ni-ti in endodontics and endodontic files and martensitic transformation

1. Dr. Ahmad Khateeb

2. Heat Treatment

3. .
. Ts
.
.
. Tf
.
Stable Austenite (L)
Unstable Austenite
T
Log t
Annealing
T E (melting temp)
start
finish
Pearlite
(Fe3C)
Quenching
M s
M f
Martensite
A + P
A + M
Time Temperature Transformation (TTT)
start
finish
Atomic
mobility
decrease
Transformation
driving force
increase
Because no atomic migration is necessary, the transformations
usually progress in a time-independent fashion,
with the speed of the interface between the two phases able to
move at nearly the speed of sound.
This type of transformation is referred to as an Athermal
transformation since it cannot progress at a constant temperature,
but rather the amount of the new phase present depends only upon
temperature, not time

4. heat
hold
cool
TE
Aging
Temp
time

5. Stress - Strain

6. Stress - Strain

7. Stress –strain curve

11. Schematic illustration of parent
and martensite with invariant
plane as a habit plane.

15. Super elasticity, or Pseudo elasticity, can be defined as the ability of certain materials to recover their original shape
after the load is removed even when they are deformed beyond their yield strength.
The driving force for the reverse-transformation is the difference
between the chemical free energy of the parent and martensitic
phases above As, and the complete shape recovery lies in that the
original orientation of the parent phase can be restored
(crystallographic reversibility).
In most metals, when an external force exceeds a given amount
mechanical slip is induced within the lattice causing permanent
deformation; however, with NiTi alloys a stress-induced martensitic
transformation occurs, rather than slip.
This effect is called Stress‐Induced Martensite (SIM) transformation.

16. Super elasticity When the stress stops without permanent deformation
occurring returning to the previous shape with a return to the
austenite phase, provided the temperature is within a specific
range austenite is transformed to martensite by stress;
for a practical example, an insertion of the instrument into a
curved root canal.
Because the stress‐induced martensitic state is not stable at the
intraoral temperature, unloading of the endodontic instrument
(e.g., withdrawal of an instrument out of a curved root canal)
leads to reverse transformation back to the austenite phase
resulting in a spring‐back of the endodontic instrument to its
original shape

18. Shape memory effect
Shape-Memory Effect (SME), which is the ability of the alloy to completely recover its original shape when heated
above the martensite-to-austenite transformation temperature
The transformation induced in the alloy occurs by a
shear type of process to a phase called the martensitic
or daughter phase ,which gives rise to twinned
martensite
The deformation can be reversed by heating the alloy
above the As with the result that the properties of the
NiTi alloy revert back to their previous higher temperature
values

19. Martensite’s crystal structure has the unique ability to undergo
limited deformation in some ways without breaking atomic
bonds, known as the twinning, which consists of the
rearrangement of atomic planes without causing slip or
permanent deformation allowing to undergo about 6–8% strain
in this manner.
When martensite is reverted to austenite by heating, the
original austenitic structure is restored, regardless of whether
the martensite phase was deformed; hence, the name “shape
memory” refers to the fact that the shape of the high-
temperature austenite phase is “remembered,” even though the
alloy is severely deformed at a lower temperature.

20. Different phases of shape memory alloy at different
temperature and their relationship with loading and unloading
Superelasticity = mechanical
(Stress Induced Martensite)
Shape memory = thermal +
mechanical stress
(Thermal Induced Martensite)

21. The differences between NiTi alloys are their nickel content and their temperature
ranges of MT.

23. The "premartensitic transition" precedes the martensitic transformation under certain conditions.
R-phase possesses a lower shear modulus than martensite and austenite, and the transformation
strain for R-phase is less than one-tenth that of martensitic transformation.
The two step transformations of :
(B2 Austenite → R-phase) -- and subsequent --- (R-phase → B19′ Martensite) ,, occurs upon cooling when :
Rs (the start temperature of As → R-phase transition) is below Ms (start temperature of martensite transformation)
A variety of possible Martensite variants are obtained by using
different twinned R-phase as matrix and by changing
the rhombohedral angle.

24. The R-phase transition usually appears prior to the martensitic transformation when Ms is more lowered by some means.
There are many factors effective to depress the Ms point as follows:
(1) Increasing Ni-content.
(2) Aging after solution-treatment
(3) Annealing at a low temperature immediately after cold work.
(4) Thermal cycling.
(5) Substitution of a third element

25. Crystal structure

26. Ti
Ni
BCC
Body centered cubic
The crystal structures of Ni-Ti
Left: Austenite, which is ordered Face-centered cubic;
Middle: R-phase, which is rhomboidal
Right: Martensite, which is Body-centered cubic

27. Transformation temperatures are highly dependent on the nickel concentration of the
alloy.
On the Ni-rich side, increasing nickel content results in a drastic decrease in transformation
temperature.
On the Ni-rich side, Ti3Ni4 precipitates can be formed from TiNi3 decomposition.
These precipitates (TiNi3) can affect the characteristics of martensitic transformation and act as
nucleation
centers for formation of the R phase
For NiTi alloys in which SME(shape memory effect) is desired, the Ni content limits
range from 48% to 52% Ni by weight

28. The heat treatments performed in NiTi alloys with the aim of improving their
properties are:
Aging :process consists of uniform heating of the alloy to about 500°C followed by rapid cooling (quenching), usuall
in water, to temporarily prevent precipitation of the alloying elements. the aging time is an important factor affecting
the Ms temperature and consequently affecting the transformation sequence of Ti49Ni51 which is aged at 400°C, × 1 h.
Annealing :Is when The material is heated to 300–500°C until the desired changes have taken place over the
entire mass of the part, which is then cooled slowly.
Recrystallization :is the replacement of a cold-deformed structure by a new set of deformation-free grains, as eviden
reduction in hardness and an increase in ductility.
To eliminate the hardening effect caused by cold forming, annealing is performed to achieve recrystallization.

30. Twisted
File

31. In the case of TF, maintaining the NiTi alloy in R-phase by heat treatment enables the twisting process.
Thus, it is easier to apply plastic deformation to R-phase because lower stress would be required.
According to the manufacturer, TF instruments are developed by:
1- Transforming a raw NiTi wire in the austenite to R-phase through a thermal process.
2- In the R-phase, the NiTi blank is twisted along with repeated heat treatment, and after additional thermal
procedures to maintain its new shape.
3- The instrument is converted back to austenite, which is super-elastic once stressed.
Attempts to twist instruments in the conventional way would probably result in instrument fracture.
R-phase possesses lower shear modulus than martensite and austenite, and the transformation strain for
R-phase transformation is less than one-tenth that of martensitic transformation.

32. The manufacturer claims that this proprietary twisting process with concurrent heat treatment and protection of the
crystalline structure imparts superior flexibility and resistance to fatigue.
TF instruments are significantly more resistant to cyclic fatigue than ground ones.
The twisting process avoids machining defects
The higher Ms temperature of TF may require the lower critical stress to induce martensitic transformation

33. CM Wire

34. CM NiTi files do not present the rebound effect after unloading, and their original shape is restored only
after autoclaving.
The behavior of these files may be explained by the presence of stable martensite, meaning that the working
temperature is below the Af.
Stable martensite is known for exhibiting the shape memory effect, which is the capacity to recover the original
shape by reverse transformation after heating the deformed martensite to temperatures above the Af
These temperatures are strictly related to the nickel content of the alloy and/or its thermomechanical
history, which is unknown for the CM NiTi wires.
The G-wire was developed in 2015, which is manufactured through several processes;
(1) by producing a mixed phase of austenitic phase, R-phase, and martensitic phase by M-wire treatment,
(2) by precipitating Ti3Ni4 in the austenitic phase,
(3) By controlling excessive growth of martensitic phase to promote the formation of R-phase
HyFlex (HF) is produced using CM wire to possess martensitic property at room temperature,
resulting in no spring back and excellent root canal trackability.

35. CM wire was a kind of Ni-rich NiTi alloy that possessed a relatively high As and Af compared with regular SE wire.
At room temperature the instrument may be a composite of R-phase martensite and austenite, unlike
conventional NiTi instruments, which are purely austenitic.
CM instruments have >300% greater resistance to cyclic fatigue when compared to SE
instruments
The transformation temperatures increase as the nickel content decreases.
Thus, with the decrease of the nickel content in the NiTi alloy, the Af increases as well as the tendency to
obtain stable martensite at the working temperature.
Furthermore, Zinelis et al showed that Hyflex CM files have a lower percentage of nickel (52.1 %wt) than do
conventional NiTi files.

36. M - wire

37. The austenite-finish temperature of M-Wire (45°C – 50°C) is much higher than that for
conventional superelastic wire (approximately 20°C or lower)
At room temperature GTP®Series X™ instruments made from M-Wire exist in the martensite phase.
This martensite phase results from extensive thermomechanical processing that occurs during the
manufacture of the starting M-Wire segments.
The special martensite structure accounts for the superior mechanical properties of M-Wire instruments.
M-Wire instruments are largely a mixture of martensite and R-phase at room temperature, with a
small amount of austenite.
EDS analysis indicate that the approximate composition of the precipitates in the M-Wire microstructure
is TiB2BNi, which indicates that the starting wire segments are Ti-rich rather than Ni-rich.

38. M-Wire instruments have significantly higher hardness compared with conventional rotary NiTi instruments
The increased hardness of the M-Wire instruments is attributed to the special work-hardened martensite
structure found by STEM examination of starting wire blanks and metallographic examination of the etched
microstructures of instruments.
M-Wire has relatively coarse grains, numerous triple-point junctions of grain boundaries, and localized
deformation bands with microtwins, indicative of extensive thermomechanical processing.

39. Hyflex EDM