Transformation Characteristics and Properties Optimization of the Aged Ni-rich Ti49Ni51 Shape Memory Alloy
Date Issued
2012
Date
2012
Author(s)
Chien, Chen
Abstract
In this study, the property improvement of shape memory effect (SME) and pseudoelasticity (PE) exhibited in Ni-rich Ti49Ni51 shape memory alloy (SMA) solid-soluted(SS) and aged at 250℃~500℃ for various time is investigated. The hardness of specimen at 900℃x 1 hr SS and water quenched is 303HV. Among all aged specimens, the maximum hardness is 374 HV for specimen aged at 250℃x25 hrs. From DSC tests, B2↔R transformation mainly appears in specimens aged at 250℃~350℃, but both B2↔R and R↔B19’/B2↔B19’ transformations occur in specimens aged at 400℃~500℃. The Rs temperature and ∆HR value of B2↔R transformation reach their maxima for specimens aged at 300℃. In the early aging, the curve of the specimen hardness at room temperature v.s. aging time has the 1st maximum, the minimum and the 2nd maximum , in which the 2nd maximum is also the highest hardness possessed in aged alloy. From DSC results and TEM observations, the formation of R phase and nano-precipitates is closely related to the hardness maximum and minimum exhibited in specimens aged at 250℃~400℃. The specimens with two step aging have 100% SME and higher damping capacity even though their the maximum hardness is 348HV. The tensile tests for specimens aged at 300℃x50 hrs (hardness 360HV) and at 250℃x5 hrs+400℃x1 hr (hardness 341HV) indicate their SME and PE properties are better than those of Ti49.3Ni50.7 due to the former has higher SS strengthening and precipitation hardening.
Subjects
Ni-rich TiNi shape memory alloy
Aging
Precipitation hardening
Phase transformation
Shape memory effect
Pseudoelasticity
Type
thesis
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