Highlights

Grain Growth in Nanocrystalline Metal Thin-Films Under In Situ Ion-Beam Irradiation

Scientific Achievement

The microstructure evolution in metal Zr, Pt, Cu and Au nanocrystalline thin films under ion-beam irradiation was studied in situ in a transmission electron microscope (IVEM).  Samples were irradiated in-situ with Ar and Kr ions to fluences in excess of 1016 ion/cm².  As a result of irradiation, grain growth was observed in all samples.  The average grain size increased monotonically with ion fluence until it reached a saturation value.  Similarly to thermal grain growth, the ion-irradiation induced grain growth curves could be best fitted with curves of the type:t. F=-KDD0

The irradiations were done at temperatures ranging from 20 to 773 K.  The results evidenced the existence of three regimes with respect to irradiating temperature: (i) a purely thermal regime, which starts above the recrystallization temperature for bulk material, (ii) a thermally-assisted regime where thermal diffusion and irradiation effects combine to increase the rate of grain growth relative to that resulting from either of these mechanisms alone, and (iii) a low-temperature regime where irradiation can by itself cause grain growth.  The transition temperature between the athermal regime and the thermally-assisted regime depends on the material, but is in the range 0.14-0.22 times the melting point.

The effect of solute addition on grain-growth was investigated by irradiating supersaturated solid solutions of Zr(Fe) and Cu(Fe).  The grain-growth kinetics decreased with respect to the pure metallic films as a result of both solute drag (Cu-Fe) and second-phase particle pinning i.e. Zener drag effect (Zr-Fe).

A theoretical model of grain-growth under ion irradiation in the temperature-independent regime was developed, based on the direct impact of irradiation-induced thermal spikes on grain-boundaries. In the model, grain-boundary migration occurs by atomic jumps within the thermal spikes biased by the local grain-boundary curvature driving force.  The model yields a power law expression relating the average grain-size with the ion dose, where the exponent is 3 in good agreement with the experimental data. In contrast with previous studies, this work showed that the grain-growth exponent of 3 is an inherent feature of grain-growth under ion irradiation.

Significance

This work has resulted in the derivation of a model which accounts for grain growth kinetics under irradiation observed in this work and in previously unexplained others.  This work also provide mechanistic insight on the stability of nanocrystalline metals under irradiation, which is a novel field and as such lacks information as pointed out in several reviews.

Performers