Cyclic oxidation response of multiphase niobium-based alloys

The nonisothermal and isothermal oxidation behavior of arc-melted, hypereutectic ternary alloys have been investigated vis-a`-vis that of Nb and Nb-10Si alloy. The nonisothermal studies carried out using simultaneous thermogravimetric and differential thermal analysis in the temperature range of 50°C to 1300°C using heating rates of 5°C, 15°C/min, 25°C/min, and 40°C/min have shown oxidation initiation at lower temperatures for slower heating rates and higher Mo content. Isothermal oxidation experiments have been carried out in dry air at 800°C, 1000°C, and 1150°C for 24 hours and the kinetics have been examined by the power-law equation. The studies show that the ternary Nb-Si-Mo alloys undergo less mass gain compared to the Nb and the Nb-10Si alloy. The hypo-or hypereutectic character of the alloys controls the oxidation behavior at 800°C, while the effect of Mo content is predominant at temperatures of 1000°C to 1150°C. X-ray diffraction and scanning electron microscopy (SEM) accompanied by energy dispersive X-ray analysis have shown that the oxide scales of all the alloys are made of Nb 2 O 5 and SiO 2 . The beneficial effect of alloying with Mo is attributed to the improved sinterability of the oxide scale and increase in the activity of Si, which assists in the formation of a stable and continuous layer of SiO 2 at the alloy-oxide interface.

Section: Introductionmentioning

confidence: 99%

“…The SiO 2 layer at the metal-oxide interface is shown with a pair of arrows.alloys, reports the presence of Nb 2 O 5 in the oxide scales [16][17][18]22,24,[30][31][32][33][34]. Thus the formation of Nb 2 O 5 in the oxide scale is kinetically driven.…”

mentioning

confidence: 99%

Nonisothermal and Isothermal Oxidation Behavior of Nb-Si-Mo Alloys

Chattopadhyay

Mitra

Ray

2008

“…Chan [14] determined the oxidation resistance of multiphase alloys containing both laves phases and silicides to be dependent on the volume fractions of solid solution, laves phase, and silicides in the alloy. Higher concentrations of chromium in the solid solution phase as well as higher volume fractions of both silicides and laves phases resulted in improved oxidation resistance.…”

Section: Introductionmentioning

confidence: 99%

Oxidation Behavior of Nb-20Mo-15Si-25Cr and Nb-20Mo-15Si-25Cr-5B Alloys

Portillio

Varma

2011

Nb-20Mo-15Si-25Cr (25Cr alloy) and Nb-20Mo-15Si-25Cr-5B (25Cr/5B alloy) alloys have been subjected to oxidation in air for 24 hours from 973 K to 1673 K (700°C to 1400°C). Even though B additions do not improve oxidation resistance at temperatures higher than 1473 K (1200°C), the lower temperature oxidation resistance is superior with B by influencing the microstructure. Porous oxide scale development at lower temperatures has been attributed to the dominant growth of Nb 2 O 5 and the vaporization of MoO 3 . An intermediate oxidation layer is developed between the scale and the metal for the 25Cr/5B alloy at temperatures above 1173 K (900°C). Scale densification at elevated temperatures results in higher stress development as a result of the mismatch of coefficients of thermal expansion, ultimately resulting in oxide spallation.

“…[10,[13][14][15][16] However, disastrous oxidation of Nb-base materials at temperatures above 700 K remains a serious problem and impedes their practical use. Although some efforts have been made to improve their inherent oxidation resistance, [8,17,18] fruitful results have not yet been attained. Accordingly, the authors believe that an oxidation-resistant coating is indispensable to give a sufficient oxidation resistance to the Nb-base materials.…”

Section: Introductionmentioning

confidence: 99%

“…The authors focus their attention on Mo(Si,Al) 2 as the Al reservoirs, because it exhibits low parabolic oxidation rate constants and low CTEs, together with a sufficiently high melting point, compared with other aluminides such as NiAl TATSUO This article is based on a presentation made in the symposium entitled "Beyond Nickel-Base Superalloys," which took place March [14][15][16][17][18]2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee. and PtAl.…”

Section: Introductionmentioning

confidence: 99%

Development of Mo(Si,Al)2-base oxidation-resistant coating on Nb-base structural materials

Tabaru

Kim

Shobu

et al. 2005

base oxidation-resistant coatings for Nb-base structural materials have been studied. The coating is composed of a Mo(Si,Al) 2 -base Al reservoir and Al 2 O 3 interlayer to suppress interface reactions between the Al reservoir and the substrate. To develop a suitable Al-reservoir material, some Mo(Si 0.6 ,Al 0.4 ) 2 -HfB 2 composites were prepared. Their oxidation resistance and coefficients of thermal expansion were investigated, in addition to their chemical reactivity with the Nb substrate at high temperatures. As a result, Mo(Si 0.6 ,Al 0.4 ) 2 -20 vol pct HfB 2 was selected as one of the satisfactory Al reservoirs. The introduction of a stable Al 2 O 3 interlayer was attempted using a novel powder metallurgical process to overlay the Nb substrates with the Al reservoir, where the Nb substrates were subjected to a slight surface oxidation prior to the coating process. The Nb specimens, which are thoroughly coated with the Al reservoir and Al 2 O 3 interlayer, can be successfully fabricated by this method. The coated Nb specimens are not damaged at all after prolonged exposure in flowing Ar-20 pct O 2 at 1673 K for 120 hours. Furthermore, the Al 2 O 3 interlayer is very effective and no reactions occur at the interface. Thus, this Mo(Si,Al) 2 -base oxidation-resistant coating is applicable to Nb. The utility of the coating system is also confirmed for a Nb SS /Nb 5 Si 3 composite.