Sinan AKSÖZ, Rıdvan ARSLAN, Nimet KARDEŞ SEVER, Bülent BOSTAN

NiTi Esaslı Saclara Uygulanan Farklı Çözündürme ve Yaşlandırma Isıl İşlemlerinin Metalografik ve Mekanik Özelliklere Etkileri

Bu çalışmada 45ºC ve 80ºC‘de faz dönüşümü gösteren 1 mm kalınlığında NiTi saclar kullanılmıştır. Öncelikle, eğilebilirliği incelemek için üç nokta eğme testleri yapılmıştır. Bundan sonra bükülmüş saclardan alınmış numunelere sırasıyla; 750ºC, 800ºC, 850ºC ve 900ºC’de 1 saat çözündürme ısıl işlemleri uygulanmıştır. Bu numunelere sırası ile 300ºC’de 15, 30, 45, 60, 75 ve 90 dakika (dk.) süreyle yaşlandırma ısıl işlemi uygulanmıştır. Isıl işlem parametrelerinin etkilerini incelemek için Optik Mikroskop, Alan Emisyon Taramalı Elektron Mikroskobu (FESEM), Element Dağılım Spektrometresi (EDS) ve Haritalama (MAP) analizleri gerçekleştirilmiştir. Mekanik etkileri incelemek için her bir ısıl işlemden sonra makro sertlik verileri alınmıştır. Ayrıca Dinamik Mekanik Analiz (DMA) ile malzemenin yük altındaki mekanik davranışı incelenmiştir. Sonuç olarak, yaşlandırma ısıl işlemi sayesinde mekanik özellikler (sertlik ve depolama modülü) iyileştirilebilmiştir. En yüksek sertlik değeri, 80°C dönüşüm sıcaklığına sahip numunede, 900°C’de çözündürme ve 300°C’de 75 dk. yaşlandırma sonrası 414HV olarak ölçülmüştür. Ayrıca, depolama modülünde artış, tanδ değerinde ise düşüş meydana gelmiştir. NiTi alaşımlarının süper elastik özelliklerinden faydalanabilmek için, dönüşüm sıcaklıklarının üzerindeki sıcaklıkların kullanılması gerekliliği depolama modülü ölçümleri ile görülmüştür. 45°C dönüşüm sıcaklığına sahip numune için dönüşüm sıcaklığı altında depolama modül değeri ~21500MPa iken dönüşüm tamamlandıktan sonra ~ 28500MPa olarak ölçülmüştür.

Anahtar Kelimeler:

NiTi alaşımlar, ısıl işlem, sertlik, depolama modülü, mikroyapı

Effects of Different Dissolution and Aging Heat Treatments on Metallographic and Mechanical Properties of NiTi Based Sheets

In this study; 1-mm-thick NiTi sheets showing phase transformation at 45ºC and 80ºC were used. First, three-point bending tests were performed to investigate the bendability. Later, solution heat treatments were applied to the samples taken from the bent sheets for 1 hour at 750ºC, 800ºC, 850ºC and 900ºC respectively. Aging heat treatment was applied for 15, 30, 45, 60, 75 and 90 minutes (min.) at 300ºC to these samples. Optical Microscope, Field Emission Scanning Electron Microscopy (FESEM), Element Dispersion Spectroscopy (EDS) and MAPPING (MAP) analyses were carried out to examine the effects of the heat treatment parameters. Macro hardness data were taken to examine the mechanical effects after each heat treatment. Moreover, the mechanical behavior of the material under load was investigated by Dynamic Mechanical Analyses (DMA). As a result, mechanical properties (hardness and storage modulus) have been improved by aging heat treatments. The highest hardness value was measured as 414HV after solution heat treatment at 900ºC and aging heat treatment at 300ºC for 75 minutes on the sample showing phase transformation at 80ºC. Furthermore, an increase in storage modulus and a decrease in tanδ value occurred. To benefit from super elastic properties of NiTi alloys, the necessity of using temperatures above the transformation temperatures was observed with the storage modulus measurements. Storage modulus value under transformation temperature was ~21500MPa while it was measured as ~28500MPa after the transformation was completed for the sample with 45°C transformation temperature.

Keywords:

NiTi alloys, heat treatment, hardness, storage modulus, microstructure,

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