Formation of an Age Hardening Structure

Cu-Ti alloys present an important category of alloys that combines good thermal resistance and high mechanical strength properties that make them attractive for different applications such as high-density miniaturized contacts. Over the years, extensive research work has been conducted to improve the preparation and performance of Cu-Ti alloys. In particular, these studies have mainly focused on the development of spinodal decomposition-based theory which has been very instrumental in the studies of Cu-Ti alloys. For instance, the currently available formation structure model comprises of spinodal decomposition in the early stages of aging followed by the transformation of the Ti-rich face-centered cubic disordered region to fine needle-shaped precipitates. Despite the remarkable research efforts, the mechanisms for the formation of the hardening structures of Cu-Ti alloys have not been fully explored.

In a new effort to provide more insights on the age hardening of Cu-Ti alloys, Dr. Kazuhiko Fukamachi from JX Nippon Mining and Metals Corporation in Japan investigated the age-hardened structure of Cu-4at Ti alloys. The main objective was to identify the hardening structures as well as the underlying formation mechanism of the alloy system, which is deemed important in enhancing the performance of such alloys. In this approach, a combination of high-resolution scanning transmission electron microscopy (STEM) and X-ray diffraction (XRD) techniques were used to identify the structures and the formation mechanisms. The research work is currently published in the journal, Materials Science and Engineering A.

Results showed that the formation of ordered phase domains took place in two main stages. During the first nucleation stage, ordered α-Cu4Ti phase D1_a domains with short-range-ordered superlattice characteristics were formed during quenching following the treatment of the solution. This was attributed to the isotropic random diffusion of Ti due to the reaction-diffusion mechanism. On the other hand, the second stage was characterized by coarsening of the structure, also known as Ostwald ripening. Ostwald process was initiated by anisotropic diffusion due to the variation in the radius of the fine domains. A sharp decrease in the number of domains was observed during the early stage of aging. The decrease continued until the peak aging and over-aging conditions were achieved. D1_a domains produced in the peak-aged sample exhibited diameters in the range 10 – 20 nm and high yield strength of 663 MPa. Furthermore, XRD observations revealed that the satellite spots and the sidebands in the diffraction patterns were produced as a result of the tetragonal distortion of the face-centered-cubic of the sublattice in the alloy. Thus, the formation of the overall structure was governed by a diffusion-controlled spontaneous process. It was worth noting that spinodal-decomposition of Ti-Cu alloys did not take place during the two primary processes. Originally, the main basis for spinodal decomposition to replace classical nucleation theory is the diffuse boundary. In this study, it is of great value to hypothesize that the domain boundary has a width and negative volume free energy, and to explain the rationale logically. This has significantly reduced the reason for the existence of spinodal decomposition.

In summary, the study analyzed the characteristics of age-hardened structure formed in the solution-treated and aged Cu-4 at% Ti alloys. Results showed that the formation of the hardened structure was due to the diffusional transformation comprising of reaction-diffusion mechanism and Ostwald ripening characterized by aging. In a statement to Advances in Engineering, Dr. Kazuhiko Fukamachi noted that the study results would provide more insights in understanding the hardening mechanism and structure of Cu-Ti alloys, that is of great significance in the development of high-performance alloys.