Challenges of Using Elemental Nickel and Titanium Powders for the Fabrication of Monolithic NiTi Parts

Due to its interesting properties and wide applications in different industries, fabrication of monolithic NiTi with cost-effective methods is an important and attractive issue. One of the economic ways to fabricate NiTi is employing elemental nickel and titanium powders. In this study, effects of using elemental powders as a precursor on the microstructure and mechanical properties of HIP-consolidated NiTi samples will be explored. The result of XRD analysis showed that after HIP process an interwoven structure which consists of NiTi2, Ni3Ti and B2-B19′ NiTi evolves. The formation of NiTi2/Ni3Ti intermetallics is thermodynamically favored which affects different aspects of this alloy: (i) it alters martensitic transformation temperatures; (ii) restricts essential properties of this alloy such as PE and SME, (iii) increases hardness, and (iv) yields to premature fracture at small strains during tensile tests

Microstructural and kinetic investigation on the suppression of grain growth in nanocrystalline copper by the dispersion of silicon carbide nanoparticles

In this paper, the thermal stability and grain growth kinetics of nanocrystalline Cu, reinforced with SiC nanoparticles and obtained using a mechanical milling process, were investigated during isothermal annealing. The presence of the nanoparticles in the nanocrystalline copper matrix resulted in a significant decrease in grain growth, the formation of partially textured microstructure and twin boundaries at higher temperatures, and an increase in the volume fraction of recrystallized grains, as estimated by grain orientation spread, in comparison to unreinforced Cu during annealing. The lower volume fraction of recrystallized grains at higher temperatures was attributed to dynamic recovery. Normal grain growth was observed in the annealing range of 400-600 degrees C, and significant abnormal grain growth was observed at higher temperatures. An analysis of the grain growth kinetics in the temperature range of 400-600 degrees C revealed a time exponent of n approximate to 3.6 and activation energy of approximate to 34 kJ mol(-1), based on the parabolic equation. The calculated activation energy for grain growth in the SiC dispersion strengthened Cu was found to be less than that of nanocrystalline Cu. The low activation energy and high thermal stability were attributed to high lattice strain and the retarding effect of nanoparticles by the Zener mechanism. (C) 2017 Elsevier Ltd. All rights reserved.116Nsciescopu

A microstructural and kinetic investigation on suppression of grains growth in nanocrystalline copper through silicon carbide nanoparticle dispersion

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Mechanical, tribological and electrical properties of Cu-CNT composites fabricated by flake powder metallurgy method

Cu-CNT composites were fabricated by a flake powder metallurgy method, and their microhardness, electrical conductivity, frictional and wear properties were investigated. Homogenous distribution of CNTs in fine-grained Cu matrix was obtained using this process. Microhardness increased with the addition of CNT vol% up to 8% to the Cu matrix, while the conductivity decreased to 79.2 IACS %. Results showed that CNTs play a major role in improving wear resistance by forming a CNT-rich film that acts as a solid lubricant layer. In the synthesized composites, Cu-4 vol% CNT composite exhibited the best wear and friction properties. The dominant wear mechanisms for the Cu-CNT composites were plastic deformation, abrasion, and flake formation-spalling. Also, a newly modified correlation was proposed for the theoretical calculation of the friction coefficient of Cu-CNT composites consisting agglomerated CNTs. (C) 2019 Politechnika Wroclawska. Published by Elsevier B.V. All rights reserved.11Nsciescopu

Effect of Composition on Mechanical Properties of Mullite-WC Nano Composites Prepared by Spark Plasma Sintering

Mullite-WC composites were prepared from Mullite and WC powders by spark plasma sintering at 1400 °C for a holding time of 180 s under 30 MPa. Microstructure, strength, and hardness of the mullite-WC composites were studied. The mullite-WC composite containing 5-20 wt% WC reached over 94 % theoretical density. The strength and Vickers hardness of mullite-(10 wt%) WC sintered composite reached maximum values of 298 MPa, and 1589 HV, respectively demonstrating that the introduction of WC significantly enhances the mechanical properties of the mullite matrix

Fabrication of NiTi and NiTi-nano Al2O3 composites by powder metallurgy methods: Comparison of hot isostatic pressing and spark plasma sintering techniques

To escape the detrimental effect of NiTi matrix decomposition on hot-isostatic-pressing (HIP)-processed NiTi-nano alumina composites, which leads to the formation of NiTi2/Ni3Ti intermetallics; in this study, spark plasma sintering (SPS) was utilized to restrict this phenomenon in NiTi and NiTi-6 wt% nano alumina composites. After optimization of the SPS processing conditions, the microstructural aspects of the SPS and HIP-consolidated samples were compared. According to X-ray diffraction analysis, the SPS-processed composites contained more B2-NiTi phase than the HIP-processed samples did. This is because of lower NiTi matrix decomposition due to formation of NiTi2/Ni3Ti phases. Scanning electron microscopy studies revealed that at the interface of alumina and the NiTi matrix, Ti-rich phases had evolved, while in the grain interiors, matrix decomposition mostly led to the formation of Ni-rich intermetallics. High-resolution transmission electron microscopy investigations confirmed an increased Ni/Ti ratio in the austenitic NiTi matrix in the vicinity of the Ti-rich phases, and the formation of martensitic NiTi near the Ni-rich phases. The results from differential scanning calorimetry indicated that the latent heat of martensitic transformation in the SPS composite is higher than that of HIP-processed composite samples due to lower matrix decomposition and higher NiTi phase, which can participate in martensitic transformation.11Nsciescopu

Compressive behavior of NiTi-based composites reinforced with alumina nanoparticles

NiTi alloys are extensively used in various industrial applications. However, to enhance the performance of this alloy in applications, its mechanical properties should be improved. In previous research, the effect of addition of ceramic microparticles on the mechanical properties of a NiTi matrix has been explored. In this study, to improve the mechanical properties of NiTi, small fractions of alumina nanoparticles were used as reinforcement. The compressive stress-strain curves of the NiTi and NiTi-2Al(2)O(3) samples demonstrated well-known four-stage behavior with double yielding. Moreover, it was found that the addition of 2 wt% alumina nanoparticles enhanced the elastic modulus, shortened the first plateau region, and reduced the fracture strain, compared to the unreinforced NiTi sample. Increasing alumina nanoparticle contents to 6 wt% completely changed the mechanical behavior of this composite: considerable elasticity, followed by a short plateau region, and fracture. The high resolution transmission electron microscopy images showed that, a high fraction of dislocations in the vicinity of nanoparticles can be generated in NiTi, which seems to lock the matrix in the NiTi-6Al(2)O(3) sample. Fracture surface analysis using scanning electron microscopy showed dimple features in the NiTi and NiTi-2Al2O3 samples and a smooth and brittle appearance in the NiTi-6Al2O3 sample. (C) 2016 Elsevier B.V. All rights reserved.1153sciescopu

Effect of High-Pressure Torsion on the Microstructure and Wear Behavior of NiTi Alloy

The wear property of NiTi is one of the most important properties of this alloy. In the current study, the effect of high-pressure torsion (HPT) process on the wear properties of an austenitic NiTi shape memory alloy is investigated. Full density NiTi samples with a composition of Ti-56 wt% Ni are fabricated using hot isostatic pressing (HIP), followed by the HPT process at room temperature, with an applied pressure of 6 GPa for 10 turns. The microstructural analysis reveals that the HIP-processed samples with a B2-NiTi phase evolve into significant grain refinement after HPT process and an interwoven B2-B19' nanocrystalline/amorphous structure formed, leading to increased hardness in these samples. The results of the wear tests using a ball-on-disc configuration at room temperature demonstrate that the wear performance of the samples is improved after the HPT process. This is due to greater hardness and better pseudo-elasticity in the HPT-processed samples.1165sciescopuskc