Nanoindentation Measurements of Tungsten at Temperatures up to 950°C

Table of Contents

Introduction
Experimental Results of High Temperature Nanoindentation Tests
Conclusion

Introduction

For the first time, Nanoindentation measurements have been carried out with the sample and indenter heated up to 950 °C within a vacuum environment. This pioneering study was carried out by taking nanoindentation measurements on a single polycrystalline tungsten sample at varying temperatures: room temperature, 500°C, 750°C, 800°C, 850°C, 900°C and 950°C.

The near surface properties of tungsten (W) based alloys have already been characterized through nanoindentation tests. These alloys are one of the most promising potential materials for nuclear fusion reactor cladding. Until now, the results that have been published are of nanoindentation measurements taken at temperatures of up to 750°C. This temperature has been regarded as the practical limit for high- temperature nanoindentation technique.

Experimental Results of High Temperature Nanoindentation Tests

Figure1 represents the relation between depth and load hysteresis, with temperature varying from room temperature to 940°C. It can be observed from the graph that the quality of data remains stable throughout the temperature range. This stability is a result of careful control of temperatures of the sample and the indenter. As per the data in Figure 1, tungsten shows rapid softening initially, which turns into a steadier decline later. Analysis of the data reveals a decrease in hardness from 6GPa at room temperature, to almost 2GPa at 950°C.

Figure 1. Indentation data on a single polycrystalline tungsten sample at 500, 750, 800, 850, 900 and 950°C within a vacuum environment using a cubic Boron Nitride indenter.

The material softens as the temperature rises, and during the dwell period at peak load, an increase in depth is measured. Figure2 illustrates this increasing depth in the dwell period of 30s.

Figure 2. Increasing depth during the peak load dwell period over 30 seconds for indents at 750, 850 and 950°C.

Room temperature Young’s modulus of W is  ̴̴ 411 GPa. Published data shows that the Young’s modulus of tungsten drops by 10% approximately over a temperature range of 20 to 1000°C. Initial experiments conducted with a dwell period of 30s showed that the modulus increased to ~550GPa at a temperature of 950°C.

In order to allow tungsten to relax more prior to unloading, the dwell period was increased to 300s. This is shown in Figure3. High stability exhibited by the instrument enabled highly reliable measurements to be recorded. The reduced modulus value reduced to 260GPa at a temperature of 950°C for a longer dwell period. The Young’s modulus reached a value of ~370GPa, which is 10% less than the value at room temperature. This value is in line with the published data.

Figure 3. Comparison of 30 and 300 second peak load dwell period of two indents at 950°C.

Conclusion

High-temperature nanoindentation tests were performed on tungsten in vacuum and at temperatures of up to 950°C, using NanoTest Xtreme high temperature hardware. Future experiments should target a maximum temperature of 1000°C for testing.

This information has been sourced, reviewed and adapted from materials provided by Micro Materials.

For more information on this source, please visit Micro Materials.

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