Investigation of Thermodynamic Properties of Ni30Ti20Cu20 Shape Memory Alloy

Year 2019, Volume: 8 Issue: 4, 1194 - 1202, 24.12.2019

Ercan Ercan , Fethi Dağdelen Mediha Kök Esra Balcı

Abstract

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 (A_s)
temperature is 23.5 ^oC,
austenite finish (A_f) temperature is 50.6 ^oC, martensite
start (M_s) temperature is 26.7 ^oC and martensite finish
(M_f) 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 (A_s and A_f) varied, from austenite phase to
martensite phase (M_s and M_f) did not. Thermal activation
energy of the alloy measured by Kissinger method was found as E_a=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 TiNi_0,8Cu_0,2
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

Investigation of Thermodynamic Properties of Ni30Ti50Cu20 Shape Memory Alloy

Abstract

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 (A_s) 23.5 ^oC, austenite bitiş sıcaklığı (A_f)
50.6 ^oC, martensit başlangıç sıcaklığı (M_s) 26.7 ^oC
ve (M_f) -0.10 ^oC olarak bulundu. Farklı ısıtma hızlarında
alınan DSC ölçümlerine göre ise alaşımın martensit fazdan austenite faza
geçerken dönüşüm sıcaklıkları (A_s ve A_f) değişirken,
austenite fazdan martensit faza geçerken dönüşüm sıcaklıklarının (M_s
ve M_f) değişmediği görülmüştür. Kissinger metodu ile bulunan
aktivasyon enerjisi E_a=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 TiNi_0,8Cu_0,2 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.

Keywords

shape memory alloy, transformation temperature, microstructure

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