Al- %3 li-%0.15 Zr alaşımının mekanik davraşına yaşlanmanın etkisi

Abstract

Enerji kısıtlaması, diğer alüminyum alaşımlarına nazaran hafif Dİan Al-Li alaşımlarını, bilhassa, uçak sanaiinde Sn plana çıkarmıştır. Al-Li alaşımları, düşük yoğunluk ve yüksek elastisite modülü değerlerine sahip olmasına mukabil düşük süneklik ve kırılma tokluğu değerlerine sahip olması dolayısıyla kırılma mekaniği bakımından dikkatli olmayı gerektirmektedir. Bu alaşım sisteminin mekanik özelliklerinin belirlenmesi ve tokluğunun yükseltilmesi amacıyla yoğun bir araştırma başlatılmıştır. Bu çalışmada, döküm yolu ile elde edilen ingot halindeki Al- %3 Li-%D,15 Zr alaşımı sıcak haddelemeyi takiben, 55D°C' de çözeltiye alma işlemine tabi tutulmuş, müteakiben de 190 C'de değişik sürelerde yaşlandırılmıştır. Yaşlandırılan alaşımın, mekanik özellikleri, deformasyon davranışı ve korozyon direnci yaşlandırma süresinin fonksiyonu olarak incelenip metalografik etüdü yapılmıştır. Mekanik özelliklerin belirlenmesi amacıyla yapılan sertlik ve çekme deneylerinden, alaşımın 0ldukça erken yaşlanıp, lityum ihtiva etmiyen alaşımlara göre daha yüksek akma, çekme ve % uzama değerlerine sahip olduğu tesbit edilmiştir. %0.15 mertebesindeki zirkonyumun yeniden kristalleşmeyi önemli derecede etkilediği ve yeniden kristalleşmemiş taneler içerisinde tali taneler oluşumunu sağladığı bulunmuştur. Çekme deneyi numunelerinin kırık yüzeylerinin SEM'de incelenmesi sonucu, alaşım içerisinde bulunan empirute elementlerinin artan yaşlandırma süresi ile intermetalik inklüzyonlarda toplanarak daha kompleks inklüzyonların oluşumuna sebebiyet verdiği belirlenmiştir. "Strain Gauge" kullanmak suretiyle alaşımın elastisite modülünün yaşlandırma süresinin fonksiyonu olarak değiştiği, maksimum sertlik ve çekme mukavemeti durumunda en yüksek değerinde olduğu tesbit edilmiştir. Alaşımın deformasyon mekanizması hakkında fikir edinmek amacıyla değişik sürelerde yaşlandırılan alaşımlarda yapılan deformasyon hızını değiştirme deneylerinden, etkin gerilme, iç gerilme ve aktivasyon hacmi gibi önemli deformasyon parametrelerinin plastik deformasyon oranı ile ilişkisi incelenmiştir. Bu incelemeler sonucunda, az yaşlandırılmış alaşımda artan plastik deformasyonla dislokasyon yoğunluğu azalırken, aşırı yaşlandırılmış alaşımda sabit kaldığı ve az yaşlandırılmış alaşımda belirli plastik deformasyon oranından sonra çökelti partiküllerinin dislokasyonlar tarafından kesildiği saptanmıştır. Potansiyodinamik polarizasyon ölçümü metodu ile, alaşımın korozyon direncinin yaşlandırma zamanının fonksiyonu olduğu ve özellikle aşırı yaşlandırma ile azaldığı belirlenmiştir.

Since lithium is the lightest metal, its addition to aluminum significantly reduces density. Lithium is one Df the just eight elements, whose solid solubilities in aluminum exceeds one atomic percent. Therefore, the production and development of Al-Li alloys has been con sidered especially in aerospace application for long times. The interest in these alloys has been increased after it has found that lithium addition to aluminum alloys not only reduce the density of alloy, but at the same time the strength and especially elastic modulus of alloy increase significantly. The first production of Al-Li alloys for active using in aerospace applica tions has been succeeded in 195B. The: research in this subject to get better results still continues in U.S.A., U. H., U. S.S.R. and in many other countries. Increased fuel costs in recent years have led to renewed interest in the weight reduction at aircraft, that might be ? achived with the use of these alloys. Precipitation reaction in binary Al-Li alloys, which are interested by metallurgists, are confined to the region of the phase diagram containing up to s^^k at.%Li. Above this composition total lithium solubility can not be maintained in the solid state. When an alloy conv taining sufficient amount of lithium is quenched from the solid solution region and subsequently aged below metastable solvus, decomposition Df supersaturated solid solution takes place by homogeneous precipitation of ordered ^'(Al3Li) phase. The phase has an L12 type superlattice structure and spherical shape Possessing a cube/cube orientation with respect to the matrix. After formation of the metastable ^' precipitation, subsequent ageing below the 6X solvus leads to the formation ofi equilibrium £ (AlLi) phase within the matrix and at the grain boundaries in plate-shape phase. The increase in strenght, specific stiffness and elastic modulus of Al-Li alloys is due to precipitation VI of a high density of coherent <$*' precipitate. But, compared to conventional aluminum alloys, Al-Li alloys have reduced combinations of strength and toughness and strenght and ductility. Although, prior cold deformation before ageing and the alloy additions in this alloy system increase tensile and yield strengths, the fracture toughness and ductility in these alloy systems still cause considerable problems. The primary phenomenon, which appears to dominate the ductility and fracture characteristics is the tendency toward strain localization. In underaged and peak aged Al-Li and Al-Li-X alloys, the shearable nature of the <£'(Al3Li) precipitates tends to localize the strain in intense bands of deformation, which acts stress concen trations at grain boundary triple points. Cracks can then nucleate at these triple points and propagate :; intergranularly. On the other hand, over ageing results in a micrDstructure, which contains precipitates free zones with coarse grain boundary precipitates. Strain localization occurs in the precipitates free zones and cracks can than nucleate at grain boundary precipitates free zones. Other factors, which are responsible for the low fracture toughness and ductility, are the inter- metallic compounds and the elements, which segregate along the grain boundaries. Depending on the, characteristics of alloying element, the strength and ductility of this allay system can be increased by reducing the local stress concentration. The alloying elements can provide this by different ways such as:. '-:.' a)- Reducing and dispersing the intense slip during deformation by dispersoids forming, b)- Inhibiting the recrystallization, c)- Decreasing the pile-up length of dislocations by reducing the grain size. The strength of an alloy is related to the resistance to the motion of dislocation. The increase in the flow stress cf a precipitation hardening alloy is due to the interaction of dislocation with zones and precipitates. The precipitates can act as strong, impenetrable noncoherent particles through which the dislocations can Vll move only by sharp changes in curvature of the dislo>-i. cation line. On the another hand, they can act as coherent and incoherent particles, through which dislo cations can pass, but only at stress levels much above those required tD move dislocations through the matrix. Second phase particles act in two distinct 1 ways to retard the motion of dislocations. The particle either may be cut by the dislocations or the particles resist cutting and the dislocations are forced to by pass them. When particles are small and /or soft, dislocations cut and deform particles. In the case of shearable particles, after a certain amount of deformation, the particles can not act as a barrier to the dislocations movements. When a load is applied to a crystalline material, it generally deforms, first elastically and then plas tically. This plastic deformation is a dynamic process which is related with mobile dislocation density and the length of dislocation motion. The plastic deformation rate depends Dn mobile dislocation velocity and density. While mobile dislocation density depends upon the amount of deformation, dislocation velocity depends upon the temperature and applied stress. Applied stress is not consumed wholly in active deformation of metals, the great part Df it is given to overcome some barriers which are called "internal stress". Thus, it is believed that crystalline materials undergo plastic deformation by means Df effective stress, which is a part of applied streBS. The internal stresses are related to micro*, structure of materials, so that the value and change in internal stresses give us some idea about the micro-,. structure and deformation behavior of materials. Thei internal stress occured during plastic deformation can be determained by making some assumptions and utilizing mechanical tests. It can be determined at tensile or creep tests by "stress reduction" and at the tensile or compression tests by "stress relaxation" and "strain rate change" tests methods. It should be mentioned that, the mobile dislocation density and internal stress have to be assumed constant during testing. In addition to effective and internal stresses, the activation volume, which is an important deformation parameter, also can be obtained by strain rate change tests. Activation volume is an important quantity, which can give valuable informations on the deformation mechanism because it has a definite value and stress dependence for each atomic process. Thus, the plastic deformation behavior of metals can be interpreted with some deformation parameters, which can be obtained from mechanical tests. The purpose of the present investigation was to study the general mechanical properties and deformation Vlll behavior af Al-3% Li-0,15% Zr by utilizing mechanical tests. The effect af little addition of zirconium to the Al-Li alloys as an alloying element also has been investigated. For this purpose, the changes in the mechanical properties of alloy with ageing time has been investigated. The elastic modulus and corrosion resis tance of the alloy with respect to ageing time, on which there are no satisfactory data in literature have also been investigated. The material used in this investigation, was hot rolled to 2.5 mm thickness, Al-Li-Zr sheet alloy with a composition of 3.00% Li, 0.15% Zr, 0,07% Fe, 0,004% l\la, 0,001% H were produced by ingot metallurgy route at ALCAIM company before hat railing. Hat rolled allay was solu tion treated at 550 C far 35 minutes and subsequently water quenched at room temperature. The ageing treatment was performed at 19Qİ1DC in protective oil atmosphere (silicon oil bath) for different times. The tensile properties of aged alloy were determined at in initial crass head speed of 0,5 mm/min. The tensile tests were carried aut by using bath instran universal testing machine and computer controlled Zwick testing machine at roam temperature tensile test spe cimens were prepared according to ASTM E8. While the elongation of specimens were determined in 1/100 sensi tivity range by using extensometer, the elastic deforma tion magnitude in order to obtain the elastic modulus of allays were determined in 5/1000000 sensitivity range by using strain gauges. The fracture surface of specimens fractured during tensile tests were examined in Scanning Electron Microscope to obtain fracture modes of deforma tion and, probable, intermetallic compounds. The inter- metallic inclusions, which were found at the fracture surfaces, were analyzed by semi-quantitative X-ray analysis, m The initial strain rate in strain rate change tests was selected as 0,05 mm/min. and kept constant for all tests. During deformation of a sample at initial strain rate, the rate was changed suddenly by keeping the the inc rement of strain rate constant ( £2/ i^ 0, ^D=CQn.) at certain elongation values in plastic deformation region and again was brought into its initial rate. The process was repeated several times until the end of test. The strain rate change tests were carried out for each specimens groups that were heat treated at spesific conditions involving#a change in strain rate by factors of 4,10,20 and 40 ( £ 2/ £] =4,10,20,40). Borne important deformation parameters were obtained from deformation rate change tests such as effective stress ( f**), average IX internal stress ( Ç~. ), dislocation velocity stress exponent (m*) and activation volume (I/*). The corrosion test were performed in the standard polarization cell and 3,5 %[\laCl solutions by potentio- dynamic polarization measurements'. The solution was deaerated by bubbling argon gas prior to and during each scanning. The potentional scanning rate was Di,i2 mv/sEC. for all measurements. The experimental results, that were obtained from this study are belou: 1)- The addition of little amount zirconium as alloying element (G,15 %Zr) to the Al-Li alloys leads to considerable amount of early ageing by affecting the ageing kinetics of this alloy systems. Al-3, DD%Li-D, 15% Zr, alloy reaches to the peak age state by ageing 6,5 hours at 19DDC. 2)- The amount of zirconium as much as D.15% retards the recrystallization in great proportions. The alloy is strengthened by subgrain formations. The aged Al-3%l_i- G,15 %Zr alloy has rather higher tensile, yield strength and elongation values than that of the zirconium free binary Al-Li alloy. 3)- With increasing ageing time the impurities, which exist in the alloy, are collected around in inter- metallic inclusions causing the formation of more complex intermetallic inclusions. These inclusions af f ect,ori Ithe strength of alloy. k)- The elastic modulus of alloy depends on precipi tates, which are responsible for strengthening mechanism. The changes in elastic modulus of alloy uith ageing time is directly proportional to changes in hardness and ten sile strength values of alloy. The elastic modulus of Al- 3, DO %Li - 0,15%Zr alloys shows maximum in peak age heat treatment condition- 5)- The alloy has 14 % 'higher elastic modulus value than that of 7D75-T^ conventional aluminum alloy in peak age heat treatment condition. 6)- The alloy has 25% higher specific elastic modu lus than that of 7G75-T, conventional aluminum alloy in peak age heat treatment condition. 7)- During the plastic deformation of underaged alloy, the increasing strain decreases the mobile dis location density in the results of dislocation-disloca tion interaction mechanism. In the case of over ageing, the mobile dislocation density stays constant with increasing plastic strain ratio. 8)- At the underaged allays, the average internal stress values show a declined increase with increasing strain. After a certain amount of strain (5-6%), the internal stress makes a sharp decrease however, in the case of averaged allay, internal strees values increase with increasing plastic strain ratio. 9)- In under-aged alloy, during deformation of the fine precipitates, which are responsible for strengthening, are sheared by dislocations and after a certain amount of deformation, these precipitates can not act as a barrier to the dislocations movements. 10)- The corrosion resistance of Al-3,00 %Li-0,15%Zr alloy is a function of ageing time. There is not a con siderable difference between the underaged and peak aged alloys in terms of electrochemical data. The corrosion resistance of allay decreases with aver -ageing.