Abstract
Ni/Al-TiB2 composite powders were deposited on the surface of 316L stainless-steel substrates by cold spraying at gas temperatures of 250 and 450 °C, respectively. Then, the as-sprayed coatings were annealed at 650 °C for 10, 20, and 30 h. The experimental results showed that the average porosity of as-sprayed coating dropped from about 0.68% to 0.054% as the cold spraying gas temperature increased. The contents of Ni, Al, and TiB2 in the as-sprayed coatings were different from that of the Ni/Al-TiB2 composite powders. The main phase compositions of the as-sprayed Ni/Al-TiB2 coatings were the same as those of composite powder, consisting only of pure Ni, Al, and TiB2 phases. TiB2 as a reinforced particle in the as-sprayed coating could obviously increase the microhardness of the coatings. NiAl3 and Ni2Al3 intermetallic compounds were synthesized in situ in all of the annealed coatings, and the average contents of NiAl3 and Ni2Al3 intermetallic compounds increased as the cold spraying gas temperature increased. The distribution of TiB2 particle was changed as the annealing times increased, which changed from more comparative uniform distribution to accumulation. The average porosity of the annealed coatings increased as the annealing time increased. The microhardness of Ni/Al-TiB2 coatings annealed at 650 °C for 10 h was increased remarkably due to the reinforcement role of TiB2 particles and NiAl3 and Ni2Al3 intermetallic compounds.
Highlights
NiAl intermetallic compounds are regarded as high-temperature structure materials, owing to their outstanding properties, such as high creep strength, high melting point (1676 ◦ C), high oxidation and corrosion resistance, low density (5.9 g/cm3 ), and high thermal conductivity (76 W/mK) [1,2,3,4,5].the inherent shortcomings of NiAl intermetallic compounds, such as poor fracture toughness at room temperature, low ductility at ambient temperatures, and inadequate strength at elevated temperatures, limit their high temperature structural applications [6]
Compared with the surface morphology of the coating sprayed at a 250 ◦ C gas temperature, the hole rate of the surface of the coating was 1.54%, which was less than that of the coating sprayed at a 250 ◦ C gas temperature (4.11%)
This illustrates that the larger deformation of Al splats could fill the holes due to the higher critical velocity during spraying at 450 ◦ C; the coating sprayed at 450 ◦ C had a denser microstructure with fewer holes
Summary
NiAl intermetallic compounds are regarded as high-temperature structure materials, owing to their outstanding properties, such as high creep strength, high melting point (1676 ◦ C), high oxidation and corrosion resistance, low density (5.9 g/cm3 ), and high thermal conductivity (76 W/mK) [1,2,3,4,5].the inherent shortcomings of NiAl intermetallic compounds, such as poor fracture toughness at room temperature, low ductility at ambient temperatures, and inadequate strength at elevated temperatures, limit their high temperature structural applications [6]. NiAl intermetallic compounds are regarded as high-temperature structure materials, owing to their outstanding properties, such as high creep strength, high melting point (1676 ◦ C), high oxidation and corrosion resistance, low density (5.9 g/cm3 ), and high thermal conductivity (76 W/mK) [1,2,3,4,5]. In order to overcome these drawbacks, the NiAl intermetallic composites using reinforcing ceramic particles (e.g., TiC, WC, TiO2 , Al2 O3 , ZrO2 , TiB2 , and CrB2 ) as an additive have been prepared to improve their mechanical properties and high-temperature strength [7,8,9,10,11,12,13,14,15,16,17]. Hawk et al [16] and Bhaumik et al [17] reported that the hardness of NiAl-TiB2 composites increased with the increasing volume fraction of TiB2
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