Hardening of Tungsten Induced by Helium Implantation

Key words: 
• He implantation /
• high purity tungsten /
• nano indentation /
• surface hardening

Abstract: Tungsten has been selected as divertor materials in fusion reactors because of its high thermal conductivity,high melting point, low expansion coefficient and high threshold energy for sputtering etc. The paper presents the hardening behaviour of high pure tungsten by 100 keV He+ with different fluences from 5x10¹⁶ ions/cm2 to 1x10¹⁸ions/cm2 at room temperature, and with fluence of 1x10¹⁸ ions/cm2 at higher temperatures (400, 600 and 800 °C). The microscopic mechanical properties of these samples were investigated by nano-indentation technology. The results show that all of the implanted samples harden obviously. The reason for hardening may be that defects of interstitial dislocation loops or dense helium bubbles etc induced by helium implantation obstacle the movement of dislocation. The peak nanohardness of the samples increased with the fluences increasing when the fluence is not more than 5x10¹⁷ ions/cm2, while the nano-hardness value of the implanted sample with the fluence of 1x10¹⁸ ions/cm2 decreases and the nano-hardness changes little in the region of 50 nm to 200 nm from surface. For all the implanted samples with 1x10¹⁸ ions/cm² at higher temperatures, their nano-hardness values are similar, but show a trend of decrease with increasing temperature.The reason may be the decrease of the defects’ density during implantation at higher temperatures. In addition, the capability of resisting deformation for the implanted tungsten reduces with increasing fluence and increases a little at higher temperatures.