Bakır-kobalt-silisyum alaşımlarında yaşlanma işleminin sertlik ve elektrik iletkenliğine etkisi

Abstract

Bu çalışmada, farklı bileşimlerde kobalt ve silis yum içeren bakır alaşımlarında yaşlanma sıcaklığı ve süresinin alaşımların sertlik, elektrik, iletkenliği ve mikroyapı üzerindeki etkileri incelenmiştir. Deney sonuçlarına göre günümüzde kullanılan diğer sert bakır alaşımlarına seçenek oluşturabilecek alaşım üretimi amaçlanmıştır. Bu alaşım sisteminde çökelme prosesi konusunda yapılan çalışmalarda çökelmenin 400-600°C sıcaklıkları arasında kobalt ile başladığı bildirilmektedir. Bu çökelme reaksiyonu silisyum varlığı ile hız kazanmaktadır. Silisyum miktarı arttıkça partikül boyutu ve partiküller arası mesefe de artmaktadır. Dökümü yapılmış alaşımlar 9D0°C de 6 saat homojenleştirildikten sonra 1000°C de 1 saat süreyle çözeltiye alınmıştır. Çözeltiye alınan numunelere 400,500,600, 700°C sıcaklıklarında yaşlandırma işlemi uygulanmıştır. Yaşlandırma 1 saatten 5 saate kadar değişen sürelerde uygulanmıştır. Yaşlandırılmış numunelerde yaşlanma koşulları ve bileşimin sertlik, elektrik iletkenliği, mikorayapı üzerindeki etkileri incelenmiştir. Yapılan çalışmalar sonucunda, incelenen alaşımlar içerisinde sertlik değeri en fazla olan alaşımların aynı zamanda maksimum elektrik iletkenliği değerinde olduğu saptanmıştır. Sertlik oluşumuna neden olan çökel ti partikülleri çok küçük boyutlu olduklarından taramalı elektron mikroskobu ile görüntülenememiştir.

There are three important general methods by which the resistance to plastic deformation of a metal crystal may be increased, namely by cold working by solid solu tion strengthening and by precipitation hardening, and may modern high strength alloys depend on the use of one or more of these effects. High strength in most coppers and copper alloys is echieved by cold working. But for certain copper alloys containing small amount of berlyllium, chromium or zirconium or nickel in combination with silicon or phosphourus, unusually high strength and hardness can be obtained by precipitation hardening. All precipitation-hardening copper alloys have similar metallurgical characteristics: they can be solution treated to a soft condition by quenching from high temperature, and then subsequently precipitation hardened by ageing i at a moderate temperature for a time usually not exceeding 3 hours. The chief advantages of these alloys are. 1- Customer fabrication is easily performed in the soft solution-annealed condition. 2- The precipitation-hardening heat treatment performed by the fabricator is relatively simple. It is carried out at moderate temperatures, usually in air; controlled cooling is not needed, and time of treatment is not of critical importance. VI 3- Different combinations of properties, including, strength, hardress ductility, conductivity, impact resistance and anelasticity, can be obtained by varying hardening times and temperatures. The particular reguirments of the application determine the type of hardening treatment. Drought beryllium coppers can develop wide ranges of mechanical properties, depending on solution treating and ageing conditions, on the amount of cold work imparted to the alloy and on whether the alloy is cold worked after solution treating and before ageing or is cold worked after ageing. cast beryllium coppers generally are not cold worked so variations in proper ties can be developed only by varying solution treating and ageing conditions. Chromium coppers containing about 1% Cr are solu tion treated at 950 to 1010 °C and rapidly quenched. Solution treating usually is done in molten salt, but may be done in a controlled atmosphere furnace to prevent surface scaling and internal oxidation. Solution treated chromium copper is soft and ductile ;- therefore it can be cold worked in a manner similar to that used for unalloyed copper. Solution treated chromium copper is aged at kDÜ to 500 °C for several hours to produce the desired mechanical and physical properties' cycle is ^55 C for kb or more. A typical ageing The Copper-Beryllium alloys known as the hardest alloys among the age hardepable Copper alloys, but it is hazardeous. Manufacturing process because of its toxicity and pollutive character. However, hardness values of other copper alloys containing nickel, chromium, silicon zirconium are below that of the copper-beryllium alloy. These alloys of high strenght and electrical conductivity are used as elec trodes in resistance welding an as contactors in electrical application. The purpose of this study is to produce alloys of suitable hardness and conductivity values as an alternative alloys to presently used hard copper alloys by using methods in which the alloys containing Vll different cobalt and silicon contents are subjected to ageing process at various temperatures for various times. A great number of studies have been done on har- denable copper- cobalt-silicon alloys. According to these studuies, the phase formed subsequent to preci pitation is detected as the orthorombic Co"5i. However, the information given for the solubility capability of Co"Si in copper not well understood. The recent studies has shown that the solubility of Co"Si is 19. at % at 1 000°C in copper. According to the studies done by others on this particular alloys, the precipitation process starts with the precipitation of cobalt between 400-600 C, The preoece of silicon in alloy increases the precipita tion rate of ; COpSi^ precipitates particle dimension and the interparticle distance increases as the amount of silicon inceases in alloys. The alloys to be subjected to the ageing treatment are formed from electrolytic copper, metallic cobalt and metallic silicon are added into the melt in the above order. The alloys compositions which is used on ageing treatment are listed below. After the melting procedure is completed the alloy is casted into a graphite mold. The so-casted alloys are covered with graphite powder and homogenized at 900DC for 6 hours. vm To be able to compare the result with similar studies by others 1 000 C is selected as the dissolution temperature. The alloys covered with graphite powder are subjected to the dissolution process for 1 hour at 1000°C. The temperatures selected ageing process are 400, 500,600, 7D0 C. The ageing time varies from 1 hour to 5 hours. The ageing process similar to the homogeniza- tion and dissolution is performed with graphite powder covering over the specimens. The hardness values obtained have been found to very considerably depending on the composition, ageing tempe rature and time. Copper alloys containing low cobalt and silcon showed low hardness values. On the other hand copper alloys containing high cobalt and silicon showed higher hardness values. After the ageing process, the highest hardness were obtained for those treatments performed at 500 C and o o 600 C. The optimum ageing time for 500 C was determined as 1 hour. The heat treatments performed at 400 C yi elded no increase in hardness. The reason is thought to be the lack of Co"Si precipitation process at this temperature samples aged at 700DC on the other hand showed minimum hardness values due to over ageing. There is no obvious correlation between ageing con dition and electrical conductivity measured, however, the low cobalt and/for silicon content alloys indicated higb electrical conductivities. Microstructural observations made with SEM and EDX indicate that hardness observed was due to the presence of particules which were rich in silicon content. Although we did not observed any Co"Si particules in those alloys contained low amount of cobalt and silicon when studied in SEM, there was an increase in hardness after ageing. This would indicate that particules formed are very small size that can not be easily ob served in SEM. Needle like particules were observed in those alloys contained higher amounts of cobalt and silicon, after dissolution process. EDX analysis indi cated that particules are rich in cobalt and silicon content, therefore, these particules may be precipitated out as the solution reach to its saturation limit for cobalt and silicon elements at heat treating tempera tures. ix Although hardness increased in these alloys,. there uas not any noticeable change in microstructure after ageing process. This situation is very similar to low cobalt and silicon content alloys which indicates an increase in hardness due to the precence of very small particules which are usually observed by trans mission electron microscope. In this study, it uas found that the hardest alloys showed the highest electrical conductivity. Copper, cobalt, silicon alloys studies showed comperable proper ties to comercially available hard copper allays shaped by casting, these alloys later can be plastically defor med during processing to cause on increase in hardness, although some decrease in electrical conductivity could be observed. Therefore, it is advisable to study the effect of this variable in Cu-Co-Si alloys in future studies.