Phase Transformations Of Cu-15Ni-8Sn-XSi And Cu-9Ni-2.5Sn-1.5Al-0.5Si Alloys And Their Effects On The Alloy's Properties

By means of measurements of mechanical and electrical properties, metallographical and transmission electron microscope observation, X-ray diffraction and selected diffraction analysis, scanning electron microscope and energy spectrum, the phase transformation process of Cu-15Ni-8Sn alloy and the relation between its microstructure and properties were studied. At the same time, the influence of Si addition on the microstructure and properties in Cu-15Ni-8Sn alloy was studied. The way of Si presence, the effect of Si addition and its mechanism in the alloy were confirmed. At the last, based on microcontent alloying principle Cu-9Ni-2.5Sn alloy with low Sn content was rebuilt by doped Al and Si. The alloy changed from solution strengthening to precipitation strengthening. The way of Si and Al presence and the effect of Si and Al addition in the alloy were analyzed. And the technological parameter of thermomechanical treatment was studied. The results were as followed,1. The phase transformation process of Cu-15Ni-8Sn alloy was studed detailedly. Phase transformation products of the alloy contained spinodal structure, DO₂2 ordered phase, L1₂ ordered phase, discontinuous precipitation Y phase and continuous precipitation. The full phase transformation process was spinodal decomposition→DO₂2 ordering— L1₂ ordering → discontinuous precipitation → continuous precipitation.Based on the phase transformation sequence in different periods and corresponding phase transformation products in the study, TTT diagram of Cu-15Ni-8Sn alloy was drawn.2. Based on the thermodynamics and the kinetics of phase transformation, the relation between spinodal decomposition and oredering and the reason of the ordering occurred after spinodal decomposition in Cu-15Ni-8Sn alloy were analyzed. It considered that the concentration of solute atom needed by the ordering was higher than the average, which can be reached by the composition fluctuation of spinodal decomposition.3. Although their chemical formulas were both the form of (Cux,Nii.x)3Sn, their X values were different. So, both orderings with no subgroup relation can coexist and DO22 ordering occurred before Ll2 ordering. Two orderings both obtained composition fluctuation of nucleation from spinodal decomposition, so there was no inherited relation in structure between them. Based on the electron diffraction feature of three variants of DO22 ordered phase and the directivity of phase transformation stress, the morphology of DO22 ordered phase was speculated in the theory that it was needle-like.4. Besides spinodal decomposition, ordering transition can also produce strengthening effect. The results in the study showed that the strengthening of the ordered phase precipitation for the alloy was muchstronger than that of spinodal decomposition. According to properties, cold deformations prior to aging can increase the hardness of Cu-15Ni-8Sn alloy and accelerate the age hardening. The more cold deformations were, the more remarkable the effect was. According to microstructure, cold deformations prior to aging can suppress the ordering transition and promote the nucleation and the growth of discontinuous precipitation. At the same time the distribution of discontinuous precipitation changed.5. The doped Si in Cu-15Ni-8Sn alloy can refine the microstructure and the grain. However, when the Si content arrived some value (0.3 Si), the action did not increase continuously. The doped-Si combined dominantly with the Ni atoms and formed Ni31Sii2phase and Ni3Si phase. Ni3Si phase had the same structure as the matrix and its lattice constant was also close to the matrix's, so it can mostly refine the dendritic. Ni3Si phase can dissolve and precipitate reversibly with the temperature changing in the alloy. During the ageing, Ni3Si phase precipitated easily in grain boundary, occupied the nucleation sites of discontinuous precipitation and suppressed discontinuous precipitation in the later stage during ageing. When o.3Si was doped, the suppressing effect was the strongest. The suppressing effect was decreased with Si content increasing. The reason was that when the doped Si was too much, Ni3iSii2 phase which difficulty dissolved was easily formed. Ni3]Si]2 phasedistributed not only in grain boundary but also intracrystalline, which provided the nucleation sites of discontinuous precipitation and accelerated discontinuous precipitation. So, properties of the alloy decreased.6. Analyzing the effect of the doped Si on the microstructure and properties of Cu-15Ni-8Sn alloy, we considered that the doped 0.3Si in Cu-15Ni-8Sn alloy was appropriate. It was appropriate for Cu-15Ni-8Sn-0.3Si alloy that the temperature of uniform annealing was 860°C ⁸80°C and the time was 4h—8ho The alloy with 0.3Si can obtain preferable mechanical and electrical properties ageing at 400°C⁴50°C for lh³h after a proper cold deformation prior to aging.7. The doped Si in Cu-9Ni-2.5Sn alloy combined with Ni and formed 8 -Ni2Si phase, which made the alloy change from solution strengtheningto precipitation strengthening. There certainly existed orientation relation between 8 -Ni2Si phase and the matrix: (no)m // (21l) ppt, [110]m//[324]ppt. 8 -Ni2Si phase strengthened the matrix following Orowan mechanism. There existed no Ni2Al phase in the alloy. The doped. Al increased the solubility of Si and improved the effect of precipitation strengthening.8. Compared with traditional technology (hot rolling + solution treatment + cold rolling, ageing), Cu-9Ni-2.5Sn-l.5Al-0.5Si alloyintroduced the simple technology of thermomechanical treatment— 960 °C⁷00°C hot rolling, then 90% cold rolling and 480°C ageing, which can evidently improve the electrical conductivity of the alloy, achieving above 15%IACS. At the same time, the same high hardness as traditional technology's was obtained (HV=380)o9. There still existed serious dendritic segregation of Sn in Cu-15Ni-8Sn and Cu-9Ni-2.5Sn-l.5Al-0.5Si alloy. It was appropriate for Cu-15Ni-8Sn alloy that the temperature of uniform annealing was 860 °C 880 °C and the time was 4h 8h 0 It was appropriate for Cu-9Ni-2.5Sn-l.5Al-0.5Si alloy to anneal uniformly at 900°C— 960 °C for lh⁴h.