TEXTURE IN TUBES AND PLATES OF THE SUPERELASTIC/SHAPE-MEMORY ALLOY
NITINOL

**S. W. Robertson\*, V. Imbeni\*\*, E. Notkina\*\*\*, H.-R.
Wenk\*\*\*\*, and R. O. Ritchie\***

*\* Materials Sciences Division, Lawrence Berkeley National Laboratory,
and Department of Materials Science and Engineering, University of
California, Berkeley*

*\*\* SRI International and The International Organization on Shape
Memory and Superelastic Technologies*

*\*\*\* Technische Universität Hamburg-Harburg, Hamburg, Germany*

*\*\*\*\* Department of Geology and Geophysics, University of
California, Berkeley*

# ABSTRACT

The superelastic/shape-memory material, Nitinol, a roughly equiatomic
alloy of Ni and Ti, is rapidly becoming one of the most important
metallic implant materials in the biomedical industry, in particular for
the manufacture of endovascular stents. As such stents are invariably
laser-machined from Nitinol tubes (or sheets rolled into tubes), it is
important to fully understand the physical phenomena that may affect the
mechanical behavior of this material. With tubing and sheet, one major
issue is crystallographic texture, which can play a key role in
influencing the mechanical properties of Nitinol. In this paper, we
present a preliminary study on how geometry and heat treatment can
affect the texture of Nitinol.

# INTRODUCTION

The production of biomedical devices made from the shape-memory and
superelastic alloy Nitinol, a roughly equiatomic alloy of Ni and Ti,
employs numerous processing techniques, e.g. tube drawing, plate
rolling, etc., and heat treatment processes in order to modify the
microstructure, transformation temperatures, and mechanical properties.
These procedures can lead to texturing (crystallographic alignment) of
the material. There are a few, mainly qualitative, reports of textures
in Nitinol sheets and plates^1-8^ but no information for textures of
tubes. Since Nitinol derives its unique nonlinear and anisotropic
mechanical behavior from stress-induced martensitic transformations,
where the resulting strains are affected by crystallographic
orientation, it is clear that texture can have a marked influence on its
mechanical properties. Indeed, Gall and Sehitoglu^9^ have concluded that
texture, and consequently variants of martensite active during the
transformation, are the source of the compression-tension asymmetry of
Nitinol. Furthermore, Vaidyanathan *et al.* ^10^ have concluded that
texturing can occur in Nitinol when martensite is stress-induced from
austenite via such compression loading. Texture can have a critical
effect in modeling the constitutive behavior of the alloy, as shown in
Figure 1^11^ where the assumption of the correct texture is shown to
markedly influence the finite-element predictions of the equivalent
stress-equivalent strain curve for Nitinol under combined tensile and
torsional loading and unloading.

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  ![](media/image1.jpeg){width="2.3694444444444445in"   ![](media/image2.jpeg){width="2.4194444444444443in"
  height="1.8305555555555555in"}                        height="1.8722222222222222in"}

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*Figure 1: Finite element simulation curves of equivalent stress versus
equivalent strain for Nitinol tubing under combined tension and torsion
loading conditions a) assuming the correct texture of the tubes, and b)
assuming incorrect texture.*

It is the goal of this preliminary study to better understand how
processing and heat treatment affect the texture in Nitinol,
specifically by examining drawn tubes in comparison to rolled plate. The
motivation for this comparison is to determine if such tubes could be
simply modeled as plates rolled into a cylindrical form. This can be
important where the flat geometry of plate specimens is preferred over a
curved geometry for certain testing conditions, such as for microscopy
studies which require a flat surface for resolution.

# 

## MATERIALS AND EXPERIMENTAL PROCEDURES

Nitinol tubing and plate (50.8 at.% Ni,) was received from Nitinol
Devices & Components, Inc. and heat-treated, as described below, to
modify the microstructure. The sample geometries (tube vs. plate) were
chosen to determine if the processing (drawing vs. rolling) has a
significant effect on the texture. The samples were heat treated to two
different specifications:

-   485°C in air furnace for 5 minutes, followed by an ice-water quench
    (on both tube and plate).

-   850°C *in vacuo* furnace for 30 minutes followed by a slow cool (on
    plate only).

The tube dimensions were 4.6 mm outer diameter and 3.9 mm inner
diameter; the plate sections were 15 mm square by 1 mm thick. The
samples were too heavily deformed for SEM-EBSP texture measurements and
therefore textures were measured with an x-ray pole figure goniometer.
To provide large enough samples, the tube was sliced into sections (see
Figure 2), the sections lined up side-by-side, cold-mounted in epoxy and
ground flat. Figure 2 shows that two slice types were used, an offset
cross-section (maximized geometry with minimum sample) and a pure radial
cross-section (ideal axial alignment for easier pole figure
interpretation). After combining the multiple tube slices into two
samples, their sizes were approximately 10 mm square, the minimum sample
size for quantitative texture analysis with an x-ray pole figure
goniometer. All samples were mechanically ground and polished to a 0.05
μm finish to permit x-ray diffraction and establish consistency among
the samples.

![](media/image3.png){width="3.6194444444444445in"
height="2.714583333333333in"}

#### Figure 2: Diagram of slices from tube taken for texture measurements at a) offset cross-section, and b) pure radial cross-section

Both the tube and plate samples were fully austenitic at room
temperature, i.e. the *A*~f~ \< 25°C. Optical micrographs of the
microstructure are shown in Figure 3. It should be noted that the imaged
martensitic grains in the 485°C heat-treated samples are polishing
artifacts; stresses from grinding and polishing cause transformation in
the exposed grains which leaves residual surface relief on etching.
Moreover, for the as-drawn conditions, the apparent grain sizes observed
in Figures 3a and b also give a false impression about the actual grain
size; the true grain size for the as-drawn or rolled tube and plate is
on a sub-micron scale and can only be revealed by transmission electron
microscopy^12^. On the other hand, the optical micrograph in Figure 3c
is representative of the grain size of the fully annealed plate, as the
annealing has allowed significant recrystallization and grain growth.

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  height="1.6333333333333333in"}                       height="1.6333333333333333in"}             height="1.6430555555555555in"}

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*Figure 3: Optical micrographs of a) tube heat-treated at 485*°*C for 5
minutes; drawing direction is vertical, b) plate heat treated at 485*°*C
for 5 minutes, and c) plate heat treated at 850*°*C for 30 minutes*

The crystallographic texture of the Nitinol was determined from three
incomplete measured pole figures (110, 200, and 211 poles, at Θ[^1]
values of 21.3°, 30.8°, and 38.9°, respectively) and using the WIMV^\*^
method in BEARTEX^\*^ to obtain the orientation distribution function
(ODF)^13^. Pole figures were measured with a Huber X-ray pole figure
goniometer with Cu-Kα radiation source at 40 kV and 14 mA, and a
graphite monochromator. From the ODF, pole figures for 100, 110 and 111
poles were recalculated following smoothing and rotation operations and
are presented in the following section, together with ODF sections.

# RESULTS AND DISCUSSION

## POLE FIGURES

The pole figures in Figure 4 reveal that the tube and plate heat-treated
at 485°C show a significant texture. After corresponding rotations, pole
figures derived from both sections of the 485°C heat-treated tube are
similar (see Figures 4a and b). The main feature is a maximum in the 111
pole in the circumferential direction as indicated by the dark spots at
6 and 12 o'clock in Figures 4a and 4b, pole direction (p.d.) 111. In
addition, texture minima occur in the 100 pole parallel to all three
primary directions (drawing, circumferential, and radial) as indicated
by white areas in Figures 4a and 4b, p.d. 100, at 3 and 9 o'clock, 6 and
12 o'clock, and center, respectively. The pole figure of the pure radial
cross-sectioned tube, Figure 4b, shows a weak tendency in the 111 pole
parallel to the radial direction.

The 485°C heat-treated plate displays a maximum in the 110 pole parallel
to the rolling direction, as indicated by the highest intensity at 3 and
9 o'clock in Figure 4c, p.d. 110. Also, there is a 111 maximum parallel
to the normal direction as shown by the dark spot concentration at the
center of Figure 4c, p.d. 111. Furthermore, the 485^o^C heat-treated
plate has texture minima at the 100 and 111 poles parallel to the
rolling direction as indicated by white areas at 3 and 9 o'clock in
figures 4c, p.d. 100, and 4c, p.d. 111. Another minimum was observed at
the 110 pole parallel to the normal (thickness) direction as indicated
by the white spot in the center of Figure 4c, p.d. 110. The pole figures
for this plate are consistent with body-centered cubic (BCC) rolling
textures^12^.

Pole figures of the 850°C heat-treated plate show stochastic variations
due to poor grain statistics (see Figure 4d). A more regular pattern
becomes apparent after additional smoothing (see Figure 4e). Scattering
of the data in Figure 4d is attributed to recrystallization and grain
growth experienced during the annealing. Figure 4e shows that the
850^o^C heat-treated texture is generally the same as the plate
heat-treated at 485°C, although the degree of texturing was much less in
the higher temperature heat-treated condition, as indicated by the lower
pole densities (the same contouring levels were used for all pole
figures).

The only apparent similarity between Nitinol tube and plate texture is
the minimum of the 100 pole in the rolling/drawing direction. There may
also be a correlation between the tube's weak, and plate's strong,
tendency to texture the 111 pole in the radial/normal direction. The
significant differences in texture indicate that plate should not be
used as a replacement for tubes when tests are geometry-limited.
Moreover, plates that have been folded into a cylinder may not be
assumed to have the same texture as tubes.

![](media/image7.png){width="6.490277777777778in"
height="6.233333333333333in"}

*Figure 4: Pole figures showing texture in a) 485^o^C H.T. Tube offset
cross-section, b) 485^o^C H.T. tube pure radial cross-section, c)
485^o^C H.T. plate, d) 850^o^C H.T. plate with one ODF smoothing
operation (same as a-c), and e) 850^o^C H.T. plate with three ODF
smoothing operations*

The observed plate textures in Figures 4c-e were similar to those
previously determined by Monasevich et al.^1^, Li et al.^2^, and Inoue
et al.^4^ on other Nitinol plate or sheet geometry. However, pole
figures calculated by Kitamura et al.^5^ were different than those
obtained in this study. Their work on much thinner Nitinol sheets (0.1
mm) revealed a texture maximum of the 110 pole normal to the sheet,
whereas our data indicated a texture minimum at that location. That
group also observed that, upon increased-temperature heat treatment
(400^o^C versus 600^o^C, both for 60 minutes), maximum intensities of
the 110 pole in the rolling direction diffuse toward the transverse
direction creating offset local maxima. This trend was also observed in
the pole figures collected in our experiment; notice how the maximum
intensity spots at 3 and 9 o'clock in the 485^o^C heat-treated plate
(Figure 4c, p.d. 110), diffuse into four spots in the plate heat-treated
at 850^o^C (Figure 4e, p.d. 110).

**Sample Orientation Distributions (SODs)**

In order to obtain further information on texture components, the ODF
was analyzed in sample orientation distribution (SOD) sections,
displaying sample orientations in terms of crystal coordinates ϕ~1~ and
Φ (Bunge nomenclature is used). Figure 5 shows four SOD sections at ϕ~1~
values of 0, 15, 30 and 45 degrees for the 485^o^C heat-treated tube and
plate. The plate shows the typical features of a BCC rolling texture
with α fiber in the ϕ~1~=0^o^ section and a γ fiber extending parallel
to ϕ~1~^14^.

In the tube sample, the α fiber is absent and the γ fiber is only weakly
developed. The most pronounced orientation concentration, labeled as
point A in the figure, is in the ϕ~1~=45º section at Φ =90^o^ and
ϕ~1~=45^o^. Interpretations of the details contained in each SOD section
is beyond the scope of this preliminary investigation, but will be
conducted in future studies to completely characterize the tube texture
and understand its development.

![](media/image8.png){width="6.490277777777778in"
height="2.5569444444444445in"}

*Figure 5: Selected Sample Orientation Distributions (SODs) for 485^o^C
heat-treaded tube (pure radial cross-section) and plate in polar
coordinates; point A is the most pronounced texture component for the
tube. The α fiber, and γ fiber for the plate are indicated.*

The results presented in this report are based upon pole figure
collection of one sample of each selected geometry and heat treatment.
Future experiments are needed to confirm the findings presented here and
to further investigate the influence of deformation conditions on
texture and also on texture changes during the phase transformation.

# PRELIMINARY OBSERVATIONS

Room-temperature fully austenitic Nitinol tubes and plates have been
shown to be highly susceptible to crystallographic texturing following
mechanical processing (drawing or rolling) and heat treatment. Specific
conclusions include:

-   The tubes were found to be strongly textured in the 111 pole, along
    the circumferential direction. They have a weak tendency to texture
    the 111 pole in the radial direction. Also, the tubes have texture
    minima of the 100 pole in the circumferential, drawing, and radial
    directions.

-   The plates correspondingly were found to be textured with α and γ
    fibers, both common in BCC rolling textures. α fibers produce 110
    poles parallel to the rolling/drawing direction, whereas the γ fiber
    is characterized by the 111 pole parallel to the normal direction
    (i.e. plate thickness). Texture minima occur in both the 100 and 111
    poles parallel to the drawing direction, and in the 110 pole
    parallel to the normal direction.

-   Drawn tubes and rolled plates were observed to have few similarities
    in texture. We conclude therefore that plates should not be used as
    a substitute for tubes when testing is geometry-limited. In
    addition, plates that have been rolled into a cylinder cannot be
    assumed to have the same texture as tubes.

-   Some degree of remnant texture was found in Nitinol plate, even
    after a full anneal at 850°C for 30 minutes. This texture is similar
    to that of plate heat-treated at 485^o^C for 5 minutes.

# ACKNOWLEDGMENTS

This work was supported in part by a gift from Nitinol Devices and
Components (NDC), Inc., Fremont, California, and in part by Grant no.
CMS-980006 from the National Science Foundation, both to the University
of California at Berkeley. Thanks are due to Drs. Tom Duerig and Alan
Pelton of NDC for supplying the Nitinol tubing and for many helpful
discussions.

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# 

[^1]: ^\*^ Θ is equal to half of the angle between the x-ray source and
    detector; WIMV calculates orientation distributions from pole
    figures; BEARTEX is a software package for quantitative texture
    analysis.