Mechanical Alloying Spark Plasma Sintering Research Articles

Influence of mechanically activated annealing on phase evolution in Al0.3CoCrFeNi high-entropy alloy

In the present work, the concept of mechanically activated annealing (MAA) has been applied to produce nanocrystalline Al0.3CoCrFeNi high-entropy alloys (HEAs) with reduced contamination levels. Phase evolution during conventional mechanical alloying (MA), MAA and subsequent consolidation by spark plasma sintering (SPS) have been studied in detail. Complete alloying is obtained after 15 h of MA, while milling time of 5 h and annealing at 1100 °C for 1 h have been used to achieve alloy formation during MAA. Both the MA–SPS and MAA–SPS routes have shown major FCC phase. The contamination of WC observed during MA was successfully eliminated during MAA, while the volume fraction of Cr7C3 reduced from 20% during MA–SPS to 10% after MAA–SPS. This method can serve as an effective way to produce nanostructured HEAs with minimum contamination.

Influence of lanthanum on enhancement of mechanical and electrical properties of Cu-Al2O3 composites

Two kinds of Cu-Al2O3 composites (with and without La) were prepared via mechanical alloying-spark plasma sintering (MA-SPS) method. Microstructure, mechanical properties and electrical resistivity were investigated systematically using metallography, scanning electron microscopy, transmission electron microscopy, mechanical and electrical properties testing. The results indicate that an appropriate amount of La can homogenize the distribution of Al2O3. As such, yield strength, ultimate tensile strength and elongation of Cu-Al2O3-La are greatly increased. Some semi-coherent interface between Cu and Al2O3 is found, which means a low interface energy. The grain shape of Cu changes to irregular band with the addition of La. This change results in a density decrease of grain boundary and reduces electrical resistance. Lanthanum may exist in the form of La2O3.

Phase evolution and stability of nanocrystalline CoCrFeNi and CoCrFeMnNi high entropy alloys

High entropy alloys (HEAs) have emerged as promising class of materials having equiatomic or near equiatomic multicomponent configurations. Nanostructured HEAs have added a new facet to the development of these alloys, showing remarkable strength and functional properties. The present work examined the phase evolution and thermal stability of nanocrystalline CoCrFeNi and CoCrFeMnNi HEAs prepared through mechanical alloying (MA) followed by spark plasma sintering (SPS). After MA, both the alloys showed single phase FCC structure with minor fractions of tungsten carbide arising due to contamination from milling media. After SPS, the major phase remained as FCC in both the alloys along with Cr7C3 evolution. Phase stability of CoCrFeNi and CoCrFeMnNi HEAs, (MA powders and SPS pellets) were investigated in the temperature range 1073–1373 K up to 96 h. Formation of Cr7C3, concomitant with Cr-depletion in FCC matrix, was observed on heat treatment of MA powders. SPS alloys retained their mixture of FCC + Cr7C3 on thermal exposure with minimal change in the fraction of carbides. The presence of single phase field in their respective phase diagrams, similar atomic sizes of constituents and non-equilibrium nature of MA combined to lend a highly stable FCC phase in the nanocrystalline HEAs studied.

Structure and properties of ultrafine-grained CoCrFeMnNi high-entropy alloys produced by mechanical alloying and spark plasma sintering

CoCrFeMnNi high-entropy alloy (HEA) materials were fabricated using mechanical alloying (MA) and spark plasma sintering (SPS). The MA time, SPS temperature, and contaminations strongly affected the final microstructure and mechanical properties. Nanocrystal face-centered cubic (FCC) solid solution was made during MA, and the FCC phase maintained as the matrix after SPS at 900 °C and 1100 °C. However, Cr carbides were transformed near the surface due to the carbon contamination. When MA time increased, phase stability of the FCC phase was improved, and the contaminant (ZrO2) from the MA balls was also increased. Ultrafine-grained microstructure was obtained at 60 min MA and 900 °C SPS. On the other hand, the higher SPS temperature and lower levels of contamination were required to achieve tensile ductility. Irregularly distributed ZrO2 particles developed bimodal microstructures.

Microstructure and sintering mechanism of C/Cu composites by mechanical alloying/spark plasma sintering

This paper investigated the microstructure transformation behavior of copper–graphite composites prepared via mechanical alloying/ spark plasma sintering (MA-SPS). The results revealed that the composite powder existed in the forms of nano/submicron composite particles and pure nanoparticles. The composite particles were totally bonded together via the MA-SPS process and showed a dense and fine microstructure. The grain shape was different from that of the milled powder particles. The microhardness values of Cu–9.7 at.% C, Cu–18.1 at.% C, and Cu–25.3 at.% C composites were enhanced greatly compared with pure copper. The enhancement is due to the combination of the MA induced by high-energy ball milling from MA and discharge activation induced by pulsed current discharge from SPS in the MA-SPS fabrication process.

Improved Thermoelectric Properties of p-Type Bismuth Antimony-Based Alloys Prepared by Spark Plasma Sintering

Bi85Sb15−xPbx (x = 0, 0.5, 1, 2, 3) alloys have been prepared by the mechanical alloying–spark plasma sintering (MA-SPS) method. X-ray diffraction and scanning electron microscopy were used to characterize the microstructure of the alloys. The effect of Pb content on the thermoelectric properties was investigated in the temperature range 77–300 K. The results showed that the electrical transport properties of the Bi–Sb alloys changed from n-type to p-type with substitution of Sb by Pb. The maximum power factor reached 1.6 × 10−3 W/mK2 at 190 K, a significant improvement on values reported elsewhere. This study demonstrated that high-performance p-type thermoelectric Bi–Sb materials can be obtained by spark plasma sintering.

Correlation between processing conditions, microstructure and charge transport in half-Heusler alloys

Abstract Five bulk samples of n-type Zr 0.25 Hf 0.75 NiSn 0.975 Sb 0.025 half-Heusler (HH) alloy were fabricated by reacting elemental powders via (1) high temperature solid state (SS) reaction and (2) mechanical alloying (MA), followed by densification using spark plasma sintering (SPS) and/or hot pressing (HP). A portion of the sample obtained by SS reaction was mechanically alloyed before consolidation by hot pressing (SS–MA–HP). X-ray powder diffraction and transmission electron microscopy studies revealed that all SS specimen (SS–SPS, SS–HP, SS–MA–HP) are single phase HH alloys, whereas the MA sample (MA–SPS) contains metallic nanoprecipitates. Electronic and thermal transport measurements showed that the embedded nanoprecipitates induce a drastic increase in the carrier concentration ( n ), a large decrease in the Seebeck coefficient (S) and a marginal decrease in the lattice thermal conductivity ( κ l ) of the MA–SPS sample leading to lower ZT values when compared to the SS–HP samples. Constant values of S are observed for the SS series regardless of the processing method. However, a strong dependence of the carrier mobility ( μ ), electrical conductivity ( σ ) and κ l on the processing and consolidation method is observed. For instance, mechanical alloying introduces additional structural defects which enhance electron and phonon scattering leading to moderately low values of μ and large reduction in κ l . This results in a net 20% enhancement in the figure of merit (ZT=0.6 at 775 K). HH specimen of the same nominal composition with higher ZT is anticipated from a combination of SS reaction, MA and SPS (SS–MA–SPS).

Comparison of thermoelectric performance of AgPbxSbTe20 (x = 20–22.5) polycrystals fabricated by different methods

Both vacuum melting and mechanical alloying (MA) were applied to the fabrication of AgPbxSbTe20 (x=20, 21 and 22.5) as a promising thermoelectric material to investigate the processing influence on its thermoelectric properties. The AgPbxSbTe20 compound powders were prepared by pulverizing the melted ingots or directly from MA, which were then consolidated to fine-grained bulk materials by spark plasma sintering (SPS). A high ZT value of 1.25 at 725K was observed in the AgPb20SbTe20 samples fabricated by both methods. Thermoelectric properties of all AgPbxSbTe20 compounds synthesized by vacuum melting method are nearly the same as that of those fabricated by MA combined with SPS in the whole measured temperature range. Considering the complexity of the melting method but with the same results as the MA–SPS method, the present work revealed that the MA–SPS process is more suitable for the fabrication of PbTe-based thermoelectric materials.

Low-temperature fabrication of high purity Ti 3SiC 2

In this study, we fabricated high purity Ti 3SiC 2 ceramic by mechanical alloying (MA) and spark plasma sintering (SPS), and investigated the effect of trace amount of Al on these processes. Our results show that addition of proper amount of Al significantly increases the purity of Ti 3SiC 2 in the MA and subsequent SPS products, and remarkably reduces the sintering temperature for Ti 3SiC 2. Ti 3SiC 2 sintered compact with a purity of 96.5 wt% was obtained by 10 h of MA and subsequent SPS from a starting mixture composed of n(Ti): n(Si): n(Al): n(c) = 3:1:0.2:2 at 850 °C. At 1100 °C, Ti 3SiC 2 with a purity of 99.3 wt% and a relative density of 98.9% was obtained.

The effects of La on thermoelectric properties of La xCo 4Sb 12 prepared by MA–SPS

Thermoelectric materials formulated with La x Co 4Sb 12 were prepared via mechanical alloying (MA) and spark plasma sintering (SPS). The regularities of their thermoelectric properties varying with La content at 100–500 °C are investigated. The experimental results show that the La doping helps achieve the P or N type semiconductor materials and improves their thermoelectric properties. The ZT value increases with La content ( x) from 0.1 to 0.9, but decreases when x changes from 0.9 to 2.0. The maximum ZT value is 0.3425 when x = 0.9 at 500 °C and the optimal doped La content is about 0.9–1.0 under the experimental conditions in this study.

Cyclic oxidation behavior of TiAl composites incorporated with TiB2 dispersoids

TiAl alloys incorporated in (0,3,5,10) wt.% TiB2 dispersoids were manufactured via mechanical alloyingspark plasma sintering (MA-SPS), and their cyclic oxidation characteristics were studied at 800, 900 and 1000°C in air. The cyclic oxidation resistance of the prepared TiAl-TiB2 composites effectively increased with increases in TiB2 content. The oxide scale formed consisted of an outer TiO2 layer, an intermediate Al2O3 layer, and an inner (Al2O3+TiO2) mixed layer. The scale adherence was relatively good, and much thinner oxide scales, when compared to TiB2-free TiAl alloys, were formed on the prepared composites. The incorporated TiB2 dispersoids oxidized to TiO2 and B2O3 which evaporated during oxidation.