Al-Fe alaşımlarının hızlı katılaşması
Rapid solidification of AI-Fe alloys
- Tez No: 68916
- Danışmanlar: PROF. DR. M. NİYAZİ ERUSLU
- Tez Türü: Yüksek Lisans
- Konular: Metalurji Mühendisliği, Metallurgical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1997
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Metalurji Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 94
Özet
ÖZET Hızlı katılaşma ile ince tane boyutu, yan kararlı fazlar, amorf fazlar, yüksek oranda aşırı doymuş fazlar ve ince dağılmış ikincil faz parçacıkları içeren fazlar elde edilebilmektedir. Hızlı katılaşma ile elde edilen nihai ürünler bileşim üniformluğu, ince tane boyutu, yan kararlı fazlar, amorf fazlar, yüksek oranda aşın doymuşluk ve ikincifaz parçacıkları ile karekterize edilebilir. İkinci faz parçacıkları tanelerin büyümesini engellediği için hızlı katılaşmış metal ve alaşımlar özellikle yüksek sıcaklık uygulamaları için ideal malzemelerdir. Hızlı katılaşmış alaşımların korozyon dirençleri geleneksel yöntemlerle üretilmiş malzemelere oranla mükemmeldir. Ayrıca hızlı katılaşmış alaşımlar doğrudan sıvı halden katı hale geçtiklerinden haddeleme, dövme gibi ara işlemler ortadan kaldırılmış olur. Genel katılaşma prensipleri çerçevesinde hızlı katılaşma olayı tanımlanarak; hızlı katılaşma teknikleri tanıtılarak, melt spinning yöntemi ayrıntılı olarak açıklanmaktadır. Alüminyum alaşımlarının mekanik özelliklerini etkileyen en önemli metalurjik faktör ikinci faz partiküllerinin şekli, boyutu ve dağılımıdır. Bu tez çalışmasında Al-Fe ikili alaşımının kokil döküm sonrası özellikleri ile hızlı katılaşma sonrasındaki özellikleri incelenmiştir. Hızlı katılaştırılmış Al-Fe numunelerin mikroyapıları, kokil haldeki mikroyapılarla karşılaştırıldığında tüm alüminyum-demir bileşiklerinin mikron boyutunda homojen olarak dağıldığı tesbit edilmiştir. X-Işınlan çalışmasından M. S. şerit numunelerinde yüksek hızlı katılaşmanın sağlanmış olduğu ve dolayısıyla Fe elementi Al içinde Al3Fe fazı şeklinde çok ince ve homojen olarak disperse olduğundan, AbFe pikleri görülmemiştir. Al-Fe alaşımı kokil numunelerinde sertlik %lFe' li numune de 115Hv, %3 Fe'li numunede 140Hv ve %5 Fe' li numunelere 185Hv' dir. Hızlı katılaştırılmış numunelerdeki mikrosertlik incelemeleri de kokil sertlik özelliklerine paralel olacak şekilde %lFe' li numune de 30,5Hv, %3 Fe'li numunede 31,3Hv ve %5 Fe' li numunelere 49,8Hvdir. Uygulanan pota üst basıncının değiştirilmesiyle elde edilen şeritlerde artan gaz debisi ile şerit kalınlığının 8 lt/dk ' lık debiye kadar 10 um ' dan 140um ' a arttığı ve artan debilerde de şerit kalınlığının 70um ' a düştüğü belirlenmiştir. Elde edilen pota üst argon debisi sabit tutularak pota-bakır disk arası mesafe değiştirilmiş ve 2 cm ' e kadar şerit kalınlığında artış gözlenmiş, 2.6 cm 'lik pota-bakır disk mesafesinden itibaren şerit kalınlığının azaldığı, 3.5 cm 'de ise ortalama şerit kalınlığının 62 um olduğu tesbit edilmiştir. ıx
Özet (Çeviri)
SUMMARY Rapid Solidification Processing has been popular recently. Novel structures are being studied and new technologies are being developed. Detailed consideration is given to the interplay of supercooling and rapid heat extraction rate in cooling the mode and kinetics of solidification processes. Achievable external heat extraction rates impose strict limitations on solid liquid interfacial velocities in the absence of bulk supercooling. On the other hand, in the case of supercooling the recalesence rate at the interface almost always predominates the external heat extraction rate. Particular attention is directed to the resulting degrees of solute partitioning at the moving solid liquid interface and also to the special conditions which achieve partitionless solidification. The required conditions for the latter are the interfacial velocity which exceeds the diffusive velocity of solute in the liquid. The complete trapping of solute elements in the solidifying phase is potentially of major significance from the standpoint of microstructural control and alloy design because extremely high supersaturations of very stable (low solid solubility) phases can thereby be obtained. Subsequent precipitation leads to ultrafine dispersions of second - phase particles which resist coalescence and resolution in the solid matrix at elevated temperatures. Such dispersions are unusually effective in pinning grain boundaries and inhibiting grain growth. In dealing with rapid solidification phenomena, it is well to recognize to conditions of extreme heat flow. In the first, the solid - liquid interface moves into a super heated melt, and the latent heat of fusion is withdrawn through the solid. Typical cases are atomization, melt spinning and surface melting when there is no bulk supercooling. The resulting achievable interfacial velocities are then limited by the rate of external heat extraction and the thickness of the material being processed. The second situation involves solid growth into a supercooled melt, where most of the latent heat of fusion can be absorbed by the liquid itself and so the external rate of heat extraction is less important. Sufficiently high interfacial velocities can be attained in this type of solidification to reach complete solute trapping. Metallurgical factors have a great deal effect on the control of microstructure of alloys. The first detailed investigation on rapid solidification has been performed by Duwez and his Co-workers. But the first study on the effect of rapid solidification on modification of alloy structures has beenaluminum alloys. The Al-Fe systems were not received any attention in this respect. In the present study, the melt spinning process were preferred to investigate microstructure and mechanical properties of rapid solidified Al-Fe alloys. The solution range of Al-Fe system is rather limited. This can not be enlarged by conventional casting methods. The enlargement is only possible by rapid solidification. The first step of the experiment is to remove residual materials from the used crucible and the stopper, standard specimens with 75 gr weight and a hole in the middle (horizontally) used for all experiments. Second step is to place the specimen into the Ffigh Frequency Unit and stopper pass through the hole and plugs the hole of the crucible. The liquid metal alloys prepared in the graphite crucible 1 cm external diameter and 10 cm in length heated by induction. By introducing Argon gases into the system, the inert welting atmosphere was obtained. The liquid alloy heated 50°C above the liquidus and kept for a period of 5 minutes in order to reach equilibrium conditions. Then lifting the stopper a stream of molten alloy ejected by pressured argon gas with 15 lt/min. The liquid metal is brought in contact with the metallographically polished copper substrate wheel rotating with a speed of 3000 rpm. The liquid metal puddle forms on the rotating copper substrate and the ribbon is formed from the puddle. The ribbons produced into the refrigerator to avoid aging. The size of ribbons is 20-60 p,m in thickness, 2-5 cm in width and 1000-1500 mm in length. The optical microstructure of the rapidly solidified Al-Fe alloys was examined after mounting longititudional section of ribbon and polishing and etching by conventional technique by using 0,5 % HF reagent. The grain structure of the rapidly solidified Al-Fe alloy was observed to be primarily columnar and tilted towards the upstream direction of the liquid metal flow. The distinct solidification microstructure zones were clearly observed. The region nearest the substrate is the chill zone is consist of fine equiaxed grains. The cellular structure grow out of the chill zone. Then the coarse equiaxed structure. These microstructures was found to depend on heat extraction rate which control the solid/liquid interface velocity and temperature gradient at solid/liquid interface and solute content. The second phase particles have been observed in rapidly solidified aluminum alloys with iron content by optical and SEM investigation. Homogenous dispersed second phase particles precipitation were determined. Because of ultrafine homogenous Al-Fe second phase, similar observations were not recorded in X-ray analysis. xiiiperformed by Folkenhagen and Hoffman. It has been shown that the possibility to reach 105K/sec cooling rate for a great deal of alloy systems. The interesting metallurgical structures and important developments for M.S (melt spinning) method has been obtained by Pond's investigation. In conventional casting processes such as sand casting, investment casting, die casting... etc. cooling rate is far less than that of rapid solidification processes. The rapid solidification can be defined as cooling rate exceeds 103K/sec. The techniques to achieve the cooling rates above 103K/sec are melt spinning, melt extraction which produce thin ribbons or flakes (25-100 urn) atomization which produces powder (10-200 j.im) surface melting and resolidification. It is very difficult to determine the exact temperature distribution during rapid solidification of crystalline alloys which involves the quick removal of the latent heat of fusion by a heat sink. Heat flow analysis for rapid solidification rates are investigated by a great deal of workers. The magnitudes of the heat transfer coefficient and the initial supercooling have been examined. Supercooling in turn depends on cooling rate alloy composition and the properties of heterogeneous nuclei. The investigation on cooling rate was the early interest in solidification studies. At the same time TTT diagrams which represantates the solidification kinetic at different supercoolings have been drown to identify the critical cooling rate. It is necessary to determine supercooling together with cooling rate because the amount of the supercooling affects the formation of stable or metastable phases. Therefore, it is necessary to control the supercooling kinetics too. It can be noted from the rapid solidification studies, rapid solidification has a positive effect on the microstructure and the properties of alloys. Rapid solidification is employed for powder or strip production techniques. The most important structural change obtained by rapid solidification is the decrease of micro and macrosegregations as a function of solidification rate. So that is possible to eliminate segregated phases by using rapid xialuminum alloys. The Al-Fe systems were not received any attention in this respect. In the present study, the melt spinning process were preferred to investigate microstructure and mechanical properties of rapid solidified Al-Fe alloys. The solution range of Al-Fe system is rather limited. This can not be enlarged by conventional casting methods. The enlargement is only possible by rapid solidification. The first step of the experiment is to remove residual materials from the used crucible and the stopper, standard specimens with 75 gr weight and a hole in the middle (horizontally) used for all experiments. Second step is to place the specimen into the Ffigh Frequency Unit and stopper pass through the hole and plugs the hole of the crucible. The liquid metal alloys prepared in the graphite crucible 1 cm external diameter and 10 cm in length heated by induction. By introducing Argon gases into the system, the inert welting atmosphere was obtained. The liquid alloy heated 50°C above the liquidus and kept for a period of 5 minutes in order to reach equilibrium conditions. Then lifting the stopper a stream of molten alloy ejected by pressured argon gas with 15 lt/min. The liquid metal is brought in contact with the metallographically polished copper substrate wheel rotating with a speed of 3000 rpm. The liquid metal puddle forms on the rotating copper substrate and the ribbon is formed from the puddle. The ribbons produced into the refrigerator to avoid aging. The size of ribbons is 20-60 p,m in thickness, 2-5 cm in width and 1000-1500 mm in length. The optical microstructure of the rapidly solidified Al-Fe alloys was examined after mounting longititudional section of ribbon and polishing and etching by conventional technique by using 0,5 % HF reagent. The grain structure of the rapidly solidified Al-Fe alloy was observed to be primarily columnar and tilted towards the upstream direction of the liquid metal flow. The distinct solidification microstructure zones were clearly observed. The region nearest the substrate is the chill zone is consist of fine equiaxed grains. The cellular structure grow out of the chill zone. Then the coarse equiaxed structure. These microstructures was found to depend on heat extraction rate which control the solid/liquid interface velocity and temperature gradient at solid/liquid interface and solute content. The second phase particles have been observed in rapidly solidified aluminum alloys with iron content by optical and SEM investigation. Homogenous dispersed second phase particles precipitation were determined. Because of ultrafine homogenous Al-Fe second phase, similar observations were not recorded in X-ray analysis. xiiiperformed by Folkenhagen and Hoffman. It has been shown that the possibility to reach 105K/sec cooling rate for a great deal of alloy systems. The interesting metallurgical structures and important developments for M.S (melt spinning) method has been obtained by Pond's investigation. In conventional casting processes such as sand casting, investment casting, die casting... etc. cooling rate is far less than that of rapid solidification processes. The rapid solidification can be defined as cooling rate exceeds 103K/sec. The techniques to achieve the cooling rates above 103K/sec are melt spinning, melt extraction which produce thin ribbons or flakes (25-100 urn) atomization which produces powder (10-200 j.im) surface melting and resolidification. It is very difficult to determine the exact temperature distribution during rapid solidification of crystalline alloys which involves the quick removal of the latent heat of fusion by a heat sink. Heat flow analysis for rapid solidification rates are investigated by a great deal of workers. The magnitudes of the heat transfer coefficient and the initial supercooling have been examined. Supercooling in turn depends on cooling rate alloy composition and the properties of heterogeneous nuclei. The investigation on cooling rate was the early interest in solidification studies. At the same time TTT diagrams which represantates the solidification kinetic at different supercoolings have been drown to identify the critical cooling rate. It is necessary to determine supercooling together with cooling rate because the amount of the supercooling affects the formation of stable or metastable phases. Therefore, it is necessary to control the supercooling kinetics too. It can be noted from the rapid solidification studies, rapid solidification has a positive effect on the microstructure and the properties of alloys. Rapid solidification is employed for powder or strip production techniques. The most important structural change obtained by rapid solidification is the decrease of micro and macrosegregations as a function of solidification rate. So that is possible to eliminate segregated phases by using rapid xi
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