The cyclic oxidation of a variety of chromia-forming ferritic stainless steels has been studied in the temperature range 700-900°C in atmospheres relevant to solid-oxide fuel-cell operation. The most detrimental environment at 800°C and 900°C was found to be air with 10% water vapor. This resulted in excessive oxide spallation or rapid scale growth. Impurities in the alloys, particularly Al and Si, were found to have a significant effect on the oxidation behavior. Oxide growth was slow at 700°C but the higher-Cr-content alloys were observed to form sigma-phase at this temperature. The sigma phase formation was accelerated by higher silicon contents, and remarkably, by the presence of water vapor in the exposure environment. Alloys containing Mn were observed to form an outer layer of MnCr 2 O 4 over the chromia scale. The potential for this overlayer to suppress reactive evaporation of the chromia scales has been analyzed.

The mechanisms whereby intermixed zones of alumina and zirconia are formed at the interface between the metallic bond coat and the ceramic top coat (yttria-stabilized zirconia) in thermal barrier coating systems have been investigated. The results lead to the following mechanism for the formation of the zones. The predominant mechanism for intermixed zone formation involves formation of a metastable alumina polymorph (h or c) during TBC deposition, with a significant amount of zirconia dissolved in it. The outward growth also begins to incorporate zirconia particles, which initiates the formation of the intermixed zone. Upon thermal exposure, the metastable TGO continues to grow outward, extending the intermixed zone, and eventually transforms to the equilibrium a-Al 2 O 3 . The transformation to a-Al 2 O 3 results in an increase in the volume fraction of zirconia in the intermixed zone as it is rejected from solution. Once the aAl 2 O 3 appears, subsequent TGO growth produces a columnar zone of the TGO without a second phase. When a-alumina was preformed on the bond coat, prior to TBC deposition, no intermixed zone was formed for Pt-modified aluminide bond coats.

Table of Contents 4 List of Graphical Materials 5 Introduction 8 Executive Summary 9 Project Description 11 Experimental 12 Results and Discussion 20 References 44 EXECUTIVE SUMMARY This project had four objectives: • To develop mechanism-based evaluation procedures for the stability of SOFC interconnect materials and to use these procedures to study and modify a group of alloys, which have already been identified as candidate interconnect materials, i.e. ferritic stainless steels. • To study fundamental aspects underlying the thermomechanical behavior of interconnect materials and develop accelerated testing protocols. (CMU Subcontract) • To investigate the potential for the use of "new" metals as interconnect materials. • Development of a durable, conductive ceramic/metal (cermet) material, suitable for long-term use as a contacting material in the cathode chamber of SOFC. (WVU Subcontract) The program consisted of four major tasks aligned with its four objectives. Task 1: Mechanism-based Evaluation Procedures A variety of chromia-forming interconnect alloys were subjected to thermal cycling in air, in simulated anode gas (Ar-H 2-H 2 O) and with simultaneous exposure to air on one side and simulated anode gas on the other. Exposure temperatures range from 700°C to 900°C. Oxidation kinetics were tracked by mass change measurements and corresponding changes in oxide scale resistances were measured. Exposed specimens were examined in cross-section by scanning electron microscopy (SEM) to document changes in structure with exposure. Methods were studied to slow the growth of chromia scales on Cr and Ferritic alloys upon exposure to oxidizing gases. The effect of alloying additions (e.g. Mn, Ti) to ferritic steels to reduce harmful CrO 3 and CrO 2 (OH) 2 evaporation by forming a sealing outer layer over the chromia scale was evaluated. The ability of chromite coatings to reduce evaporation from chromia-forming interconnect alloys was investigated. Task 2: Fundamental Aspects of Thermomechanical Behavior (CMU) Understanding the resistance of growing chromia scales to spallation requires a fundamental understanding of the mechanics of chromia adhesion. From a fracture mechanics standpoint, the adherence of protective oxide scales to alloy substrates is governed by 1) the stored elastic energy in the scale, which drives delamination and 2) the fracture toughness of the alloy/oxide interface, which quantifies the resistance to fracture. The stored elastic energy in the scale is increased by increases in the scale thickness and increases in the residual stress in the scale. In this task, x-ray diffraction (XRD) was used to measure stresses in chromia films formed on pure chromium and chromia-forming alloys after the exposures described for Task 1. An indentation test was also used to measure the fracture toughness of chromia/alloy interfaces for the same exposures. In the test, the chromia scale is penetrated by the indenter and the plastic deformation of the underlying substrate induces compressive radial strains in the...

Knowledge of creep properties is vital in determining the allowable stresses for rotating equipment design at high temperatures. Unfortunately, the traditional method to generate creep data requires several long term tests; in some cases, upwards of 100,000 hours are needed. These tests are often time and cost prohibitive to perform. Some data is available from sources such as the ASME Boiler and Pressure Vessel Code, but these are limited to commonly available materials in set processing conditions. They speak very little to the properties for new materials, alternate processing of existing materials, and properties for weldments and heat affected zones (HAZ) that occur in the fabrication of large equipment. Due to this, several methods have been developed for accelerated creep testing. One such method is the Stress Relaxation Test (SRT) developed by Woodford. This high precision stress relaxation test can generate five decades of creep data in a single, one-day test. This paper discusses the use of the SRT method to evaluate two different materials and their weldments used in the manufacturing of steam turbine casings. The first material is cast 9Cr-1Mo-V (SA-217, Grade C12A). In this first case, material from two different foundries was tested at temperatures between 550°C and 700°C. Specimens consisting entirely of matching weld metal and those that that include the HAZ centered between weld metal and the base casting were also tested as a means to verify fabrication and casting upgrade procedures. In this case, the data generated for all three sample types very closely match those given in literature. In a nearly identical test program, testing was performed on cast 1Cr-1Mo-¼V steel (ASTM A356, Grade 9). In this second case, the base casting closely matched literature data, while the weldments did not. In one instance, through a significant reduction in properties of the weld metal specimen, the SRT method was able to detect that an under matching filler metal was used. In another instance, the HAZ specimen, from a weldment using matching filler metal, failed during the test. It was found that the welding procedure resulted in overheating the sample. These two case studies illustrate the ability of the SRT method to accurately predict creep properties and its sensitivity to detect variations in properties, which can make it useful for rapid verification of welding procedures for high temperature applications.

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