Direct observation of deformation-induced grain growth during the nanoindentation of ultrafine-grained Al at room temperature

Abstract

In situ nanoindentation within a transmission electron microscope is used to investigate the deformation mechanisms in ultrafine-grained Al films. Deformation-induced grain growth resulting from grain boundary migration, grain rotation and grain coalescence is commonly observed as the indentation proceeds. In situ studies of nanograined films suggest that the same mechanisms are operative, though the difficulty of imaging nanosized grains makes the evidence less clear. The results suggest that grain growth and coalescence are important modes of response in the deformation of ultrafine- and nanograined materials.

Introduction

Understanding the mechanisms that allow plastic deformation of polycrystalline materials is a longstanding challenge in materials science. Grain size is one of the most important variables. The yield strength ordinarily increases with decreasing grain size according to the classic Hall–Petch relationship [1], [2]

$${\sigma }_{\mathrm{y}}={\sigma }_0+K_{\mathrm{y}}{d}^{-1/2},$$

where d is the mean grain size and σ₀ and K_y are constants. While there are several possible explanations for the Hall–Petch relation [3], the most widely accepted involve direct interactions between dislocations and grain boundaries. The Hall–Petch relationship shown above is almost always obeyed in ductile metals with grain sizes larger than about a micrometer. In fact, most of the available data follow Hall–Petch down to grain sizes of a few tens of nanometers, below which the grain size exponent changes from near −0.5 to a value near zero [4]. At even smaller grain sizes (typically below 20 nm), the results are controversial. Some experiments show an inverse Hall–Petch softening effect [5], [6], but others confirm an increasing strength with decreasing grain size [7], [8]. At present, both the strengthening trends and the deformation mechanisms in the deep nanocrystalline regime remain open questions.

As the grain size decreases the fraction of the atoms that lie close to grain boundaries increases dramatically. For a fixed grain geometry the atom fraction in the grain boundaries changes from about 0.01% for 3 micrometer grains, to 0.1% for 300 nm grains and to 1% for 30 nm grains [9]. Moreover, when the grain size reaches the nanometer scale many of the dislocation mechanisms that govern plasticity in the bulk are no longer geometrically possible. It follows that the mechanisms of deformation must change when the grain size reaches well into the nanoscale and that grain boundary processes play an important role. This issue has been addressed in a number of theoretical models and computational simulations. The alternate modes of deformation that have been proposed include twinning [10], grain boundary sliding [11], [12], [13], grain rotation [14], [15], grain boundary migration and grain growth [16]. However, the relative importance of these various mechanisms in the low-temperature deformation of nanograined materials remains unclear. Recently, several uniaxial straining experiments have been carried out on nanocrystalline materials in situ in a transmission electron microscope (TEM) to investigate the possible deformation mechanisms. Each has shown intense dislocation activity ahead of a crack tip [17], [18], [19]. However, there is no unambiguous documentation of the grain boundary processes that have been proposed.

To address this issue, we have used an in situ nanoindentation stage in a transmission electron microscope to explore the behavior of grain boundaries during the indentation of ultrafine-grained and nanocrystalline Al. The observations we have made to date illustrate the importance of grain boundary processes in indentation, but specifically reveal the common occurrence of a mechanism that has not been widely discussed: the deformation-induced coalescence and coarsening of grains. The principal purpose of the present paper is to demonstrate and describe this behavior.