Martensitic Transformations in TiNi Alloy Subjected to High Pressure Torsion and Subsequent Heating

Thursday, May 23, 2013
OREA Pryamida Hotel
Dr. Sergey Belyaev , Saint-Petersburg State University, Saint-Petersburg, Russia
Mrs. Maria Drozdova , Saint-Petersburg State University, Saint-Petersburg, Russia
Dr. Vitaly Pilyugin , Institute of Metal Physic, the Ural branch of RAS, Yekaterinburg, Russia
Dr. Natalia Resnina , Saint-Petersburg State University, Saint-Petersburg, Russia
Mr. Viacheslav Slesarenko , Saint-Petersburg State University, Saint-Petersburg, Russia
An evolution of the kinetics of martensitic transformations during heating of a Ti – 50.2 at.% Ni alloy subjected to high pressure torsion (HPT) were investigated with the help of differential scanning calorimetry (DSC). HPT was carried out at the room temperature under a pressure of 8 GPa up to angles of 15, 90 and 1260 degrees. Then HPT-processed samples were heated up to 550 °C with the intermediate interruptions at the some characteristic temperatures, determined in the previous study. DSC and resistivity measurements were carried out after each interruption to clarify the kinetics of martensitic transformations.

It was determined that martensitic transformations were completely suppressed in all samples processed by HPT regardless of the rotation angle. DSC and resistivity measurements after each interruption revealed significant change of the kinetics of martensitic transformations during heating. After heating up to temperature which was just little higher than the temperature of the start of crystallization/recrystallization process, the martensitic transformations were observed at very wide temperature range and had low transformation enthalpy. The samples deformed up to 90 and 1260 degrees underwent two direct transformations: B2→R and B2→B19', while the sample deformed up to 15 degrees underwent only one transformation B2→R without the formation of B19' phase at the temperatures higher than -100 °C. Further heating up to the higher temperatures caused changes in the kinetics of the transformations and after complete crystallization direct B2→R→B19' and reverse B19'→B2 transformations took place in the samples deformed up to 15 and 90 degrees, while additional direct B2→B19' transformation occurred simultaneously with B2→R→B19' transformation in the sample deformed up to 1260 degrees. As far as these changes of the kinetics of martensitic transformations took place during crystallization/recrystallization and grain growth processes, they might be related to some structural changes such as alteration of grain size.

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