טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentErlich Chen Korine
SubjectThe Influence of Heat Treatments on Properties of Ni-rich
Nitinol Weldments
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Emeritus Menachem Bamberger
Full Thesis textFull thesis text - English Version


Abstract

During the last few years there has been an increasing interest in Nitinol Shape Memory Alloys (SMAs), due to their special functional properties - the Superelastic Effect (SE) and the Shape Memory Effect (SME). Suitable welding techniques must be used to obtain devices and components with complex geometries. The heat invested in the welding process causes a change in the microstructure of the weld and in the heat affected zone. This change leads to a decrease in the mechanical properties of the component. For mechanical applications, a desired set of transformation temperatures with minimal change in mechanical properties is required.

The aim of this work was to observe the effect of 3 different heat treatments, in Ni-rich (50.8 at.% Ni) Nitinol Base Metal (BM) and welded material . The effects on the phase transformation, mechanical properties and microstructure were observed. Multiple experiments were conducted before and after the heat treatments. The results showed that the low temperature heat treatments led to the appearance of Ni4Ti3 precipitations which affected the phase transformation behaviour by nucleating an R-phase. As the aging time increased, the transformation temperatures decreased due to the decrease in the Ni concentration of the matrix. As the aging temperature increased, the formation of Ni3Ti2 and Ni3Ti precipitations occurred. Those particles are larger and less coherent particles than the Ni4Ti3, which led to a decrease in the transformation temperatures. All heat treatments in the base metal caused a decrease in the Ultimate Tensile Stress (UTS) and increase in the fracture strain due to the formation of the Ni-rich precipitates.

Thermal simulation of the welding process was conducted. Based on the calculations, the temperatures developed in the HAZ were high, which explains the grain coarsening and the decrease in the microhardness in that area (Hall-Petch effect).


  Observing the effect of the Post Weld Heat Treatments (PWHT) it was noticed that the phase transformation behaviour was altered as in the base metal, but the mechanical properties showed a different behaviour which was not consistent. The heat treatment for 10min@803K decreased the mechanical properties (UTS and fracture strain) while the treatment of 30min@833K had the same mechanical properties as those the weld.

The experimental results of this work correspond to previous publication in the literature. In order to alter the phase transformation temperatures and preserve the mechanical properties of the weld, a heat treatment for 30min@833K is recommended.