7075 alüminyum alaşımında RRA ısıl işleminin mikroyapıya etkisi ve kinetik analiz

Abstract

Uçak endüstrisinde 7075 Al alaşımı yaygın bir şekilde kullanılmaktadır. Bu alaşıma uygulanan T6 ısıl işlemi ala şımı gerilmeli korozyon çatlamasına karşı duyarlı kılmakta dır. T73 ısıl işlemi ise gerilmeli korozyon çatlamasına karşı direnci artırırken, mukavemet T6 ya göre % 10-15 kadar düşmektedir. T 6 ve T73 ısıl işlemleri ile kazanılan özellikleri bir arada sağlamak amacıyla son yıllarda yeni bir ısıl işlem olan `Yeniden çözeltiye alma+Yeniden Yaşlan dırma (Retrogression and Reaging) kısaca RRA işlemi dikkat leri üzerine çekmiştir. RRA ısıl işleminde, T 6 durumundaki numuneler önce 160-280°C gibi düşük sıcaklıklarda kısa sürelerde çözeltiye alınmakta (Retrogression) sonra T6 iş lem koşullarında tekrar yaşlandırıl mataktadır (Reaging). Bu çalışmada, yeniden çözeltiye alma sıcaklığı, daha önce yapılmış çalışmalarda özellikleri en iyi şekilde opti mize ettiği saptanan 200°C ve 220°C sıcaklıkları seçi lerek, RRA ısıl işleminin mikroyapı üzerinde oluşturduğu değişim ile yeniden çözeltiye alma işleminin kinetiği araş tırılmıştır. RRA işlemi sonucu oluşan mikroyapı (matriks ve tane sınırları) TEM kullanılarak, çökelti boyutları, çökelti mor-folo jisi, tane sınırlarındaki çökeltisiz zonun genişliği incelenmiştir. TEM mikrograf larının MOP-VîDEOPLAN ile yapılan ölçüm lerinde, yeniden çözeltiye alma süresine bağlı olarak, RRA 200 işlemleri esnasında matriks çökeltilerinin ortalama bo yutlarının 7 nm de 9 nm ye, tane sınırı çökeltilerinin ortalama boyutlarının 28 nm den 40nm ye ve RRA 220 işlemlerinde ortalama matriks çökelti boyutlarının 8 nm den 12 nm ye, ortalama tane sınırı çökelti boyutlarının ise 27 nm den 47 nm ye ulaştığı, tane sınırlarındaki çökeltisiz zonun genişliğinin 30-50nm arasında değiştiği, partikül yoğunluğunun azaldığı, çökeltilerin küresel, levha ve çubuk şeklinde 1' ve 2 çökeltileri olduğu tespit edilmiştir. Bu çalışmada elde edilen sonuçlar diğer araştırmacı ların sonuçlarıyla birarada değerlendirilerek, yeniden çö zeltiye alma esnasında yeralan yayınma kontrollü olaylara açıklık getirilmiştir. Bu sentez, atom boşluğu kümeleri nin RRA işlemi ve malzemenin nihai özelliklerinde önemli bir rol oynadığını ortaya çıkarmıştır. <. ı ;?;

Aluminium alloys of the 7000 series (Ai-Zn-Mg) have been used in air-frame structures -for over 30 years. They provide very high strength and stiffness, but are prone to ex-foliation and stress corrosion cracking (SCO, particularly when aged to the near peak strength T& condi tion. Their resistance to exfoliation and stress corrosion cracking can toe increased by over-aging to the T76 and T73 tempers respectively, but with a progressive loss o-f stregth. For example, the yield strength o-f 7Ö75-T73 is about 10-15 % lower than that o-f 7075-T6. In 1974, a new heat treatment is described, known as ` Retrogression and Reaging ` (RRA) which is thought to be capable o-f providing strength eguivalent to that o-f the 16 temper, together with stress corrosion resistance eguivalent to that o-f the T73 temper. This treatment is app 1 ied ta the material in the 16 condition and consists o-f a short time retregression, or partial solut ion treatment at a temperature in! the range 200-220°C, -followed by reaging using the same conditions used -for the original lb age. Since the importance of the RRA treatment has been recognized only recently, little is known o-f the mecha nism responsible for its benefical effects on both strength and SSC resistance of the 7075 Al alloy. Microstructural studies `of 7075 subjected to RRA heat treatments have been rather limited in both number and scope. The purpose of this work is to examine microstruc tural changes occur ing in the matrix and grain boundary region during RRA treatments in relation to mechanical pro perties of the T6 tempered 7075 alloy. Comparisions bet ween our results and those o-f recently published research are presented and an information on the kinetics of RRA treatment and causes -for the improvement in strength and BCC resistance was discussed. The material used in this study was an extruded com mercial 7075 Al alloy in the T 6 condition obtained -from Seydişehir Aluminum Factory. The nominal alloy composition was 5.30 pet Zn-2.3 pet lig-1.3 pet Cu- 0.25 pet Cr- 0.44 pet Fe-Al. The retrogression heat treatment was performed in the silicon oil bath at 200 and 220 °C for various times up to 40 mine. Reaping at 121DC in a stirred silicon oil bath for 24 hours was done immediately after the retrogres sion treatment. Electrical conductivity measurements were made as a function of retrogression time after the reaging treatment. The conductivity measurement of F;RA samples are higher than that of T6. The conductivity value of RRA which includes retrogression at 200°C for 15 min. and more, and at 220°C for 6 min. and more ' reached the value T73. Hardness measurements were also made as a function of retrogression time before and after the reaging treat ment. During retrogression, hardness falls very rapidly to reach a minimum at about 5 min. for 200°C and 4 min. for 220QC, and then increases to a secondary peak and decreasing again at long times as the material overages. The hardness curve of 220QC was stepper and deeper than 200°C retrogression curve. The general shape of the retrogression curves is similar to the hardness curves obtained by the earlier investigators. It was observed that, hardness of RRA samples is equal to T6 hardness during the first 15 min. for RRA 200 and 6 min. for RRA 220. When retrogression period is increased the hardness value of RRA samples are approach to T73 value. In this investigation the transmission electron microscopy was used. The technique described by Gjonnes ?and S i men sen. and DeArdo and Simensen was used to identify the precipitates. Each types of precipitate can be iden tified by analysing a. series of dark-field images taken using various reflections in the indexed difraction pat terns. The analysis of various difraction patterns fromsamples in the RRA temper reveals that the basic patterns are the same as those.from samples in the T 6 and T73 tempers. With increased retrogression time, the difraction pattern of RRA samples shows more distinct spots surrounding the matrix spots. By the analysis of bright and dark field micro graphs belonging to RRA samples, it was observed that the precipitates sizes increase and the number of precipitates decrease with retrogression time. It was also found that the precipitates were nucleated in heterogeneous along dislocations. Around the precipitetes which settles on the dislocation lines, a precipitate free zone could be seen c lear ly » The RRA matrix structure includes 1 and 1 partic les. The morf ologies of precipitates are spherical » plate shape and lath type. It seems that spherical shaped particles become coarse with increasing retrogression time: under the higher magnification it was observed clearly that the plate shaped particles become larger than the spherical ones « The RRA matrix precipitates size were measured by using dark-field micrographs. For each sample at least four dark-field migrographs were evaluated and 250 particles were measured by using MOP-VIDEOPLAN. By evaluation of the histograms it has been determined that the average size of particles from 7,1' nm to 9 nm for RRA 200 for 5 min. and 30 min. retrogression time respectively, and from 8 nm to 12 nm for RRA 220 for i min. and 20 min. retrogression time respectively. Since it is known that the resistances to stress corrosion cracking at the tempered condition is dependent on grain boundary precipitates, and precipitates free zone (PFZ) width. The grain boundary precipitates have been further investigated. RRA samples were tilted in transmission elctron microscopy so that grain boundaries appeared and then grain boundary precipitates at RRA temper were analised in compare to retrogression time. Brain boundary precipitates consisting to 1 particle have been formed. During the tempers no major change has been noticed in precipitate free zone width. Precipitates free zone width had changed between 30-50 nm. The grain boundary precipitate sizes rapidly increased in short time while number of grain boundary precipitates decreased. ; 1 1For each RRA samples at least -five grain boundary micrographs were evaluated by using HOP-VIDEDPLAN. As a result of this study, a particle size distribution histog ram was obtained from this histogram, it seems that average size of grain boundary particles -from 28 nm to 40 nm for RRA 200 -for 2 man» and 5 min. retrogression time respectively, and from 27 nm to 47 nm for RRA 220 for 1 min. and 20 min. retrogression time respectively. The parameter which have equal importance on resistance of stress corrosion cracking compared to grain boundary precipitates sise are the number of particles per unit area (Nq), and the areal fraction of the grain boundary covered by particles (Aq) was measured at the same a.re&« The volume of grain boundary precipitates per unit grain boundary area, Vq, can also be calculated using the measured parameter A^ and N^. The kinetics of retrogression has been investi gated with the help of the relation between retrogression stage and temparature. Based on the results of the hardness measurements, reciprocal of the time to reach the in the minimum in retrogression curve is plotted as a function of reciprocal retrogression temperature» From this plot the apparent activation energy has been calculated as a 11.5 kcal/mol for the initial stages of retrogression. This value is close to the migration energy of ` Zn-vacancy ` pair and also activation energy for the migration of single vacancies in aluminium. The same graph had been obtained considering the inflection points from minimum to secondary peak of retrogression curve which gave an activation energy approximately 17.5 kcal/mol a value which compares closely to that of migration energy for `Mg-vacancy ` pair and formation energy of vacancy in pure aluminum. Arrhenius plot for secondary peak was also considered which yielded an activation energy of 30 kcal/mol, a value which compares closely to that of 2n diffusion and for Mg diffusion energy in aluminum based all oys. The data obtained in this study shows1 that coherency between precipitates ( SP or 1' ) and matrix in T6 structure has been decreased by Zn-vacancy motion, and same time because of high temperature many vacancies have migrated and gathered and then become the nuclation sites during initial retrogression stage. Since retrogressed structure has included many vacancies for nu.cleation, reaging temper has made the partide distrubited more -finely in comparision to T6, and there-fore the strength and hardness of the RRA tempered samples is expected higher' than T& in during the initial stag© of retrogression » According to data obtained -from this study and the earlier investigations it is believed that mechanical properties of RRA temper depends on size of precipitates (matrix and grain boundary ) and stress corrosion resistance to cracking is related to the amount and distrubition of vacancy clusters. This approach supports that a hydrogen embrittlement mechanism of stress corrosion cracking may be assin.qed to this alloy system. '. ).; i v >