The mechanical behavior and the change of retained austenite of nanocrystalline Fe-Ni alloy have been investigated by considering the effect of various Ni addition amount. The nanocrystalline Fe-Ni alloy samples were rapidly fabricated by spark plasma sintering (SPS). The SPS is a well-known effective sintering process with an extremely short densification time not only to reach a theoretical density value but also to prevent a grain growth, which could result in a nanocrystalline structures. The effect of Ni addition on the compressive stress-strain behavior was analyzed. The variation of the volume fraction of retained austenite due to deformation was quantitatively measured by means of x-ray diffraction and microscope analyses. The strain-induced martensite transformation was observed in Fe-Ni alloy. The different amount of Ni influenced the rate of the strain-induced martensite transformation kinetics and resulted in the change of the work hardening during the compressive deformation.
New graphite tools were designed and produced to fabricate a semi-finished product from which nine cutting inserts were obtained in one spark plasma sintering process. As a result, WC-5Co cemented carbides were spark plasma sintered and the effect of various sintering parameters such as compacting pressure, heating rate and holding time on the main mechanical properties were investigated. It was shown that WC-5Co cemented carbides spark plasma sintered at 1200°C, 80 MPa, 400°C/min, for 5 min are characterized by the best relation of hardness (1861 ±10 HV30) and fracture toughness (9.30 MPa·m1/2). The microstructure of these materials besides the WC ceramic phase and Co binder phase consists of a synthesized Co3W3C complex phase. Comparison with a commercial WC-6Co cutting insert fabricated by conventional powder metallurgy techniques shows that spark plasma sintering is a very effective technique to produce materials characterized by improved mechanical properties.
Fe-40wt% TiB2 nanocomposites were fabricated by mechanical activation and spark-plasma sintering of a powder mixture of iron boride (FeB) and titanium hydride (TiH2). The powder mixture of (FeB, TiH2) was prepared by high-energy ball milling in a planetary ball mill at 700 rpm for 3 h followed by spark-plasma sintering (SPS) at various conditions. Analysis of the change in relative sintered density and densification rate during sintering showed that a self-propagating high-temperature synthesis reaction occurs to form TiB2 from FeB and Ti. A sintered body with relative density higher than 98% was obtained after sintering at 1150°C for 5 and 15 min. The microstructural observation of sintered compacts with the use of FE-SEM and TEM revealed that ultrafine particulates with approximately 5 nm were evenly distributed in an Fe-matrix. A hardness value of 83 HRC was obtained, which is equivalent to that of conventional WC-20 Co systems.
In this study, precisely controlled large scale gas atomization process was applied to produce spherical and uniform shaped high entropy alloy powder. The gas atomization process was carried out to fabricate CoCrFeNiMn alloy, which was studied for high ductility and mechanical properties at low temperatures. It was confirmed that the mass scale, single phase, equiatomic, and high purity spherical high entropy alloy powder was produced by gas atomization process. The powder was sintered by spark plasma sintering process with various sintering conditions, and mechanical properties were characterized. Through this research, we have developed a mass production process of high quality and spherical high entropy alloy powder, and it is expected to expand applications of this high entropy alloy into fields such as powder injection molding and 3D printing for complex shaped components.
The microstructure and corrosion properties of spark plasma sintered yttria dispersed and yttria free duplex and ferritic stainless samples were studied. Spark plasma sintering (SPS) was carried out at 1000°C by applying 50 MPa pressure with holding time of 5 minutes. Linear sweep voltammetry (LSV) tests were employed to evaluate pitting corrosion resistance of the samples. Corrosion studies were carried out in 0.5, 1 and 2 M concentration of NaCl and H2SO4 solutions at different quiet time of 2, 4, 6, 8 and 10 seconds. Yttria dispersed stainless steel samples show more resistance to corrosion than yttria free stainless steel samples. Pitting potential decreases with increase in reaction time from 2 to 10 seconds. Similarly, as concentration of NaCl and H2SO4 increases from 0.5 M to 2 M the corrosion resistance decrements due to the availability of more Cl¯ and SO4 2¯ ions at higher concentration.
The β-phase Titanium (β-Ti) alloys have been under the spotlight in the recent past for their use as biomedical prosthetic materials owing to their excellent properties such as low elastic modulus, high corrosion resistance and tensile strength. Recently, Niobium (Nb) has gained a lot of attention as a β-phase stabilizing element in Ti alloys to replace Vanadium (V) due to its excellent solubility in Ti, low elastic modulus and biocompatibility. In this work, low cost Ti-20Nb binary alloy has been fabricated via powder metallurgy procedures. The blended powder mixtures of Ti and Nb were sintered at 900°C for 20 mins by the Spark Plasma Sintering (SPS) with an applied uniaxial pressure of 40 MPa. The heating rate was fixed at 50°C/min. The sintered alloy was subject to heat treatments at 1200°C in vacuum condition for various time durations. The characterizations of microstructure obtained during this process were done using FE-SEM, EDS and XRD. By increasing heat treatment time, as understood, the volume of residual Nb particles was decreased resulting in accelerated diffusion of Nb into Ti. Micro hardness of the alloy increased from 340 to 355 HV with the increase in β phase content from 30 to 45%. The resultant alloys had relatively high densities and homogenized microstructures of dispersed lamellar β grains in α matrix.
The dispersion of nanoparticles in the host matrix is a novel approach to enhance the thermoelectric performance. In this work, we incorporate the TiC (x = 0, 1 and 2 wt.%) nanoparticles into a p-type Bi0.5Sb1.5Te3 matrix, and their effects on microstructure and thermoelectric properties were systematically investigated. The existence of TiC contents in a base matrix was confirmed by energy dispersive X-ray spectroscopy analysis. The grain size decreases with increasing the addition of TiC content due to grain boundary hardening where the dispersed nanoparticles acted as pinning points in the entire matrix. The electrical conductivity significantly decreased and the Seebeck coefficient was slightly enhanced, which attributes to the decrease in carrier concentration by the addition of TiC content. Meanwhile, the lowest thermal conductivity of 0.97 W/mK for the 2 wt.% TiC nanocomposite sample, which is ~16% lower than 0 wt.% TiC sample. The maximum figure of merit of 0.90 was obtained at 350 K for the 0 wt.% TiC sample due to high electrical conductivity. Moreover, the Vickers hardness was improved with increase the addition of TiC contents.