Preparation and properties of ultra-fine-grained and nanostructured copper alloy with the addition of P
Graphical abstract
Introduction
Copper is widely used in the field of electronic appliances because of its good conductivity and fatigue resistance [1]. However, high mechanical strength is also a necessary condition for the Cu alloys' further application. So Cu-Fe-P alloys are widely concerned in the lead frames of integrated circuit because of their excellent electrical conductivity, good combination of mechanical and physical properties, ease of manufacture, and low cost [[2], [3], [4], [5], [6]]. It accounts for more than 65% of the total amount of lead frame materials. It has been frequently reported that Cu−Fe−P alloys are strengthened by precipitation hardening. Though the iron doped in the alloy can increase the tensile strength, it will also lead to a decrease of the conductivity. Therefore, it is very important to reduce as much iron in the copper matrix as possible [7].
Adding other alloying elements may be one way to reduce the content of iron in copper and improve the conductivity of the Cu alloys. P is a comparatively cheaper alloying element (compared with Ag, Ni, etc) in industry. And some studies found that Fe and P can form precipitates such as Fe3P or Fe2P during aging, which increases the strength and conductivity [5]. Despite all this, the contents of Fe and P should be limited to a very low level to maintain a high conductivity, which will result in low volume fraction of precipitates in the matrix and therefore low response to precipitation strengthening [3]. So many investigations of assisted enhancement methods have been conducted to increase the mechanical strength with a relatively high conductivity such as cold working and fine particles strengthening methods [8].
Fine-grain strengthening can be used as an assisting strengthening method because of its high response to strengthening and little harm on conductivity. And more attention has been paid to the ultra-fine grain structure recently because it has much more potential to improve the mechanical properties than the coarse-grained materials [[9], [10], [11], [12], [13], [14]]. Previous studies have shown that metallic oxide and alloys with ultra-fine grain and nanocrystalline structure can effectively improve the properties [[15], [16], [17], [18], [19], [20], [21]]. And for Cu-Fe-P alloys, the electrical conductivity is just slightly reduced with a significant increase in strength. The ultrafine grain structure can be obtained by mechanical alloying (MA) and reactive hot-pressing (RHP) [22,23]. Some researchers also use green synthesis methods to prepare nanomaterials [[24], [25], [26], [27]].
MA is a solid powder processing technique in a high-energy ball mill which involves repeated welding, fracturing, and re-welding of powder particles. It can be used to produce nanocrystalline materials due to the remarkable deformation introduced into the powders and the low working temperature essential in keeping the nanocrystalline structure [28,29]. However, there will be iron introduced into the alloy during in the process of MA. So trace amounts of phosphorus were used to improve the performance. And as a sintering method, the advantages of reactive hot-pressing (RHA) lie in the short sintering time and low sintering temperature. So it could be used to prepare compact alloy block with ultrafine microstructure through inhibiting grain growth [[30], [31], [32], [33], [34], [35], [36]].
The main objectives of the present work are: (a) to synthesize Cu-Fe-P powders with ultra-fine grain by MA, (b) to sinter nanostructured Cu-Fe-P alloys by HRP, (c) to study the effect of trace addition of P, content of Fe and sintering temperature on the microstructures of Cu-Fe-P alloys, and (d) to obtain the optimum experimental conditions through analyzing the relationship between microstructure and performance.
Section snippets
Preparation
Copper (48 μm, 99.9 wt.% purity) and phosphorus (3 μm, 99.9 wt.% purity) were used as raw materials. Then the powders were processed by mechanical alloying. MA was conducted in a high-energy ball mill (Model: GN-2). The powders and steel balls were placed in a stainless steel jar under argon atmosphere. The process control agents (PCA) were added to minimize agglomeration. The ball-to-powder weight ratio was 15:4. The milling time speed of jar was 532 rpm. The milling time of the powders was
Analysis of powder
Fig. 1 shows the XRD patterns of the mixed powders with different P content. It can be seen that only diffraction peaks corresponding to copper are observed in the patterns. And there is no obvious difference in peak width and intensity of all copper diffraction peaks. As the phosphorus content increases from 0 to 0.040 wt.%, the average grain size of the mixture powder changes a little (31–37 nm). This indicates that trace addition of phosphorus will lead to a little increase in grains. The
Conclusions
Nanostructured Cu-Fe-P alloy with crystallite size less than 10 nm was successfully prepared by mechanical alloying (MA) and reactive hot-pressing (RHP). The SEM and TEM results confirmed the formation of ultra-fine grain and nanoscale structure. The addition of P could reduce the content of iron doped in the Cu matrix. It did not form precipitates such as Fe3P or Fe2P with Fe. And it showed a positive effect on the performance of Cu-Fe-P alloy. The optimum content of P was 0.025 wt.%. The
Declarations of interest
None.
Acknowledgements
This work was supported by the National Natured Science Foundation of China [grant number 51702314]; Science and Technology Development Project in JiLin Province [grant number 20170520154JH]; and the Funds for Creative research Groups of China [grant number 21521092]; Key Scientific Technological Project of JiLin Province [grant number 20150204002GX].
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