ABSTRACT

For automotive exhaust valve applications, future vehicles will need affordable, durable materials capable of operating at higher temperatures with predictable response to severe oxidizing environments. Both commercial and model Ni-based alloys were tested in 1-h cycles at 800-950°C in wet air, and the oxide scales formed on wrought Ni-(14-25)wt%Cr binary alloys were characterized to gain a better understanding of the behavior of chromia-forming alloys under these conditions. The mass change curves were used to quantify the behavior of the tested alloys and fit growth and spallation rates using the kp-p model. We systematically analyzed the correlation between elemental alloy compositions and the manually fitted kp and p values to select high-ranking features to be included in a machine learning analysis. The machine learning models for the rate, kp, could be trained with a surprisingly high accuracy even with limited data, while only modest fitting was obtained for p, the spallation parameter. A preliminary theoretical framework that can predict kp and p of hypothetical alloys was established, however, improving the accuracy of surrogate models is needed to assist in alloy development for this transportation application.

INTRODUCTION

The efficiency of automotive engines can be improved by increasing the operating temperature. Exhaust gas temperatures are anticipated to increase to approximately 950°C by 2025, from the current 870°C regime1. This higher operating temperature, however, results in severe oxidation and oxide scale spallation of exhaust valves and other components made of Ni-based alloys, threatening long-term reliability. One possibility is that new alloys need to be developed with the required combination of creep strength, fatigue and oxidation resistance, fabricability and cost. In order to take advantage of the current computational tools for alloy development, a model to account for Ni-Cr alloy oxidation is needed for this temperature range. The oxidation behavior of Ni-based alloys at higher temperature, therefore, should be further explored to provide experimental data for the validation of the oxidation model.

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