mixed Ti, Si and C powders. The mechanical, electric and thermal properties of the obtained Ti3SiC2 polycrystals were investigated. Commercial Ti (16μm, >99.6% purity, Toho Titanium Co., Japan), Si (70μm, >99.9% purity, Rare Metallic Co., Japan) and C (5μm,>99.7% purity, High Purity Chemicals Co., Japan) powders were used in the present study. The powder mixtures were prepared in three ways: hand-mixing for 30 min using a mortar and pestle; wet ball-milling in ethanol solution for 100 h; dry ball-milling in an argon atmosphere for 500 h. The ball milling used a jar and balls made of 97% purity Si3N4 ceramics. The slurry obtained by the wet ballmilling was vacuum dried and then pulverized with a mortar and pestle. The mixed powders were formed into billets 10 mm in diameter and∼10 mm in height, by die pressing at 130–150 MPa and subsequent cold-isostatic pressing at 200 MPa. To investigate the formation temperature of Ti3SiC2, the green compacts of mixed powders were fired within a BN crucible in an atmosphere of Ar (99.9995% purity, O2< 0.2 ppm) at 0.1 MPa in an electric furnace with a carbon heater. The heating and cooling rates were 15 ◦C min−1 and 20 ◦C min−1, respectively. The resultant products were examined by X-ray diffraction (XRD) with CuKα radiation. Dense Ti3SiC2 samples were then prepared by so-called reactive hot-isostatic pressing (R-HIP) process. The details of the R-HIP process are as follows. First, the green compacts of mixed powders were coated with BN powder to a thickness of about 1 mm, and then inserted into a borosilicate glass tube used as the capsule. Here BN powder was used to prevent any reaction between the specimen and glass capsule. The tube was evacuated and heated up to the softening temperature of the glass (∼780 ◦C), then sealed and cut with a gas-flame burner when the specimen was closely enveloped in the glass capsule. The glass-encapsulated specimens were hot-isostatically pressed under Ar gas as a pressure-transmitting medium. They were heated up to the softening temperature under a low pressure of about 1 MPa; pressure and temperature were then concurrently increased to the predetermined peak values with soaking time of 2 h. Density was determined by the Archimedes method. Microstructures were observed by scanning electron microscopy (SEM). Fracture strength was measured in a 4-point bending test with lower and upper spans of 20 and 10 mm, respectively. The bending tests were performed at a crosshead speed of 0.5 mm min−1 using specimens of nominal dimensions 24× 2.0× 1.5 mm. The prospective tensile surfaces were ground and polished to achieve a mirrorlike surface finish prior to mechanical testing. Young’s modulus and Poisson’s ratio were measured at room temperature by a static method using a cross-shaped electrical-resistance strain gauge mounted onto a bend specimen. Vickers microhardness was measured under a load of 4.9 N. Electrical conductivity was measured using two-probe method from room temperature to 800 ◦C under vacuum. Thermal expansion coefficient was measured under vacuum using a dilatometer. Fig. 1 shows XRD patterns of the starting powders and the products obtained by heating the mixed Ti, Si and C powders with a molar ratio of 3 : 1 : 2, in an

Synthesis of Ti3SiC2 polycrystals by hot-isostatic pressing of the elemental powders

As a model for composite materials of piezoelectric ceramic and metal, lead zirconate titanate (PZT) and platinum (Pt) particulate composites were fabricated by power processing. The electrical and mechanical properties of the PZT–Pt composites were measured as a function of the Pt volume fraction. The relative dielectric constants of the PZT–Pt composites increased markedly, while the piezoelectric constants and electromechanical coupling coefficients decreased with increasing Pt content. When the Pt volume fraction exceeded 30%, the PZT–Pt composite became electrically conductive because of percolation of the Pt particles. The Pt-dispersed PZT composites enhanced the mechanical properties, particularly the high fracture resistance, compared to the monolithic PZT ceramics.

Ryuzo Watanabe

Papers

Thermal fracture behavior of metal/ceramic functionally graded materials

Mechanical properties of polycrystalline Ti3SiC2 at ambient and elevated temperatures

Synthesis of Ti3SiC2 polycrystals by hot-isostatic pressing of the elemental powders

Electrical and mechanical properties of piezoelectric ceramic/metal composites in the Pb(Zr, Ti)O3/Pt system

Finite Element Analysis of Thermal Stress of the Metal/Ceramic Multi-Layer Composites with Controlled Compositional Gradients