Investigation of wear and depth-sensing indentation behavior of thermally stabilized Fe-Ni-Zr alloys

For structural applications, ultrafine grained and nanostructured materials are hopeful due to their high strength compared to coarse grained ones. Although they are promising for structural applications, their widespread usage is limited even at low temperatures by an intrinsically high driving force due to thermally induced grain growth. Therefore, understanding and controlling of grain growth are perfect scientific and technological challenge which directly influence the mechanical properties of these materials. Zirconium has been proven as an important grain size stabilizer in pure Fe and Fe-based systems. Due to the precipitation of intermetallic compounds for high Zr containing alloys at higher temperatures, grain growth can be kinetically suppressed. The alloys used in the present thesis have been prepared by high energy ball milling and equal channel angular extrusion in a previous study (Kotan H., Thermal stabilization and mechanical properties of nanocrystalline Fe-Ni-Zr alloys, PhD Thesis, North Carolina State University, Raleigh, North Carolina, 2013). In this thesis study, effect of Zr addition (1 at. % Zr and 4 at. % Zr) on the mechanical properties of Fe-Ni-Zr alloys were investigated. For this purpose, reciprocating wear tests and depth-sensing indentation tests were performed on Fe-Ni-Zr alloys, which have been previously produced. Wear test results revealed that wear track areas of both alloys increased with increasing normal load. In the view point of wear rate, which is normalized wear volume by normal load and sliding distance, on the other hand, wear rate of 1 at. % Zr containing alloy does not follow a systematic trend with respect to normal load, while wear rate of 4 at. % Zr containing alloy increased with increasing normal load. In addition, wear rate of 4 at. % Zr containing alloy was lower than that of 1 at. % Zr containing alloy at lower normal loads (1 N and 2 N). However, at higher loads (3 N and 4 N), wear rate of 4 at. % Zr containing alloy is higher than that of 1 at. % Zr containing alloy. Indentation hardness of the alloys revealed that 4 at. % Zr containing alloy was harder than 1 at. % Zr containing alloy. Also, hardness of both alloys were sensitive to loading rate, i.e. they tend to be increased with increasing loading rate. However, loading rate dependency of hardness in 4 at. % Zr containing alloy was more significant. Hardness of this alloy increased about 30 % with increasing loading rate from minimum to maximum. When hardness and wear tests results are comparatively evaluated, it is seen that wear rate of the alloys is mainly controlled by their hardness, and harder alloy exhibited lower wear rate (higher wear resistance) at lower loads. At higher loads, 1 at. % Zr containing alloy with less amount of intermetallics and oxide precipitates in its microstructure has a higher wear resistance. Friction coefficient of 4 at. % Zr containing alloy was always higher than that of 1 at. % Zr containing alloy. Creep behavior of the alloys were investigated by room temperature creep test conducted by depth sensing indentation tests. Test were conducted at two different maximum load and with a constant loading rate. At the end of the tests, displacement versus time plots were obtained and the slope of these plots were taken as a measure of creep rate. Results showed that creep rate increases with increasing Zr addition (with decreasing grain size) at lower load (50 mN), but decreases with increasing Zr addition at higher load (200 mN). It suggests that larger grains (accompanied by small amount of intermetallics and oxide precipitates) are effective for improved creep resistance only at higher loads. At lower load, smaller grains (accompanied by large amount of intermetallics and oxide precipitates) are better for improved creep resistance. Beside creep rate, as the slope of displacement versus time plots, stress exponent is another measure of creep phenomena indicating stress dependency of the creep rate. Stress exponent decreased with increasing Zr addition at lower load and increased with increasing Zr addition at higher load. It can be finally concluded that creep behavior of Fe-Ni-Zr alloys are very sensitive to Zr addition and the test load. Creep rate and stress exponent have opposite variations with respect to the test load. At lower load, 4 at. % Zr containing alloy which has smaller grains and large amount of intermetallics exhibits better creep resistance. At higher load (200 mN), 1 at. % Zr containing alloy which has larger grains and less amount of intermetallics and oxide precipitates exhibited better creep resistance. Stress exponent values are higher than 10 in the present study and this indicates that the dominant creep mechanisms of the alloys used in the present study is a dislocation based mechanism such as including dislocation climb and dislocation glide. To our knowledge, this is the first study in which wear and depths sensing indentation tests were performed and the results were comparatively evaluated on the basis of increasing Zr content.