Study of Pre-Precipitation in Ti-Cu and Ti-Cu-Al Alloys and the effect of Aluminum additions on the Kinetics of formation of Guinier-Preston Zones

The study of the beginning of pre-precipitation and clustering of atoms in metal alloys allows a better comprehension of the phenomena of precipitation and phases transformations in these alloys as well as the role of each chemical element and of each stage of precipitation in the formation of the various phases. In the case of titanium alloys Ti-Cu and Ti-Cu-Al the aim of the investigation is to determine if the participating atoms (cupper) in the formation of the Guinnier-Preston zones (GPZ) tend to form clusters precursor of the formation of these GPZ or not, and to determine the influence of aluminum on the formation of these clusters and consequently on the kinetics of formation of GPZ.The experimental results of the atom probe analysis are studied statistically in the form of distributions. This study consist to compare the atom distribution (in block which contain some atoms) of cupper to one theoretical distribution (random distribution the one of Poisson valid for events with smallness probability) in return of χ² test which permit to decide if the distribution of copper in the material is effectively in random or not, of course after calculation of standard deviations and to compare them with tabulated values of χ² of the Poisson Law. The study shows, indeed, that the alloy, after solution treatment and quenching, is quite homogeneous, i.e. that no tendency to the clustering of cupper atoms for this state is observed, but after ageing the material seems to contain small clusters of cupper atoms precursory to the GPZ formation. These clusters have not a sufficient size for to be detectable and observable in experimental analysis by atom-probe. As for aluminum, we have confirmed that it plays an unfavorable role in the formation and the precipitation of GPZ by slowing down the clustering of Cu atoms in small clusters and consequently slowing down the kinetics of precipitation of these GPZ.