NewNew ProductsProducts

Development of a Cu-Al-Mn Shape-Memory Alloy andits Application to an Ingrown Toenail Correcting Clip

Furukawa Techno Material Co. Ltd. has commercialized aCu-Al-Mn shape-memory alloy which has been developedby Tohoku University as an ingrown toenail correcting clip
and it has been launched by Reckitt Benckiser Japan Ltd.
The shape-memory effect has been found in a numberof alloy systems and it had been actively researched on acopper-alloy system, such as a Cu-Al-Ni alloy. However,
because of its poor workability and its insufficient shapememory property, the reality is that a Ti-Ni alloy is the only
major shape-memory alloy which has been put to practical use. Although the Ti-Ni alloy has a wide field of application, it has not been put to practical use except for ashape of wire and tube because of its poor workability.
In consultation with Tohoku University, we have
improved the shape-memory property of the Cu-Al-Mn
alloy and have succeeded in putting it to practical use as
an ingrown toenail correcting clip. That is the world’s first
application of this alloy.
An ingrown nail is a symptom where a nail curves in atransverse direction. It commonly appears in a first toe.
The nail bites into a toe and it often accompanies pain.
Although there are various treatments such as a correcting method using a Ti-Ni alloy superelastic wire, the burden on the patients, the complicated medical procedures
and the high cost were the problems.
The newly developed Cu-Al-Mn alloy has a good workability and can easily be processed into a plate and athree-dimensional shape. Also, as it requires no forming
block during a shape-memory treatment, it can cut costssignificantly. For these reasons, it has been commercialized after a number of experiments and monitor tests.

2.

ALLOY DESINGNING AND TEXTURE
CONTROL

Tohoku University’s research shows that a high-workability Cu-Al-Mn shape-memory alloy can be achieved by thefollowing alloy designing and the texture control.
By adding Mn to a ternary alloy of Cu-Al-Mn, β phase,
which shows martensitic transformation, comes to exist
widely. The β phase shows order transformation and theorder-transformation temperature is highly dependent
upon Al content. When the Al content decreases, its cold
workability significantly increases. The reason of it isexplained by the fact that the decrease of the Al content
brings the decrease of the order-transformation tempera-ture.
On the other hand, the shape-memory effect shows aproperty decrease at less or equal to 16 at%Al, which
generates an A2 disordered structure. As explained
above, the optimum composition which has a good
shape-memory property and a high workability at thesame time is confirmed to be a Cu-17 at%Al-Mn (Mn is 10– 13 at% for the sake of adjusting martensitic initiation
temperature (Ms)).
Next, this alloy requires texture control to achieve agood shape-memory property. Figure 1 shows the relation between the grain sizes (d), which are standardized
by the wire diameters (D), and the superelasticity after
tensile tests. Figure 1 (a), in which crystal grains penetrate the diameters, shows a superior superelasticity
property. Also, this alloy is confirmed to be effective forthe increase of a superelasticity property because the{112} <110> recrystallization texture is generated by athermomechanical treatment.
Table 1 shows the properties of the Cu-Al-Mn alloy andother shape-memory alloys. The prime property is that
the Cu-Al-Mn alloy shows greater or equal to twice asmuch cold working rate in comparison to other alloys.
That brings not only the improvement of manufacturability
but the achievement of a good superelasticity strain rate
which is comparable to that of the Ti-Ni alloy by enablingtexture control.
Test temperature =Af + 30℃
Stress (MPa)INTRODUCTION

Stress (MPa)1.

Strain (%)
Figure 1

Superelastic behavior of Cu-Al-Mn alloys in
different grain sizes.

Furukawa Review, No. 41

201222

Development of a Cu-Al-Mn Shape-Memory Alloy and its Application to an Ingrown Toenail Correcting Clip

Table 1100Maximum
Temperature dependency
Superelasticity
cold workingof superelasticity stressstrain (%)
rate (%)(MPa℃-1)

80

Cu-Al-Mn>60

Simplicity ofshape-memory
treatment

7.5

2.4

○No forming block

Ti-Ni

30

85.7

×Forming block required

Cu-Al-Mn

102––Cu-Zn-Al

30

2––

Another practical advantage is that the Cu-Al-Mn alloy
can show more stable superelasticity to temperaturechanges because its superelasticity stress is less dependent on temperatures than in the Ti-Ni-alloy.

3.

PROPERTIES OF THE CLIP(a)

Recovery force caused by superelasticityFulcrum
First toe

Clip

Figure 2(a) Ingrown toenail correcting clip.
(b) Correcting method.

10ClipTi-Ni wire

Load, N864
200123Displacement, mmFigure 3

6040
200Figure 4

As explained above, the Cu-Al-Mn shape-memory alloy
has superior workability. Therefore, it is applicable tocomplex-shape products which are difficult to be manufactured using existing the Ti-Ni alloy. As shown in Figure
2 (a), we have commercialized a new ingrown toenail correcting device. This product is attached to a nail with four
double-folded hooks as shown in Figure 2 (b). It corrects
an ingrown nail with the recovery force caused by thesuperelasticit(b)

Breakage rate, %Comparison of Cu-Al-Mn and Other Shape-Memory Alloys.

Superelastic properties of theCu-Al-Mn clip and the Ti-Ni wire.

4Non-coating

Coating

Coating

Result of coating in the stress corrosion test.

Different from usual spring materials, because thedeforming stress of the superelasticity is not highly
dependent on the strain, it can give nearly-constant correcting force to the nail regardless of the degree of curvedness. That causes less cracks and discomfort to thenail. Therefore, it is suitable for correcting ingrown toenails. The clip-on style enables the easiness of putting on
and taking off, and does not necessarily require a treatment by the specialist. Therefore, it can cut treatment
costs.
Different from the Ti-Ni alloy, this alloy’s shape-memory
treatment needs only a heat treatment, and doesn’t need
to be fixed by a die. Therefore, it can easily be shapedand memorized in a complex shape which has double
folds as shown in Figure 2. For the shape-memory treatment, a heat treatment was conducted at 900℃ for about
10 minutes. When cooling it, rapid cooling was necessary
to restrain α phase precipitate. Also, it is desirable toconduct an aging treatment at 100 - 200℃ for 15 minutes
after the shape-memory treatment to stabilize the transformation temperature. At the time of commercialization, thereverse transformation end temperature (Af) was controlled at about -20℃ to achieve good superelasticity inthe usage environment.
To evaluate the elasticity of the clip, a three-point bending test was conducted. Figure 3 shows the chart resulted
from the test. Although the Cu-Al-Mn alloy clip had someamount of residual strain, it had a sufficient displacement
field with small amount of load variation after unloaded.
Therefore it was confirmed to be sufficiently usable forcorrecting ingrown nails.
Because this clip is attached to a nail and used inside asock and a shoe, it is exposed to a humid environment.
Such an environment can generate a problem of corrosion. Because the load on the hook part which clips thenail becomes heavy during use, a rare phenomenon thatthe hook part is broken by stress corrosion was confirmed. Therefore, we concluded that surface coating isnecessary at the time of commercialization. Coating wasapplied by electrodeposition to coat even inside the double-folded hook part. Then, it was submitted to a stresscorrosion test. Figure 4 shows the percentage of the samples of the hooks that were broken by absence of coat-Furukawa Review, No. 41

201223

Development of a Cu-Al-Mn Shape-Memory Alloy and its Application to an Ingrown Toenail Correcting Clip

ing. Coating reduced the breakage rate significantly andimproved stress-corrosion cracking.

4SITUATION OF PRACTICALAPPLICATION AND PATENTIn the clinical trial of the ingrown toenail correcting clip at
the Department of Dermatology, Japan Red Cross Sendai
Hospital, an early-stage of pain disappearance and an
improvement in curvedness were confirmed. That means
it is not only easy to put on and to take off, but has asuperior correcting effect. It has been launched sinceMarch 2011 by Reckitt Benckiser Japan Ltd. as a brand“Dr. Scholl” for medical institutions. It has already beensold to about 1000 medical institutions and obtained ahigh reputation. Also, we are in the course of applying forpatents of this technology both domestically and internationally.

5.

CONCLUSION

We have commercialized an ingrown toenail correcting
clip using a high-workability Cu-Al-Mn shape-memory
alloy. We expect this clip to be widely adopted as a newand easy treatment for correcting ingrown nails. The
Cu-Al-Mn shape-memory alloy will be an option for thematerials of products which require formability andmachinability.
This article is edited based on the paper, posting toMateria Japan vol.51, No.3, (2012), “Development ofCu-Al-Mn shape memory alloy and its application toingrown toenail correcting clip” by Toyonobu Tanaka,
Sumio Kise, Toshihiro Omori, Ryosuke Kainuma andKiyohito Ishida.
For more information, please contact:
Technology Research Section
Research & Development Department
Special Metals Division
Furukawa Techno Material Co., Ltd.
TEL: +81-463-21-7316 FAX: +81-463-21-7385

Furukawa Review, No. 41

201224