Cu ve Ti ile alaşımlandırılmış çinko levhaların (titanzink) kısa dönem testlerle sürünme özelliklerinin belirlenmesi
Obtaining creep behavior and properties of Cu-Ti alloyed zinc sheets  by conducting short term creep and stress relaxation tests
Yazar: OLCAY TÜRKOĞLU
Danışman: PROF. DR. MEHMET DEMİRKOL
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Malzeme ve İmalat Bilim Dalı
Derece: Yüksek Lisans, 2013
Konu: Makine Mühendisliği = Mechanical Engineering ; Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Çinko uygulama alanı çok geniş bir malzemedir. Demir, alüminyum ve bakırın ardından en çok kullanılan dördüncü metaldir. Diğer yaygın metallerle karşılaştırıldığında atmosferik şartlarda korozyon dayanımı ve ekonomiklik bakımından ön plana çıkar. Bu nedenle duvar ve çatı kaplamalarında sıkça kullanılır. Ayrıca parlak yüzeyi de dekoratif bir görünüm sağlar.
Ancak düşük ergime sıcaklığı nedeniyle oda sıcaklıklarında 0.4 benzeş sıcaklık mertebelerindedir. Bu nedenle malzeme oda sıcaklığında sürünme (sünme- creep) davranışlarını da göstermektedir. Ayrıca soğuk şekil görmüş malzemede oda sıcaklığı rekristalizasyona sebep olmakta, soğuk şekil değişimi sonucu meydana gelecek olan dayanım artışını engellemektedir.
Saf çinko malzemenin yukarıda bahsedilen özelliklerini geliştirmek için genellikle alüminyum ve/veya bakır  ile alaşımlandırıldığı görülmektedir. Son zamanlarda daha gelişmiş ve üstün özellikler için çinko alaşımlarında bakır ve titanyumun da kullanıldığı dikkati çekmektedir. (Ticari olarak bu tür alaşımlara Titanzink adı verilmektedir.) Bu alaşımlar sayesinde olası tane irileşmesinin önüne geçilmekte ve belli bir oranda dayanım artışı sağlanmaktadır. 
Genel itibariyle malzemelerin sürünme özelliklerinin belirlenmesi uzun süreler alabilmektedir. Bu durum özellikle çelik esaslı alaşımlar için on binlerce saate varan deneylerin yapılmasını gerektirmektedir. Ancak gerilme gevşemesi deneyleri sayesinde sürünme deneylerinden elde edilebilecek bilgilerin daha kısa sürelerde ortaya çıkarılabildiği de görülmektedir.
Bu çalışmada bakır ve titanyum ile alaşımlandırılan çinko alaşımının oda sıcaklığındaki gerilme gevşemesi ve sürünme özellikleri, saf çinko alaşımlarıyla karşılaştırılmaya çalışılmıştır. Ayrıca kısa dönem sürünme testleri farklı sıcaklıklarda Titanzink malzemeye uygulanmış ve bu malzemeye ait sürünme verileri elde edilmiştir. Sonuç olarak Titanzink malzemeye ait sürünme aktivasyon enerjisi 23 -85 sıcaklık aralığında belirlenmiş ve bu malzemeye ait sürünme bünye bağıntısı elde edilmiştir.

Abstract:
After iron, aluminum and copper; zinc is one of the most common metals relatively. Especially in roofing and siding applications zinc sheets have an importance for many years. Zinc is a common, an economic material with great atmospheric corrosion resistance. All these properties are reasons of why zinc is preferred for roofing and siding applications.  
Although zinc is common, an economic, a corrosion resistive material, some of its properties must be improved. It has lower yield point, very low creep strength, lower melting point among the other metals mentioned above. It can be recrystallized at room temperature and this prevents strain hardening behavior.  In order to develop alloys that have superior properties than pure zinc, researchers have been alloyed zinc with different materials.  Most common alloying materials for zinc are aluminum, lead, copper and titanium. New trend is alloying zinc with copper and titanium. 
A Holland manufacturer has been accomplished to improve new zinc alloy which has 0.08 ? 0.17 % Cu and 0.07-0.12 Ti and called it Titanzink. Also there are few manufacturers uses Cu and Ti to alloy Zinc but in different chemical composition ratios. Recent studies showed that Titanzink has improved yield strength, creep strength and recrystallization behavior of pure zinc. 
First of all, commercially pure zinc sheets which are rolled to 0.6mm thickness and Cu-Ti addited zinc alloy (Titanzink) sheets which are rolled to 0.8mm thickness were procured by both domestic and international vendors. According to literature, test specimens were prepared from these sheets. All specimens were prepared parallel to roling direction. 
In this study, creep behavior of commercially pure zinc and Titanzink are investigated at the room temperature. Because, zinc has relatively lower melting point that let the zinc creep at the room temperature. Also Titanzink can be creep at the room temperature. In addition to this,  creep behavior of Titanzink was investigated in other temperatures.
Creep testing of materials is costly and takes a lot of time up to 100000 hours. But in stress relaxation tests, cost and time are reduced. By conducting stress relaxation tests some creep properties of materials like steady state creep strain rates can be determined. In this study, this relation of creep and stress relaxation is observed and compared with others metals. It is seen that there are lots of studies in the literature which shows this relationship.  
Mechanical properties of these two materials which are yield point, tensile strength, elasticity modulus were achieved through mechanical tests had taken from literature and former study of a colleague. 
Chemical analyses were made to obtain weight percentages of the alloying elements and other additives in commercially pure zinc and Titanzink called Cu-Ti alloyed zinc sheets in Faculty of Chemical and Metallurgical Engineerin of Istanbul Technical University. Also microscopic examinations of these two materials were observed.  Grains and phases also observed from these microscopic analyses. Secondary phase of TiZn15 that precipitated at grain boundaries of Titanzink was observed.   All these chemical and microscopic properties were taken from former study that was conducted by a colleague.
To provide creep properties near the room temperature, creep tests were conducted at the room temperature. Both materials; commercially pure zinc and Titanzink were used in creep tests. At the room temperature these materials tested upon different range of stresses. Creep curves had established. Steady-state creep rates were calculated. Comparison of commercially pure zinc and Titanzink was provided.  
Also, stress relaxation tests were conducted to these materials. Stress relaxation tests held on at the room temperature as well. Stress relaxation curves were achieved and settling stresses were obtained for both materials. Comparison of commercially pure zinc and Titanzink was provided. 
Effect of alloying zinc with Cu and Ti on creep properties was discussed through creep tests and stress relaxation tests. Enhancements of these mechanical properties are observed. Precipitates of TiZn15 in the zinc matrix and their effects to creep were discussed. 
Another creep property of the materials is that activation energy for creep of Titanzink was investigated and obtained. To obtain this property, creep tests were conducted at different temperature conditions and different stress loadings.  48°C and 85°C were provided with a chamber that was built in Material Laboratory of IŞIK University for this thesis as well as any other tests that conducted for this study.  By using results of these tests Qc value is derived in selected range of stresses and temperatures. 
The steady-state creep behaviors of these two materials were investigated. Through the literature researches, Power-Law creep was seen suitable to present this behavior. Simplified Power-Law curves are fitted for both materials and stress exponents are derived for both materials by using test datas. Datas were analyzed by MATLAB and Excel softwares
Another important point of this study was to see a relation of creep and stress relaxation behavior of Titanzink. According to literature same mechanisms takes place both creep and stress relaxation. This relationship was investigated. It hasseen that steady state creep rate can be derived through stress relaxation tests by using numerical differentiation. 
To sum up, results of this study can be summarized as follows;  
	Titanzink material has higher creep strength at the room temperature than commercially pure zinc thanks to the TiZn15 precipitates in the microstructure.
	According to results of stress relaxation tests,  Titanzink material has higher settling strength than commercially pure zinc. This shows bolts and nuts joints have better rigidity.
	Activation energy for creep value of Titanzink material can be determined by conducting creep tests at the same stress value and different temperature conditions.
	Simplified Power-Law representation of commercially pure zinc and titanzink material can be derived from creep tests. 
	Stress exponents of Power-Law representation for both materials can be derived from creep tests for both materials. 
	By conducting creep tests both at different temperature and at different stress conditions, consecutive equation that belongs to Titanzink can be derived at a specific loading range.  
	Steady state creep behavior of commercially pure zinc and Titanzink can be derived from stress relaxation test results. This shows that steady state creep strain rates can be predicted by using stress relaxation tests.