No abstract

Oxide dispersion strengthened (ODS) alloys are desirable for high temperature applications, as dispersed oxide particles within the metal matrix act as barriers to dislocation motion and grain growth at elevated temperatures. Traditional processing routes for ODS alloys are powder metallurgy based, utilizing mechanical alloying by high energy ball milling to mix metal and oxide powders, necessitating compaction techniques such as hot isostatic pressing or hot extrusion to create a dense final product. Recent studies have investigated production of dispersion strengthened alloys without powder metallurgy by introducing a dispersed ceramic phase to a bulk alloy via a physical application process, such as spraying [1], followed by severe plastic deformation to refine grain size. This study proposes an ODS synthesis method of introducing oxides via selective internal oxidation, followed by equal channel angular pressing (ECAP) to refine grain size.In this work, Fe-1.5 wt% Y alloys were studied. The as-cast microstructure consisted of an a-Fe matrix and Fe 17 Y 2 intermetallic, as shown in Figure 1a. Oxides were introduced into the bulk alloy when the material underwent selective internal oxidation, during which the Fe 17 Y 2 intermetallic phase oxidized to Y 2 O 3 particles and pure Fe, leaving the a-Fe matrix unaltered [2].Internal oxidation was followed by ECAP in order to refine the matrix Fe grain size while also dispersing the Y 2 O 3 particles. During ECAP, as illustrated in Figure 1b, strain is introduced as the ingot is pressed through a die at an angle, which for this study was 90°. Because the dimensions of the cross section are the same before and after deformation, the piece can undergo multiple consecutive passes, each introducing additional plastic deformation to the sample. The processing variables studied in this work were the number of ECAP passes and the temperature at which ECAP was performed. In this study, ECAP was performed at room temperature (25°C) and an elevated temperature (350°C), for four ECAP passes and for eight ECAP passes.After ECAP, Fe grains in the matrix were imaged using electron channeling contrast via backscattered electron detection in a FEI Quanta 600 FEG-SEM. Post-ECAP grain size of the Fe matrix was measured using the ASTM E112 linear intercept method [3], finding that increased number of ECAP passes had no significant impact on grain size. A series of thermal stability anneals at 250°C, 400°C, and 1000°C were performed, and Vickers microhardness measurements taken before and after annealing found that the high-temperature ECAP samples maintained similar hardness after 400°C, while room temperature ECAP samples exhibited loss of thermal stability based on an observed significant drop in hardness after 400°C anneals.An automated high-throughput dispersion analysis program was developed to quantify particle dispersion using the quadrat method [4]. Applying the dispersion program to the ECAP samples, it was found that the room temperature ECAP samples had more dispersed particl...

Fe-Y binary alloys underwent thermogravimetric analysis while exposed to low oxidant partial pressures using a mixture of 5 pct H 2 -95 pct Ar gas at two flow rates. The alloys experienced in situ internal oxidation of Y-rich intermetallic phases. Kinetics and microstructures were both affected by flowrate. Lower flow rates resulted in coarser oxides and decreased oxidation rates. Results show the possible impacts of rate-controlling processes in the gas phase on internal oxidation with dilute oxidants.