Effect of Zr Addition on Microstructure, Hardness and Oxidation Behavior of Arc-Melted and Spark Plasma Sintered Multiph
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UCTION
IN recent years, there has been a significant thrust on the development of new types of refractory intermetallic alloys and their composites for applications at temperatures exceeding 1100 C, that is, beyond the upper limit of the temperature for applications of both Ni and Co-based superalloys.[1–10] The primary objective of this effort is to raise the operating temperature of jet engine materials, so that the requirement of air-cooling can be avoided and thereby the efficiency can be enhanced significantly. Since the late 1990s, the Mo-rich Mo-Si-B alloys containing homogeneous distribution of brittle Mo3Si with cubic A15 structure and tetragonal structured Mo5SiB2 (T2) phases along with the ductile bcc
N.K. KUMAR, J. DAS, and R. MITRA are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal 721302, India. Contact e-mail: [email protected] Manuscript submitted 26 August, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
a-Moss (Mo-rich solid solution) phase have received world-wide attention due to the possibility of achieving desirable combinations of oxidation resistance, creep resistance, high strength, and phase stability at high temperatures.[11–15] The presence of ductile a-Mo solid solution phase contributes to raising the fracture toughness of the Mo-Si-B alloys up to the range of 8–15 MPa m1/2,[16–18] whereas the harder Si-containing intermetallic phases such as Mo3Si and Mo5SiB2 aid in improving high-temperature strength retention as well as in forming a borosilicate scale for protection against oxidation. A change in composition from Mo-6Si-5B to Mo-13Si-12B leads to reduction of the a-Mo volume fraction in the microstructure by half from 70 to 35 pct, which in turn leads to decrease in fracture toughness from 13 to 6 MPa m1/2 with accompanying increase in microhardness from 500 to 1150 Hv.[11] It has been reported that both yield and flexural strength (from 671 to 1137 MPa) are increased significantly through mechanical alloying, which causes the refinement of microstructure and strengthening of the a-Moss phase.[12] The room temperature compressive strengths have been found as 2.64, 2.48, and 1.77 GPa
for powder metallurgy-processed Mo-Si-B alloy + La2O3 composites with Si/B ratios of 2.4, 1.4, and 0.7, respectively.[16] Presence of La2O3 has been found to retard grain growth during processing, and thereby aid in strengthening. For high-temperature creep resistance, directionally solidified, near eutectic Mo-Si-B alloys have been found to be very promising.[19] In the earlier studies, Mo-14Si-10B alloy processed by reactive hot-pressing has shown hardness and fracture toughness of 8.4 ± 0.3 GPa and 5.2 ± 1.0 MPa m1/2, whereas 7.4 at. pct Al addition has led to marginal decrease in hardness with increase in indentation fracture toughness to 8.7 ± 1.7 MPa m1/2.[18] However, addition of Al has been found to lower the high-temperature oxidation resistance.[20] In contrast, addition of Fe or Zr to the Mo-Si-B alloys is
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