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TitleENGINEERING THE MARTENSITIC TRANSFORMATION HYSTERESIS OF NI-RICH NITI ALLOYS
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LanguageEnglish
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Total Pages82
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Page 1

A Thesis

by

BRIAN EELAN FRANCO



Submitted to the Office of Graduate and Professional Studies of
Texas A&M University

in partial fulfillment of the requirements for the degree of


MASTER OF SCIENCE





Chair of Committee, Ibrahim Karaman
Committee Members, Miladin Radovic
James Boyd
Head of Department, Andreas Polycarpou


December 2014



Major Subject: Mechanical Engineering





Copyright 2014 Brian Eelan Franco

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ii



The shape memory behavior in NiTi alloys can be exploited for a wide variety of

applications that require active materials. The application dictates the transformation

temperatures and hysteresis of the alloy. NiTi alloys with high Ni contents can be

precipitation heat treated to produce large changes in the transformation temperatures, as

well as increases in strength and dimensional stability. The effect of aging on the Ms

temperature has been previously studied in the literature; however, few studies have

investigated long duration aging, and little attention has been paid to the effect of

precipitate formation on the thermal hysteresis.

In the current work, a systematic study of heat treatments was performed to study

the effect of aging time, aging temperature, and initial Ni composition on the

transformation temperatures and the thermal hysteresis under zero stress conditions

using differential scanning calorimetry. Heat treatments and NiTi compositions were

chosen in order to ensure that only Ni4Ti3 type precipitates formed during the aging

process.

The results showed aging led to three different transformation paths; single step

B2-B19`, B2-R-B19`, and multiple step transformation. At low aging temperatures, the

transformation temperatures were initially suppressed but increased after sufficient aging

durations. At higher aging temperatures the transformation temperatures only increased

with aging time. In high Ni content materials the transformation temperatures were

suppressed more than in the low Ni content materials.

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detected for aging durations at 10 hours and above; for 450ºC at 1 hour and above. For

aging at 500ºC and 550ºC transformations are observed for all durations.

At any given aging temperature only increases in Ms with subsequent aging are

observed. However, because of the -80ºC lower limit of the DSC equipment it is possible

that a decreasing trend exists after very short duration aging. The Ms curves for 400ºC

and below continue to increase until 100 hours of aging is reached; but the Ms

temperatures for 450ºC and 500ºC saturate after 72 hours of aging. Additionally as the

aging temperature increases the concavity of the curves also increase.

The Ms temperature increases with aging temperature from 350ºC to 450ºC;

however, from 450ºC to 500ºC there is very little change in Ms. When the aging

temperature is further increased to 550ºC the Ms temperatures decrease.

Figure 3.3 shows the Ms temperatures for the 52.5 (at%) NiTi material aged at

various temperatures and times. When aged below 375ºC no transformation is detected;

again, this indicates that the Ms temperature for these conditions is below -80ºC. For

aging at 375ºC no transformation is detected below 72 hours of aging, for aging at 400ºC

no transformation is detected below 10 hours of aging, for aging at 425ºC no

transformation is detected below 1 hours of aging, and for aging at 450ºC no

transformation is detected below 5 hours of aging. Samples aged at 500ºC show

transformation at and above 0.5 hours of aging.

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Figure 3.3 Ms temperatures for 52.5 (at%) NiTi after aging



Similarly to the 51.2 (at%) NiTi material only increases in Ms with subsequent

aging are observed. The only exception was aging under 400ºC from 10 to 24 hours.

Again, it is possible that decreasing trends are present with short duration aging, but

cannot be observed because the Ms temperatures are extremely low. At all aging

temperatures no saturation behavior was observed; even after 100 hours of aging the Ms

temperatures continue to increase.

-80

-60

-40

-20

0

20

40

60
T

e
m

p
e
ra

tu
re

(
°C

)

120100806040200

Time (hr)

375°C
400°C
425°C
450°C
500°C

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