2010
Dependence of Transformation Temperatures of Shape Memory Alloys on the
Number and Concentration of Valence Electrons
Alloying is one of the most significant factors to alter the phase transformation temperatures
of alloys. Depending on the applications, SMAs possessing largely different actuation
temperatures are required. As a result, a large number of SMAs have been developed either
through addition of ternary and quaternary elements to NiTi, or through searching different
alloy systems, e.g., Cu-based, Fe-base, NiMnGa-base alloys, etc. Since transformation
characteristics are among the most important properties of SMAs when applications are
concerned, numerous investigations on modification of the transformation temperatures have
been carried out. Despite many efforts to relate transformation temperatures to the chemical
composition, this dependence has remained unclear, and the design of SMAs, is rather
empirical, due to the lack of an in-depth understanding on the effect of composition change.
In the present work some of the key influential factors (electronic structure, magnetic
properties and local atomic structure) which relate the alloy chemistry to the transformation
temperatures are introduced and discussed for a more robust alloy design and application.
The dependence of transformation temperatures of a large number of SMAs in nearly all
alloy systems on the number and concentration of valence electrons is examined. It is found
that the SMAs can be categorized into low (ev/a < 5), medium (5 ≤ ev/a ≤ 7.50), and high
(ev/a > 7.50) valence electron groups. For majority of alloys in medium and high valence
electron group, clear correlations between transformation temperatures and their valence
electron concentration (cv) are found. The martensite starting temperature (Ms) and austenite
starting temperature (As) both decrease from 1100˚C and 1150˚C to as low as -206˚C and 153˚C respectively, with increasing cv from 0.10 to around 0.30. Limited alloys that deviate
from this trend were further discussed. For NiTi-based alloys, two distinct trends of
transformation temperatures with respect to the number of valence electrons per atom are
found depending on whether ev/a = 7 or ev/a ≠ 7. Furthermore, for these alloys, the changes
of Ms as a result of second phase precipitation are mainly due to the change of electron
concentration of the matrix because of its composition change.
1000
ev/a=7
600
400
200
400
200
0
-200
-200
6.0
6.4
6.8
7.2
7.6
ev/a=7
600
0
5.6
ev/a<7 or ev/a>7
800
As Temperature/ C
800
Ms Temperature/ C
1000
ev/a<7 or ev/a>7
8.0
5.6
8.4
6.0
6.4
6.8
7.2
7.6
8.0
8.4
ev/a
ev/a
(a)
(b)
Figure. 1: Variations of Ms and As with the number of valence electron per atom (ev/a) of NiTi-based
ternary and quaternary alloys, (a) Martensite start, (b) Austenite start temperatures [1].
1200
1000
800
800
600
600
400
5<ev/a<7.50
ev/a>7.50
ev/a<5
1000
As/ °C
Ms/ °C
1200
5<ev/a<7.50
ev/a>7.50
ev/a<5
400
200
200
0
0
-200
-200
0.06
0.12
0.18
0.24
0.30
cv
0.36
0.42
0.06
0.12
0.18
0.24
0.30
0.36
0.42
cv
(a)
(b)
Figure 2: Variations of (a) Ms and (b) As with the valence electron concentration [2].
Related Publications:
[1] Zarinejad M., Y. Liu. Dependence of transformation temperatures of NiTi-based shape memory
alloys on the number and concentration of valence electrons, Advanced Functional Materials
18/18 (2008) 2789-2794.
[2] Zarinejad M., Y. Liu. Dependence of transformation temperatures of shape memory alloys on the
number and concentration of valence electrons, Book Chapter in Shape Memory Alloys:
Manufacture, Properties and Applications, Nova Science Publishers, Inc. 2010, ISBN: 978-160741-789-7, pp. 339-360.
[3] Zarinejad M., Y. Liu, Y.X. Tong. Transformation temperature changes due to second phase
precipitation in NiTi-based shape memory alloys, Intermetallics 17 (2009) 914–919.
[4] Zarinejad M., Y. Liu, T. White. The crystal chemistry of martensite in NiTiHf shape memory
alloys, Intermetallics 16 (2008) 876–883.
[5] Zarinejad, M. PhD thesis, Nanyang Technological University, Singapore, 2009.
Contact: [email protected] (Prof. Yong Liu)