Additively Manufactured NiTi and NiTiHf Alloys: Estimating Service Life in High-Temperature Oxidation

In this study, the effect of the addition of Hf on the oxidation behavior of NiTi alloy, which was processed using additive manufacturing and casting, is studied. Thermogravimetric analyses (TGA) were performed at the temperature of 500, 800, and 900 °C to assess the isothermal and dynamic oxidation behavior of the Ni50.4Ti29.6Hf20 at.% alloys for 75 h in dry air. After oxidation, X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were used to analyze the oxide scale formed on the surface of the samples during the high-temperature oxidation. Two stages of oxidation were observed for the NiTiHf samples, an increasing oxidation rate during the early stage of oxidation followed by a lower oxidation rate after approximately 10 h. The isothermal oxidation curves were well matched with a logarithmic rate law in the initial stage and then by parabolic rate law for the next stage. The formation of multi-layered oxide was observed for NiTiHf, which consists of Ti oxide, Hf oxide, and NiTiO3. For the binary alloys, results show that by increasing the temperature, the oxidation rate increased significantly and fitted with parabolic rate law. Activation energy of 175.25 kJ/mol for additively manufactured (AM) NiTi and 60.634 kJ/mol for AM NiTiHf was obtained.

High Temperature Oxidation Behaviors of CrNx and Cr-Si-N Thin Films at 1000 °C

The high temperature oxidation performance of nitride thin films has become an important issue when they are used as protective coatings on dry cutting tools or on die casting molds. In this study, the high temperature oxidation behaviors of CrNx and Cr-Si-N thin films were investigated at 1000 °C for 6 h in ambient air. The CrNx and Cr-Si-N thin films were prepared by a bipolar asymmetric pulsed direct-current (DC) magnetron sputtering system. Cr-Si-N films with silicon content ranging from 3.9 to 12.2 at.% were deposited by adjusting the Si target power. A thermogravimeter was adopted to study the oxidation kinetics of thin films. The weight gains were measured to calculate the parabolic rate constants of thin films. X-ray diffraction, X-ray mapping, and Auger electron spectroscopy were employed to study the microstructure and elemental redistributions of oxidized thin films. The as-deposited CrNx and Cr-Si-N thin films consisted of CrN and Cr2N mixed phases. The faceted Cr2O3 surface oxides, porous inner oxide layer, and oxygen-containing CrSi2 phases were found for the CrN film after oxidation test. On the other hand, the Cr-Si-N film containing 12.2 at.% Si showed a dense surface oxide layer and a thick and compact nitride layer, which indicates its best oxidation resistance. The high temperature oxidation resistance of Cr-Si-N thin films was improved by increasing Si content, due to the amorphous matrix contained nanocomposite microstructure and the formation of amorphous silicon oxide to retard the diffusion paths of oxygen, chromium, silicon, and nitrogen. The lowest parabolic rate constant of 1.48 × 10–2 mg2/cm4/h was obtained for the 12.2 at.% Si contained Cr-Si-N thin films, which provided the best oxidation resistance at 1000 °C for 6 h in this work. It should be noted that the residual tensile stress of thin film had a detrimental effect on the adhesion property during the oxidation test.

Air-thermal oxidation of zirconium

Zirconium is one of those few metals which are capable of dissolving relativelylarge quantities of oxygen. When heated in air at elevated temperatures, anoxide layer is built up at the metal surface. The oxidation of mechanicallypolished zirconium was studied in the range 500-900oC in air atmosphere, attemperature intervals of 100oC, for exposure times from 5 min to 1860 min foreach temperature. The weight gain of the oxidized species was the reactedamount of oxygen with zirconium for the formation of the ZrO2. The weight gainand thickness of the oxide film increase with the increasing of the oxidation timeand temperature. The oxidation initially followed a parabolic rate at alltemperatures. At temperatures higher than 700oC oxide “breakaway” appears atthe longest oxidation times. Microstructural investigations have shown that theoxide layers are compact and with good adhesion to the metal surface, but attemperatures over 700oC, radial micro-cracks appear. Raman spectra of theformed oxides at the investigated temperatures are characteristic for monoclinicphase.

Oxidation of Fe-Al Alloys (5-40 at.% Al) at 700 and 900 °C

Fe-Al alloys with Al contents between 5 and 40 at.% Al were oxidised for 1000 h in synthetic air at 700 and 900 °C to determine their oxidation behaviour. The minimum Al content which is necessary for the formation of protective Al2O3 scales decreases with increasing temperature from about 17 at.% Al at 700 °C to about 12 at.% Al at 900 °C. Established parabolic rate constants for the steady state growth of Al2O3 indicate formation of γ-Al2O3 at 700 °C while at 900 °C α-Al2O3 + Θ-Al2O3 scales form. At lower Al contents scales are predominantly formed by Fe2O3 as revealed by GI-XRD. It is also found that the oxidation behaviour is independent of the crystallographic order of the alloys, i.e. whether they are disordered A2 or ordered B2.

Effects of Yttrium on Cyclic Oxidation Resistance of Co-10Cr-5Al Alloy at 700 °C

The cyclic oxidation behavior of Co-10Cr-5Al alloys in atmosphere at 700 °C was investigated. The addition of 0.3 at.% Y changed the oxidation behavior from the approximate parabolic rate law to complex mode. The scale grown on the surface of Co-10Cr-5Al cracked seriously, while the oxide scale the Y doped alloy had better adhesive property. Yttrium doped in the sample promoted the forming of continuous Al2O3layer and decreased the oxidation rate of Co-10Cr-5Al alloys.

Effects of Yttrium on Cyclic Oxidation Resistance of Co-10Cr-5Si Alloy at 700 °C

The cyclic oxidation behavior of Co-10Cr-5Si alloys with and without Y in atmosphere at 700 °C was investigated. The addition of 0.3 at.% Y decreased the average parabolic rate constant from 4.39×10-10g2cm-2s-1to 3.42×10-10g2cm-2s-1. The surface components with and without Y were both composed of an outer CoO and inner protective SiO2. There were more SiO2formed when Y doped into the alloy. The addition of 0.3 at.% Y improved the cyclic oxidation resistance of the Co-10Cr-5Al alloy.

Effects of Yttrium on Cyclic Oxidation Resistance of Co-10Cr-5Al Alloy

The cyclic oxidation behavior of Co-10Cr-5Al alloys with and without Y in atmosphere at 800 °C was investigated. The addition of 0.3 at.% Y increased the oxidation rate of the alloy and changed the behavior from irregular oxidation kinetics to approximate parabolic rate law. The scales grown the alloys with and without Y were both composed of an outer Co2O3layer and an inner complex layer of Al2O3, Co2O3and Cr2O3, except that the addition of Y impaired the adhesion of the scale. Over-doped Y agglomerated in local zone plays an adverse role in the oxidation process by accelerating the oxidation rate.

Effects of Yttrium on Cyclic Oxidation Resistance of Co-10Cr-5Si Alloy

The cyclic oxidation behavior of Co-10Cr-5Si alloys with and without Y in atmosphere at 800 °C was investigated. The addition of 0.3 at.% Y decreased the average parabolic rate constant from 4.45×10-10g2cm-2s-1to 3.58×10-10g2cm-2s-1. The thin scales grown on Co-10Cr-5Si alloys were mainly composed of an outer CoO layer, followed by an inner protective SiO2layer. However, the scales can not be observed for Co-10Cr-5Si-0.3Y alloy, probably due to the spallation of the scales during the cooling process. The addition of 0.3 at.% Y improved the cyclic oxidation resistance of the Co-10Cr-5Al alloy.