Laser-Induced Reversion of δ′ precipitates in an Al-Li Alloy

Abstract

Atom probe tomography (APT) technique has been improved significantly, making it a wellestablished nano-analysis tool in the field of material science. It has been extensively applied to the investigation of different types of materials due to its ability to map the distribution of single atoms in a material in real space on a nearly atomic scale [1]. In this paper, we investigate the details of the laser pulse mode APT analyses, in particular the laser induced specimen-heating effect using an interface reaction in an Al-Li alloy as a model system. This alloy is known to have a low-temperature, metastable, miscibility gap [2]. It has been shown that under classical conditions of ageing of this alloy, including solution treatment, fast quench to room temperature and thermal ageing at intermediate temperatures (e.g., 100-200°C), the precipitation behavior was dominated by the presence of the metastable phase with an L12 structure [3]. The influence of the laser power on the morphology, the composition, and the diffusion of the constituents of the precipitates in the aluminum-lithium-based alloy is identified during an APT analysis. Prior to the laser exposure all sample were prepared by ageing an Al2wt.%Li binary specimen at 190 °C for 3h to produce comparable initial state with an average diameter of (14.2±3) nm and a number density of (10±0.1)x10 22 m -3 as confirmed by APT-voltage pulsing analysis. A simple model is used to explain the observed experimental behavior and to estimate the corresponding tip-apex temperature for various laser energies. APT analyses were performed with both a CAMECA laser assisted wide angle tomographic atom probe (LAWATAP) for the field ion microscopy (FIM) mode and the CAMECA local electrode atom probe (LEAP 4000X HR). Data were acquired utilizing either the voltage-or the laser pulse mode. A diode-pumped (Nd: YAG) solid-state laser operating in the frequency tripled ultraviolet region with a wavelength of 355 nm, a pulse duration of approximately 12 ps and a repetition rate of 200 kHz was used. The laser pulse energy was systematically varied through the following values: 10, 30, 40, 50, 60, 80, and 100 pJ. The reconstruction algorithm used was the standard evolution algorithm [4]. Figure 1 shows APT analyses by using laser pulse mode with various laser pulse energies. These analyses were performed to monitor the effect of thermal processes induced by laser on the morphology and composition of the phase. The figure shows a series of reconstructed volumes of the tips analyzed by laser pulses at the following laser energies: 10, 30, 40, 50, 60, 80, and 100 pJ. From Figures 1a-1e, spherical precipitates in the microstructure are clearly visible in the range of laser energies from 10 to 60 pJ. Conversely, Figure 1f shows that the precipitates begin to lose their distinctive shapes at 80 pJ. At an even higher energy of 100 pJ, precipitates are no longer detected as individual particles (Fig. 1g). Moreover, some enriched Li regions can be seen in the top view of the reconstructed volu...