Advances in laser assisted 3D atom probe and its applications to light metals

The three dimensional atom probe (3DAP) is the only tomography technique with an atomic resolution. Due this unique feature, it has been widely used in semi-quantitative compositional analyses of solute clusters and nanosized precipitates in alloys. Various microalloying effects in the precipitation processes of aluminum alloys have been investigated using the 3DAP. The most well-known example is the effect of trace additions of Ag and Mg in Al-Cu based alloys [1]. In this system, Ag and Mg form co-clusters in the early stage of aging, which incorporate Cu to form {111} plates. As they grow, Ag and Mg segregate to the interface between the {111} precipitate denoted as  and the -Al matrix, which reduces the strain energy at the interface. From the earliest stage of the solute clustering to the coarsening stage of the , the distribution of all solute atoms were visually monitored by the atom probe tomography [1]. While the combined addition of Mg and Ag leads to the formation of co-clusters, the microalloyed elements such as Sn and In are known to form fine precipitates that act as heterogeneous nucleation sites for the {001}  plates [2]. Recently, the microalloying effects have also been recognized as one of the effective methods to enhance the age hardening processes in many magnesium alloys, [3]. Although magnesium alloys had not received serious attention as structural materials except in cast products, the attention towards the use of wrought magnesium alloys as the lightest structural metal for the weight reduction of automobiles and high speed trains has been increasing due to the ever increasing demand for carbon dioxide reduction in the transportation sector. The major obstacle against the use of Mg alloys as a structural component is the lack of wrought alloys that have competitive mechanical properties, formability and corrosion resistance with those of wrought Al alloys. The development of commercially viable low-cost, high strength wrought Mg alloys may expand the application areas of Mg alloys significantly considering that only a very small amount of Mg alloys is currently used as wrought products. Although rare earth containing Mg alloys are known to show pronounced age hardening [4], the application areas of rare earth containing alloys is rather limited due to the cost of materials. Thus, rare earth free alloys that exhibit pronounced age hardening effect must be sought for wider industrial applications. The yield strengths of conventional commercial wrought Mg alloys such as Mg-Al based AZ31 and AZ61 and Mg-Zn based ZK60 are only about ~240 MPa, which is less than 2/3 of that of wrought Al alloys. Commercial wrought Mg alloys have rarely been strengthened by precipitation although most of them are age hardening systems in view of phase equilibrium. Since most of the wrought Mg alloys are strengthened by the refinement of the crystal grain size that occurs during dynamic recrystallization in extrusion processes, subsequent artificial aging usually leads to a degradation of strength. However, if the age hardening response of an Mg alloy can be substantially enhanced to surpass the softening with the T6 or T5 treatment, the Mg alloy may also be precipitation hardened after the wrought processes like many Al wrought alloys. Our goal is to explore new Mg 1361