College of Engineering
Materials Engineering Department
BS in Materials Engineering
Prof. Blair London
This study investigates the effect of cooling rate on the yield strength of NARloy-Z. NARloy-Z is a copper-based alloy with 3 wt.% silver and 0.5 wt.% zirconium. The types of NARloy-Z were classified by the ingot processing (old or new) and the material lot (old or new). There were three variations of NARloy-Z in this study: old processing and material (Old/Old); old processing and new material (Old/New); and new processing and material (New/New). NARloy-Z undergoes a braze thermal cycle and age (BTCA) heat treatment for its application, and a single cooling rate within the BTCA was manipulated in this study. The three cooling rates used were -5.4ºF/min (fast), -2.2ºF/min (moderate), and -1.1ºF/min (slow). Each material and cooling rate combination was tensile tested according to ASTM E8/E8M. Because of the limited number of Old/Old material, only the fast and slow cooling rates were used for the Old/Old material. The average yield strengths for the fast and slow Old/Old material were 11.2 ksi and 11.3 ksi, respectively. The Old/New material had average yield strengths of 10.7 ksi, 9.0 ksi, and 8.9 ksi for the fast, moderate, and slow cooling rates, respectively. The New/New material showed average yield strengths of 12.1 ksi with the fast cooling rate, 11.3 ksi with the moderate cooling rate, and 12.7 ksi with the slow cooling rate. The tensile data showed that the cooling rate analyzed did not have a significant effect on the NARloy-Z yield strength. The low yield strength values were due to exposure to high temperatures for an extended period of time during the BTCA heat treatment. Metallography was performed on one sample from each material and heat treatment combination. The Old/New material consistently had larger grains than the other materials, regardless of the heat treatment. Conventional understanding of materials engineering would suggest that the larger grains found in the Old/New material is an explanation for its low yield strength. However, this cannot solely be attributed to its grain size because material with finer grain structures had similar yield strengths.