Ercan ERCAN, Fethi DAĞDELEN, Mediha KÖK, Esra BALCI

Investigation of Thermodynamic Properties of Ni30Ti50Cu20 Shape Memory Alloy

Şekil hatırlamalı NiTiCu alaşımı ark-ergitme yöntemi ile üretildi ve faz dönüşüm sıcaklıkları, bazı termodinamik parametreleri ile mikroyapısal özellikleri araştırıldı. 10°C/dak. ısıtma-soğutma hızı ile alınan DSC sonuçlarına göre; austenite başlangıç sıcaklığı (As) 23.5 oC, austenite bitiş sıcaklığı (Af) 50.6 oC, martensit başlangıç sıcaklığı (Ms) 26.7 oC ve (Mf) -0.10 oC olarak bulundu. Farklı ısıtma hızlarında alınan DSC ölçümlerine göre ise alaşımın martensit fazdan austenite faza geçerken dönüşüm sıcaklıkları (As ve Af) değişirken, austenite fazdan martensit faza geçerken dönüşüm sıcaklıklarının (Ms ve Mf) değişmediği görülmüştür. Kissinger metodu ile bulunan aktivasyon enerjisi Ea=63.208 kJ/mol olarak bulunmuştur. Gibbs serbest enerjisi ısıtma-soğutma hızlarıyla küçük değişimler göstermiştir. DSC eğrilerinden tek-adımlı B2 ↔B19 faz geçişi görülmüş ve bu fazların kristal yapıları XRD analizi ile belirlenmiştir. Bununla birlikte Ti2(Ni, Cu) çökeltilerinin yanı sıra NiTi alaşımının interfazlarında çözünen Cu elementlerinin TiNi0,8Cu0,2 formundaki matrislerine rastlanmıştır. SEM-EDX sonuçları ile alaşımdaki Ti2(Ni, Cu) çökeltilerinin atomik yüzdeleri belirlenmiştir. Alaşımın mikrosertliği 219 HV olarak bulunmuştur. Bu değer, artan Cu miktarının geleneksel ikili NiTi alaşımlarını daha yumuşak material haline getirdiğini göstermiştir.

Anahtar Kelimeler:

shape memory alloy, transformation temperature, microstructure

Investigation of Thermodynamic Properties of Ni30Ti20Cu20 Shape Memory Alloy

Phase transformation temperatures, certain thermodynamic parameters and microstructural features of Ni30Ti50Cu20 shape memory alloy produced by arc-melting process were examined. Owing to DSC results obtained at 10°C/min heating-cooling rate, it was determined that austenite start (As) temperature is 23.5 oC, austenite finish (Af) temperature is 50.6 oC, martensite start (Ms) temperature is 26.7 oC and martensite finish (Mf) temperature is -0.10 oC. It was observed that according to DSC measurements obtained at different heating rate, while transformation temperature of the alloy from martensite phase to austenite phase (As and Af) varied, from austenite phase to martensite phase (Ms and Mf) did not. Thermal activation energy of the alloy measured by Kissinger method was found as Ea=63.208 kJ/mol. Moreover, Gibbs free energy was observed to display slight increases with the heating-cooling rates. Also, from the results of DSC curves, it was seen that at one-step B2 ↔B19 occurred and crystalline structures of these phases were determined at room temperature with the help of XRD analysis. Besides, precipitations of Ti2(Ni, Cu) as well as matrices in the form of TiNi0,8Cu0,2 of element Cu dissolved in interphases of NiTi were encountered. With the results of SEM-EDX, atomic percentage of precipitations (at.%) of Ti2(Ni, Cu) formed in the alloy were determined. The microhardness value of the alloy was measured as 219 HV. It was concluded that this value makes increased Cu amount to be softer metal than traditional binary NiTi alloys.

Keywords:

shape memory alloy, transformation temperature, microstructure,

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