Titanyum ve alaşımları sahip oldukları üstün mekanik özellikleri, düşük yoğunlukları ve yüksek korozyon dirençleri sayesinde havacılık, otomotiv, kimya ve biyomedikal endüstrilerinde yaygın olarak kullanılmaktadırlar. Titanyum ve alaşımlarının yüksek sıcaklıkta oksijene olan yüksek afiniteleri kullanımları sınırlamaktadır. Titanyum ve alaşımlarının yüksek sıcaklıkta oksijene maruz tutulmaları yüzeyde oksit tabakasının ve hemen altında ise oksijen difüzyon bölgesinin oluşmasını sağlamaktadır. Bu işleme termal oksidasyon adı verilmektedir. Bu çalışmada aşınma ve korozyon direncinde önemli artışlar sağlayan termal oksidasyon işleminin Ti6Al7Nb alaşımının yorulma davranışı üzerine etkisinin incelenmesi amaçlanmıştır. Bu amaçla 600 °C’de 60 saat tutulmasıyla gerçekleştirilen termal oksidasyon işlemi sonrasında numunelerin yüzey karakterizasyonu, kesit mikroyapısının incelemesi ve yüzey mikrosertlik değerinin ölçülmesi ile yapılmıştır. Numune yüzeylerinin mikrosertlik değerleri, termal oksidasyon öncesi ve sonrasında, numuneler çekme deneyine tabi tutularak, akma ve çekme dayanımı ile kopma uzaması ve kesit daralması değerleri belirlenmiştir. Dönel eğmeli yorulma deney düzeneği ile orijinal ve termal oksidasyon uygulanmış numuneler, 25 Hz frekansta farklı gerilme genliği değerlerinde yorulma deneylerine tabi tutularak S-N eğrileri elde edilmiştir. Dönel eğmeli yorulma deneyleri sonrasında, numunelerin kırık yüzeyleri stereo mikroskop ile incelenmiştir. Termal oksidasyon sonucunda Ti6Al7Nb alaşımının yüzeyinde ince bir oksit tabakası ve hemen altında oksijen difüzyon bölgesi oluşmuştur. Termal oksidasyon sonucu yüzey sertliğinin Ti6Al7Nb alaşımında % 170 gibi yüksek bir oranda arttığı belirlenmiştir. Termal oksidasyon işlemi sonrasında Ti6Al7Nb alaşımının mekanik özelliklerinde kayda değer bir değişiklik görülmemiştir. Termal oksidasyon sonrasında Ti6Al7Nb alaşımının yorulma dayanım sınırı düşmüştür.

Musculoskeletal system diseases cost in the countries around the globe a significant amount of money annually. The occurrence of bone fractures has also increased due to an increase of the number of traffic accidents and the increase of life expectancy almost twice as compared with past centuries. It is expected that one-third of European citizens will be soon over the age of 60. Hence, healthcare costs will be an increasing burden for society. Therefore, the need for economically feasible biomaterials for fracture healing will increase. For decades, researchers have focused on devoloping a viable and cost effective alternative materials for the dental applications instead of nickelchromium and cobalt-chromium alloys which have been popular for the denture frameworks since 1970s. Usage of alloys containing nickel was limited due to doubts related with biological safety. Titanium, which eliminates persisting doubts as to biological safety of alloys containing nickel, was introduced in the 1970s. Titanium alloys are attractive materials for many engineering applications, which require excellent combination of mechanical properties, corrosion resistance and biocompatibility along with a low weight. Their excellent corrosion resistance and biocompatibility are assumed to be due to the formation of a dense and stable TiO2 layer, which rebuilds spontaneously after being damaged, even in solutions with low oxygen contents. Unfortunately, in applications where contacting motion of counterparts is maintained, titanium and its alloys have limited usage owing to their poor tribological performance. Poor wear resistances of titanium and its alloys can be enhanced by surface modification techniques. Among the surface modification techniques, thermal oxidation is one of the simplest and cheapest process and appears as it is very promising way to produce hard surfaces on titanium and its alloys. Surface modifications like surface roughening, oxidation or coating techniques also often improve the bioadhesion and the corrosion behaviors of titanium implants. Although many types of titanium alloys are commercially available, since when compared with commercially pure titanium Ti6Al4V alloys have superior physical and mechanical properties and almost same of nickel-chromium and cobalt-chromium alloys, alfa/beta alloy Ti6Al4V is the most commonly used in the biomedical applications. Sometimes usage of Ti6Al4V alloy can be problematic due to the toxicity of vanadium in the body. In order to eliminate this negative effect of vanadium, in the 1980s new vanadium-free alloy containing niboium Ti6Al7Nb was devoloped. The aim of this study was to examine the effect of thermal oxidation on rotating bending fatigue behavior Ti6Al7Nb alloy. Thermal oxidation was conducted at 600°C for 60 h in air. Characterization of the oxidized alloy was made by microstructure examinations, hardness measurements, tensile and fatigue tests. Microstructure survey was conducted on the cross sections of the oxidized alloy by an optical light microscope after etching with 2% HF. Hardness measurements were made on the surfaces of the samples by a micro hardness tester in the unit of Vickers hardness under different indentation loads. Before and after thermal oxidation, tensile tests were performed in order to measure the yield and tensile strengths, percent elongation and reduction in area. Fatigue tests were performed under rotating bending condition at a frequency of 25 Hz. Fractured surfaces obtained from rotating bending fatigue tests were investigated utilizing a stereo microscope. In this study, the effect of thermal oxidation on the rotating bending fatigue behavior of Ti6Al7Nb alloy was investigated. Thermal oxidation, which was performed at 600°C for 60 h in an air furnace, resulted in the formation of 0.7 μm thick oxide layer with a 7 μm oxygen diffusion zone beneath it. Thermal oxidation considerably improved surface hardness to 929 HV0.1. Among the tensile properties only the yield strength was affected from the thermal oxidation. Yield strength of the examined alloy increased from 947 MPa to 1019 MPa, which corresponds to about 8 % improvement. Other tensile properties including tensile strength, elongation at fracture and reduction in area did not show any significant variation with thermal oxidation. Rotating bending endurance limit at 5x106 cycles decreased from about 560 MPa to about 500 MPa upon oxidation. The loss in fatigue resistance corresponds to 12%.