Mechanical Behavior of Nanocrystalline Materials and Application of Shear Punch Test

Abstract

Nanocrystalline (nc) materials show superior strength compared to conventional grain size materials. Synthesizing bulk nc materials often results in small quantities, which limits making large scale specimens for conventional testing such as tensile, compression etc. In such instances, miniaturized specimen test techniques are very useful. The intent of the present work was to study the mechanical behavior of nc materials and also to develop a miniaturized specimen testing procedure -"shear punch test (SPT)" for nc material testing.

A new setup for SPT was built and the testing procedure was standardized using different conventional metals. A linear correlation between SPT measurements and tensile data was established. The effect of test setup parameters such as specimen thickness and the die-punch clearance was studied in order to rationalize the technique.

Using Finite Element Analysis, SPT was modeled and the physical basis of through-thickness plastic zone development at shear yield measurements was examined. The deformation behavior and the state of stress at different stages of the punching operation during an SPT test were studied.

In order to understand the mechanical behavior of various materials from conventional to amorphous through nano regime, studies were done on investigating the mechanical properties of a Zr — based amorphous alloy (BMG — 11 alloy) using SPT. Strain rate dependent deformation behavior was studied and at high strain rates reduced strength was attributed to thermal softening.

In order to examine the applicability of SPT for testing of nc materials, initial experiments were carried out on electrodeposited nc Cu. The SPT and mini tensile experiments were conducted and compared. An attempt was made to examine the strain rate effect. The SPT technique was extended to measure the strain rate sensitivity and the activation volumes and compared with the isostrain rate tensile test measurements.

The nc materials pure Fe, Fe-Pb alloys and Fe-Al2O3 composites were processed using high energy ball milling technique and were characterized by means of X-ray diffraction, transmission electron microscopy, scanning electron microscopy and differential scanning calorimetry. The mechanical properties were evaluated using the nanoindentation technique. Solid solution strengthening of Fe-Pb system was studied. Enhanced strengthening of Fe-Al2O3 composites was attributed to the refined Fe grain size.