ساخت و ارزيابي خواص ابر كشساني و حافظه داري آلياژ TiNiCo نانو ساختار شده به روش ترموديناميكي

چكيده انگليسي :

Fabrication and evaluation of the superelastic and shape memory properties of the TiNiCo alloy nanostructured by thermomechanical processing Ehsan Mohammad Sharifi e mohamadsharifi@ma iut ac ir Date of Submission Mar 5 2014 Department of Materials Engineering Isfahan University of Technology Isfahan 84156 83111 Iran Supervisor Fathollah Karimzadeh Associate Professor karimzadeh f@cc iut ac ir Supervisor Ahmad Kermanpur Associate Professor ahmad k@cc iut ac ir Department Graduate Program Coordinator Dr Keyvan Raeissi Abstract In the current research the effect of thermomechanical processing on the microstructural evolution transformation behavior superelastic and shape memory properties of the Ti50Ni50 and Ti50Ni48Co2 shape memory alloys was investigated In view of drawbacks in vacuum induction melting and vacuum arc remelting methods in this study Ti50Ni50 and Ti50Ni48Co2 cast ingots were produced by a new process called copper boat vacuum induction mel ting CBVIM The as cast ingots were then homogenized hot rolled and annealed to prepare the suitable initial microstructure Thereafter annealed specimens were cold rolled up to 70 thickness reduction at room temperature Post deformation annealing was conducted at 400 C for 1h Transmission electron microscopy TEM observations revealed that the initial deformation mechanism of Ti50Ni50 and Ti50Ni48Co2 alloys during cold rolling was stress induced martensitic transformation followed by plastic deformation of martensite via dislocation slip and subsequent martensite to austenite transformation via the reverse transformation after unloading Microstructural investigations showed that by increasing the cold deformation a high density of dislocations is accumulated leading gradually to nanocrystallization and amorphization After annealing at 400 C for 1h the amorphous phase formed in the cold rolled specimens was completely crystallized and a fully nanocrystalline structure with the grain size between 10 and 70 nm was achieved in Ti50Ni50 and Ti50Ni48Co2 alloys Results showed that the stress level of plateau SIM in the stress strain curve of the Ti50Ni50 and Ti50Ni48Co2 alloys could be significantly improved through the formation of the nanocrystalline structure It is noteworthy that in the 70 cold rolled annealed specimens the value of SIM was measured to be 615 and 730 MPa respectively for the Ti50Ni50 and Ti50Ni48Co2 alloys With increasing thickness reduction during cold rolling the irrecoverable strain of Ti50Ni50 and Ti50Ni48Co2 alloys was decreased during superelastic experiments so that the 70 cold rolled annealed specimens exhibited about 12 of recoverable strain Observed improvement in the superelastic behavior of the Ti50Ni50 and Ti50Ni48Co2 alloys can be attributed to increase of the critical dislocation induced slip stress as a result of thermomechanical treatment and formation of nanocrystalline structure Comparing the superelastic and shape memory behavior of Ti50Ni50 and Ti50Ni48Co2 alloys showed that the addition of Co resulted in higher plateau stresses and recoverable strain when compared to the binary alloy Keywords Shape memory alloys Superelasticity TiNiCo alloy Thermomechanical processing Nanocrystallization 1 Introduction Owing to their unique properties such as shape memory effect superelasticity biocompatibility and corrosion resistance near equi atomic NiTi alloys have been widely used in aerospace and bioengineering applications It is well established that shape memory and superelastic properties of NiTi are due to the martensitic transformation and its reversion which is strongly dependent on alloy composition and processing conditions 1 1

محمدشريفي، احسان

ساخت و ارزيابي خواص ابر كشساني و حافظه داري آلياژ TiNiCo نانو ساختار شده به روش ترموديناميكي