Microstructure Evolution And High-temperature Oxidation Behavior Of Ni-Co-Cr-Al Alloy Sheet Fabricated By EB-PVD

The microstructure evolution and mechanical properties of Ni20.3Cr4.6Co2.1Al alloy sheet about 0.3mm thickness deposited by electron beam physical vapor deposition (EB-PVD) technology were systematically investigated by means of X-ray diffactometry (XRD), sanning electron microscopy (SEM), atom force microscopy (AFM) and transmission electron microscopy (TEM) as well as mechanical test method. In addition, oxidation behaviors of as-deposited and annealed Ni-Cr-Al alloys at 800°C, 900°C and 1000°C were investigated.The results show that the average sizes of as-deposited Ni20.3Cr4.6Co2.1Al alloy sheet composed only ofγphase was about 500nm.The substructures were (111) twins mainly without dislocations.For the as-deposited alloy sheet, the nano-hardness, elastic modulus E, room temperature tensile strengthσb and elongationδwas 5.57GPa, 221.33GPa, 1033MPa and 0.6%, respectively. Obviously, it was brittle fracture. Andσb tensile strength and elongation of as-deposited alloy sheet at 800℃was 107 MPa and 1.02%, respectively.The heat-treated Ni-11.5Cr-4.5Co-0.5Al sheet by EB-PVD at 600°C for different time consists ofγ-Ni-based single-phase with twins sub-structure and without dislocation. The grain size grows up with the extension of holding time. And the strain stripes disappear in TEM Micrograph. XRD analysis shows there is (220) crystal face of preferred orientation. When the heat-treatment temperature reaches 800°C, the grain grew up significantly, the sub-structure for the twins and dislocations. And the second phase precipitation was found when the sheet annealed at 1000℃.The Ni20.3Cr4.6Co2.1Al alloy sheet by EB-PVD annealed below 600°C, the fracture strength slightly reduced and the elongation slightly increased with temperature increasing. Annealed at 600℃for 16 h, the fracture strengthσb of sheet reduced to 780 MPa, and elongationδup to only 4%. Annealed at 800℃for 16 h, the room temperature tensile strengthσb and elongationδof alloy sheet increased to 825 MPa and 14%, respectively. The fracture belongs to ductile fracture. Subsequently the fracture strength slightly reduced and the elongation slightly increased with temperature increasing. The alloy sheet annealed at different temperature for 16 h, the 800°C tensile strength and elongationδof sheet annealed at 800℃for 16 reached 63 MPa and 246%, respectively. But annealed at 900℃for 16 h, the 800℃tensile strengthσb has begun to drop 24 MPa, and elongationδhas grown to 336%. And they were ductile fracture.The oxidation dynamics was different for the microcrystalline Ni20.3Cr4.6Co2.1Al sheet and conventional nickel-base alloy. Exposed at 800℃or 1000℃in air, the oxidation kinetics of microcrystalline Ni-11.5Cr-4.5Co-0.5Al alloy sheet followed a parabolic power law at initial oxidation stage and cubical power law for long oxidation terms. And the oxidation kinetics follows a cubical power law at initial oxidation stage and fourth power law for long oxidation terms at 900℃in air.As-deposited Ni20.3Cr4.6Co2.1Al alloy sheet exposed at 800℃in air, the oxidation products are mainly NiO and a small number of Cr2O3 at early stages, mainly NiO, NiCr₂O₄ and Cr2O3 after prolonged oxidation. Exposed at 900℃in air, the oxidation products are NiO, Cr2O3 and NiCr₂O₄ at initial stage. And the scale grain boundary formed Al-regional at the initial stage. After prolonged oxidation, the oxidation product is mainly NiO. The oxidation products are NiO and Cr2O3 at early stages, mainly by NiO, Cr2O3 and secondarily of NiCr₂O₄ composite layer after prolonged oxidation at 1000℃in air.After annealed of microcrystalline Ni20.3Cr4.6Co2.1Al sheet, the oxidation kinetic curves of alloy sheet at 800℃in air follow a fourth power rate law at initial oxidation stage, then very slow oxidation, and the final stage of the alloy sheet is almost not oxidized. The oxidation products are mainly NiO, Cr2O3 and NiCr₂O₄. The oxidation kinetic curves of alloy sheet at 900℃in air follow a cubic power rate law. And the oxidation products are mainly NiO, Cr2O3 and NiCr₂O₄. Exposed at 900℃in air of annealed sheet, the oxidation product is a single needle Al2O3, and Oxide layer peeling occurred.The oxidation kinetics of microcrystalline Ni20.3Cr4.6Co2.1Al failed to obey the parabolic law at 800°C,900°C and 1000°C in air. It was suggested that the oxide scale turned in to microcrystalline state since the nuclear probability of oxide was extremely increased for this kind of microcrystalline alloy. The short circuit diffusion through oxide scale was dominant. An expression, x2=At1-n has been derived for the oxidation kinetics of microcrystalline alloy based upon the model of short circuit diffusion.