Pulse Plasma Sintering of NiAl-Al2O3 Composite Powder Produced by Mechanical Alloying with Contribution of Nanometric Al2O3 Powder

NiAl-Al2O3 composites, fabricated from the prepared composite powders by mechanical alloying and then consolidated by pulse plasma sintering, were presented. The use of nanometric alumina powder for reinforcement of a synthetized intermetallic matrix was the innovative concept of this work. Moreover, this is the first reported attempt to use the Pulse Plasma Sintering (PPS) method to consolidate composite powder with the contribution of nanometric alumina powder. The composite powders consisting of the intermetallic phase NiAl and Al2O3 were prepared by mechanical alloying from powder mixtures containing Ni-50at.%Al with the contribution of 10 wt.% or 20 wt.% nanometric aluminum oxide. A nanocrystalline NiAl matrix was formed, with uniformly distributed Al2O3 inclusions as reinforcement. The PPS method successfully consolidated NiAl-Al2O3 composite powders with limited grain growth in the NiAl matrix. The appropriate sintering temperature for composite powder was selected based on analysis of the grain growth and hardness of Al2O3 subjected to PPS consolidation at various temperatures. As a result of these tests, sintering of the NiAl-Al2O3 powders was carried out at temperatures of 1200 °C, 1300 °C, and 1400 °C. The microstructure and properties of the initial powders, composite powders, and consolidated bulk composite materials were characterized by SEM, EDS, XRD, density, and hardness measurements. The hardness of the ultrafine-grained NiAl-Al2O3 composites obtained via PPS depends on the Al2O3 content in the composite, as well as the sintering temperature applied. The highest values of the hardness of the composites were obtained after sintering at the lowest temperature (1200 °C), reaching 7.2 ± 0.29 GPa and 8.4 ± 0.07 GPa for 10 wt.% Al2O3 and 20 wt.% Al2O3, respectively, and exceeding the hardness values reported in the literature. From a technological point of view, the possibility to use sintering temperatures as low as 1200 °C is crucial for the production of fully dense, ultrafine-grained composites with high hardness.

Mechanism of structural formation in the combustion wave of the Ni-Al system with strengthening additives

In the work, microstructures formed in the combustion wave of the Ni-Al system with hardening additives of high-temperature ceramic particles consisting of titanium diboride and corundum were studied. Microstructures and shapes vary depending on the content of ceramic additives in the NiAl matrix. Particles of TiB2 take the most diverse elementary forms, such as bars, plates, herringbones, regular cubic structures and cuboids. These results outline a real-time strategy of self-assembly processes to create diversified microstructures. Some grains of titanium diboride 2-5 m in size are embedded in corundum clusters, and a small number of TiB2 particles are dispersed in the NiAl matrix. It is assumed that the higher the content of reinforcing additives, the more uniform the distribution of the ceramic skeleton will be present in the NiAl matrix.

Combustion synthesis of NiAl/TiC composite from mechanically activated elemental powders

The aim of this study was the synthesis of a homogenous distributed NiAl/TiC composite by combustion synthesis of elemental powders. The effect of the mechanical activation process was evaluated. The phase characterization and evaluation of the microstructure during milling and combustion synthesis were carried on by XRD and SEM-EDS and TEM-EDS, respectively. The XRD result of as-mixed powder compact after combustion synthesis showed that in addition to TiC0.75 and NiAl, some undesired phases like Ni3Al and Ni2Al­3 formed. While after combustion synthesis of 3h mechanically activated powder compact, only AlNi and TiC formed. The microstructural evaluations by SEM-EDS and TEM-EDS analysis confirmed the synthesis of truly homogenized NiAl/TiC composite. The mechanism of NiAl/TiC formation consists of dissolution of nickel, titanium, and graphite in molten aluminum and precipitation of TiC primarily and then the formation of NiAl. The value of microhardness of synthesized NiAl/TiC was 1070±12 HV. The main reason for this result is a uniform distribution of spherical TiC reinforcement in the NiAl matrix. Therefore, the mechanical activation process facilitated the reactions and improved the mechanical properties of the final composite.

Microstructure and Tribocorrosion Properties of Ni-Based Composite Coatings in Artificial Seawater

NiAl matrix composite coatings were prepared using atmospheric plasma spraying (APS). The mechanical and tribocorrosion properties of the NiAl matrix composite coatings, incorporated with Cr2O3 and Mo, were investigated, and the synergistic effect between corrosion and wear was studied in detail. The microhardness of the composite coating improved from 195.1 to 362.2 HV through the addition of Cr2O3 and Mo. Meanwhile, the Cr2O3 and Mo phases were distributed uniformly in the composite coatings. The X-ray diffraction (XRD) peaks of Ni-based solid solution slightly shifted to the right after adding the Mo. This was probably due to the solid solution of Mo into the matrix. The NiAl–Cr2O3–Mo composite coating had the lowest corrosion current density, wear rate and friction coefficient of 9.487 × 10−6 A/cm2, 3.63 × 10−6 mm3/Nm, and 0.18, in all composite coatings as well as showing excellent tribocorrosion properties.