Development of the Ball Indentation Test to Evaluate Mechanical Properties of Nanostructured Materials.

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Title: Development of the Ball Indentation Test to Evaluate Mechanical Properties of Nanostructured Materials.
Author: Trichy, Gopinath R
Advisors: Prof. R.O. Scattergood, Committee Chair
Abstract: The goal of this thesis was to standardize the ball indentation test and subsequently apply it to evaluate the mechanical properties of nanostructured Zn. The ball indentation test is an efficient test technique for mechanical characterization when material availability is limited, as often the case with nanostructured material. A ball indentation test set up was built in our laboratory and was used to evaluate mechanical properties of conventional materials like steels (A533B, A588B, A516 steels), Al 6061 and pure Zn. The theoretical background necessary for the BI methodology was briefly described. The effectiveness of ball indentation technique in determining the flow curve was demonstrated by comparing it with tensile results for the materials mentioned above. The influence of fixture compliance and indenter ball size on the plastic flow curve was discussed. The mechanical response of an amorphous material, like Zr52.5Ti5Cu17.9Ni14.6Al10 bulk metallic glass, during the ball indentation test was studied. The flow curve obtained by the ball indentation test showed significant strain hardening. This was attributed to the multiaxial state of stress and the intersection and multiplication of shear bands. There was little or no effect of strain rate on the flow curve. Optical microscopy around the indent revealed intersecting radial and concentric shear bands. The included angle between the intersecting shear bands along the radial direction deviates from 90°, consistent with an influence of normal stress on the slip plane during yielding. Ultra fine-grained and nanostructured Zn was produced by ball milling at liquid nitrogen temperatures. Variation of grain size and hardness as a function of cryo-milling time was studied. The hardness increased steadily at lower cryo-milling times. At longer milling times (6hr and more) the hardness reaches a steady state. The hardness values obtained were higher than that predicted by a Hall-Petch strengthening effect. Transmission electron microscopy studies were done to estimate the average grain size and grain size distributions. TEM studies also revealed significant dislocation activity. The miniaturized disk bend test (MDBT) and the ball indentation (BI) test were used to mechanically characterize the 14 hr cryo-milled and subsequently compacted sample with the average grain size = 42 nm and hardness = 864 MPa. MDBT results revealed poor ductility. BI test results showed low strain hardening and low strain rate sensitivity when compared to conventional coarse-grained Zn. In-situ consolidated ultra fine grained (UFG) Zn was produced by room temperature ball milling. The in-situ consolidated Zn balls show a variation in the hardness from the outer surface to the core. TEM studies revealed a wide grain size distribution. The mechanical properties were evaluated by small specimen tests such as the ball indentation test, the shear punch test and the miniaturized disk bend test. MDBT results and corresponding fracture surfaces showed that the UFG Zn is ductile. Six times the shear yield obtained by the shear punch test agreed well with the average hardness value. Ball indentation test results revealed negligible strain hardening and the flow curve seemed to soften at higher strains. Ball indentation test results for coarse-grained (d=20 mm) and nanostructured Zn (d=42 nm) have been compared and discussed. The strain rate sensitivity obtained for nanostructured Zn is lower than that obtained for coarse-grained Zn. At a particular strain the activation volume for nanostructured Zn is less than that for coarse-grained Zn. The activation volume decreases with increase in strain for coarse-grained Zn and for nanostructured Zn the activation volume tends to remain constant with increase in strain.
Date: 2005-03-02
Degree: MS
Discipline: Materials Science and Engineering
URI: http://www.lib.ncsu.edu/resolver/1840.16/1063


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