The crystallographic and morphological evolution of the strengthening precipitates in Cu–Ni–Si alloys

Abstract

High-resolution transmission electron microscopy and first-principles energy calculations reveal that, upon formation, the hardening precipitates in Cu–Ni–Si alloys are unchanged δ-Ni₂Si nanocrystals. However, their crystallographic and morphological features evolve during the precipitation process. It is shown that, in terms of crystallographic orientation relationships, there are basically two types of δ-Ni₂Si precipitates in the alloys, referred to as δ₁-Ni₂Si and δ₂-Ni₂Si respectively. In the early stages of aging (including peak aging), the precipitates are small and belong to the δ₁ type, with the following orientation relationship with the Cu matrix: [0 1 0]_δ||[1 1 0]_Cu and (0 0 1)_δ||(0 0 1)_Cu. In the late stages, the precipitates are clearly larger and become the δ₂ type, with the orientation relationship: [0 1 0]_δ||[1 1 0]_Cu and approximately $(301{)}_{\mathrm{\delta }}\Vert (1\overline{1}1{)}_{\text{Cu}}$. Governed by the minimization of its overall energy, a developing δ precipitate has to evolve from an almond-like δ₁ particle with a low-index coherent habit plane to a French baguette bread-slice-shaped δ₂ particle that has a high-index broad interface. This evolution is found to be in excellent agreement with predictions provided by the invariant line theory. Intermediate stages exist for a particle to accomplish such an evolution, leading to many different crystallographic and morphological appearances of these δ-Ni₂Si particles being observed in the alloys.

Introduction

Cu–Ni–Si alloys are important industrial materials that are widely used for electrical connectors and lead frames, owing to their high electrical conductivity combined with high strength. They can be manufactured with relatively easy processes. They are typical of Cu-based alloy systems with a precipitation hardening effect, due to the formation of Ni–Si precipitates in the Cu matrix upon heating. Understanding the precipitation characteristics, such as the structure, morphology, crystallographic orientation relationship (OR) and evolution, of the strengthening precipitates in these alloys is of crucial importance for making improvements in the synthetic processes, and has therefore long been a serious subject for study.

However, there is still debate about the structures of the precipitates in age-hardened Cu–Ni–Si alloys. The hardening precipitates in the alloys were first identified as the δ-Ni₂Si phase according to a quasi-binary section of the ternary Cu–Ni–Si phase diagram [1], [2]. However, this was later refuted following the identification of the precipitates as the γ-Ni₅Si₂ phase [3] and the β-Ni₃Si phase [4], respectively. Further, the precipitates were found to be an unknown orthorhombic intermetallic nickel–silicon compound with a lattice parameter of about four times that for copper [5]. Nonetheless, a number of studies support the existence of δ-Ni₂Si precipitates in the alloys, though other types of precipitates, such as β-Ni₃Si, (Cu,Ni)₃Si, and even Si-rich and Si-poor regions, might coexist [6], [7], [8], [9], [10], [11], [12], [13], especially in the early stages of aging [13].

In addition to the above suggestions about the precipitate structures in the alloys, different suggestions about the crystallographic ORs between the precipitates and the Cu matrix have also been made. The first OR proposed is the following [6]: (0 0 1)_ppt‖(1 0 0)_m and [3 1 0]_ppt‖[0 3 1]_m (where ppt denotes precipitate phases and m stands for matrix), which is similar to the OR of the precipitates in the Cu–Be alloys [14]. The second OR suggested is as follows [9], [15]: (0 0 1)_ppt‖(1 0 0)_m and [0 1 0]_ppt‖[0 1 1]_m, which was claimed to be applicable not only to the late-stage precipitates in the alloy (aged for 1000 h at 450 °C), but also to the early stage precipitates as well (aged for 1 h at 450 °C). Furthermore, a few recent studies have suggested another OR, specifically for δ-Ni₂Si precipitates [11], [12], [16]: (2 1 1)_δ‖(1 1 0)_m and $[\overline{3}24{]}_{\mathrm{\delta }}\Vert [\overline{1}12{]}_{\text{m}}$.

Such significant controversies about the precipitation behaviors of the alloys exist largely due to the following technical problems encountered in the characterization of the small-sized precipitates embedded in the Cu matrix. (i) Nearly all the characterization tools used in these investigations were conventional methods, such as X-rays diffraction, selected-area electron diffraction and conventional transmission electron microscopy (TEM), used in the mass-contrast and diffraction-contrast imaging modes without sufficient resolution. These tools are unable to resolve the finest details of small precipitates. (ii) The early stage precipitates in the alloys are very small (1–3 nm), and orient in several different directions. They often generate very weak diffused diffraction streaks rather than sharp spots, or generate secondary diffraction spots because they are often covered by the matrix. Thus accurate structural information about these particles is difficult to obtain. (iii) The precipitates change from the early stage to the late stage at their own speeds of evolution, so different types of precipitates may coexist in the samples under investigation. This would result in confusing diffraction spots and morphological images that could easily be misinterpreted in terms of structures, crystallographic ORs and morphological features. Hence a systematic study of this important alloy system by advanced high-resolution TEM (HRTEM) is needed in order to understand the precipitation behavior and to clarify the existing controversies about the strengthening precipitates in the alloys.

In the present study, HRTEM was employed in association with image simulations and first-principles energy calculations to investigate the strengthening precipitates in a typical Cu–Ni–Si alloy. Similar methods have successfully been used in the studies of the tiny hardening precipitates in Al alloys [17], [18], [19], [20]. The precipitates in this alloy were studied by following their evolution path from the early stage to the late stage, in order to characterize all their changes in structure, crystallographic OR and morphology. It is shown that, upon their formation at a very early stage (<10 min at 450 °C), the strengthening precipitates already possess a δ-Ni₂Si crystal structure. These particles keep this crystal structure unchanged until the very late stage (>100 h at 450 °C). Nevertheless, they change in orientation, size and shape, leading to their rich appearances observed in the precipitation process.