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

https://mech.pg.edu.pl/documents/14057592/14057638/1611.pdf

Fabrication methods of porous metals for use in orthopaedic applications

Discrete particle simulation of particulate systems: A review of major applications and findings

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

Effectiveness of a densification process to create strong and durable bonding in densified products such as pellets, briquettes, and cubes can be determined by testing the strength (i.e., compressive resistance, impact resistance, and water resistance), and durability (i.e., abrasion resistance) of the densified products. These tests can indicate the maximum force/stress that the densified products can withstand, and the amount of fines produced during handling, transportation, and storage. In this article, the procedures used for measuring the strength and durability of the densified products are discussed. The effects of constituents of the feed such as starch, protein, fiber, fat, lignin and extractives; feed moisture content; feed particle size and its distribution; feed conditioning temperature/preheating of feed; added binders; and densification equipment variables (forming pressure, and pellet mill and roll press variables) on the strength and durability of the densified products are reviewed. This article will help select process parameters to produce strong and durable densified products from new biomass feedstocks or animal feed formulations. Guidelines for developing standards on criteria for the acceptance levels of strength and durability of the densified products are presented.

Factors affecting strength and durability of densified biomass products.

Field assisted sintering technique (FAST) is a non-conventional powder consolidation method in which densification is enhanced by the application of an electrical discharge combined with resistance heating and pressure. Interest in FAST is motivated by its ability to consolidate a large variety of powder materials to high densities in short times. Full densification of metal and ceramic powders has been achieved within minutes, with a reduced number of processing steps, no need for sintering aids and more flexibility in powder handling. Although the electrical discharge effects have not been completely elucidated, distinct surface effects created by micro-discharges have been noticed in FAST consolidated specimens such as atomically clean grain boundaries and new resistivity peaks in superconductors. On-going experimental and theoretical studies to provide more quantitative insight into the relevant FAST mechanisms are presented.

Sintering activation by external electrical field

In this article, the second part of a two-part study, we report on the mechanical behavior of Ti3SiC2. In particular, we have evaluated the mechanical response of fine-grained (3–5 μm) Ti3SiC2 in simple compression and flexure tests, and we have compared the results with those of coarse-grained (100–200 μm) Ti3SiC2. These tests have been conducted in the 25°–1300°C temperature range. At ambient temperature, the fine- and coarse-grained microstructures exhibit excellent damage-tolerant properties. In both cases, failure is brittle up to ∼1200°C. At 1300°C, both microstructures exhibit plastic deformation (>20%) in flexure and compression. The fine-grained material exhibits higher strength compared with the coarse-grained material at all temperatures. Although the coarse-grained material is not susceptible to thermal shock (up to 1400°C), the fine-grained material thermally shocks gradually between 750° and 1000°C. The results presented herein provide evidence for two important aspects of the mechanical behavior of Ti3SiC2: (i) inelastic deformation entails basal slip and damage formation in the form of voids, grain-boundary cracks, kinking, and delamination of individual grains, and (ii) the initiation of damage does not result in catastrophic failure, because Ti3SiC2 can confine the spatial extent of the damage.

Processing and Mechanical Properties of Ti3SiC2: II, Effect of Grain Size and Deformation Temperature

Microstructural observations of damage around indentations in Ti 3 SiC 2 are presented. The Vickers hardness decreased with increasing load and asymptotically approached 4 GPa at the highest loads. No indentation cracks were observed even at loads as high as 300 N. Preliminary strength versus indentation plots indicate that, at least for the large-grained material (100 μm) studied here, Ti 3 SiC 2 is a damage-tolerant material able to contain the extent of microdamage to a small area around the indent. The following multiple energy-absorbing mechanisms have been identified from scanning electron micrographs of areas in the vicinity of the indentation: diffuse microcracking, delamination, crack deflection, grain push-out, grain pull-out, and the buckling of individual grains.

Damage Mechanisms around Hardness Indentations in Ti3SiC2

The microstructure of alloys with a near-equiaxed microstructure, produced by spray casting, magnetohydrodynamic (MHD) casting and the stress induced, melt activated (SIMA) process, as it evolves within short times in the semisolid state, is examined by rapid quenching and isothermal soaking experiments. Quenching experiments reveal the morphology and distribution of solid phase at high and medium volume fractions of liquid. At medium liquid content, the microstructure of spray-cast and SIMA alloys consists of discrete equiaxed grains uniformly dispersed in the liquid phase, while the corresponding microstructure of MHD-cast alloys exhibits extensive agglomerates consisting of incompletely spheroidized grains. The connectivity of solid phase and the formation of a solid skeleton in the semisolid state are discussed in terms of grain misorientation. Isothermal soaking experiments investigated grain growth and degree of spheroidization as a function of soaking time and liquid content in the semisolid state. Results demonstrated that MHD-cast microstructures are less equiaxed compared with SIMA and spray-cast alloys even after 5 min of soaking in the semisolid state. It is also shown that the grain growth rate is smaller in spray-cast alloys than in SIMA alloys. The role of coalescence and the effects of alloying elements are also discussed.

Evolution of near-equiaxed microstructure in the semisolid state

Experiments and finite element simulations have been conducted to characterize the temperature distribution in the specimen/die/punch setup and its evolution during field activated sintering. Significant temperature gradients were found in specimen in both radial and axial directions. The punches experience the highest temperatures, while the minimum is on the outer die surface, which is where the temperature is monitored for control purposes. The temperature difference between the specimen and die surface increases with temperature. In order to know the true specimen temperature, a calibration-based correction with respect to the typically used surface temperature is necessary.

Antonios Zavaliangos

Papers

Sintering activation by external electrical field

Processing and Mechanical Properties of Ti3SiC2: II, Effect of Grain Size and Deformation Temperature

Damage Mechanisms around Hardness Indentations in Ti3SiC2

Evolution of near-equiaxed microstructure in the semisolid state

Temperature evolution during field activated sintering