Date of Final Oral Examination (Defense)

5-2010

Type of Culminating Activity

Thesis - Boise State University Access Only

Degree Title

Master of Science in Materials Science and Engineering

Department

Materials Science and Engineering

Major Advisor

Megan E. Frary, Ph.D.

Abstract

Alloy 230 is a nickel-based superalloy that is a candidate material for heat exchangers in very high temperature nuclear reactors. A wide range of operating temperatures may be found in the operation of the heat exchanger. In these kinds of applications, fatigue damage arises from the fluctuation of stress, strain, and temperature. Therefore, it is important to understand the mechanical behavior of the material under stresses at high temperatures and the microstructural evolution during deformation. In general, structures contain microstructural defects can lead to the formation of cracks if the service loading exceeds a certain level. To investigate the crack growth behavior in alloy 230, crack growth testing under a controlled stress intensity factor of 30, 35, and 40 MPa⋅m1/2 at a constant temperature of 650 to 800ºC with 50ºC increments was carried out. The tests were performed using both fatigue and static loads with frequencies of 1, 0.1, and 0.01 Hz and stress ratios (σminmax) of 0.5 and 0.7. The crack growth rate was affected by temperature, load frequency, stress ratio, and hold time. Crack growth rates per cycle increase with decreasing load frequency, increasing temperatures and increasing stress intensity. The effect of stress ratio on crack growth depends on temperature and stress intensity level. Under the different testing conditions, the damage mechanism is dominated by different phenomena.

After crack growth testing, electron backscatter diffraction was used to characterize microstructure and texture at the crack tip. Slight changes in the microtexture from the [101] to the [111] direction can be observed. The level of internal misorientation in grains near the crack tip is measured using the grain average misorientation maps. The average misorientation within individual grains increases to as much as 5º and is indicative of local deformation. Near the crack tip, voids along grain boundaries are found. The cracking modes observed are a mix of transgranular and intergranular. Developing a better understanding of the fatigue properties and microstructural evolution in alloy 230 will support the materials selection and design of the heat exchanger for future nuclear power plants.

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