Through single‐crystal x‐ray diffraction methods, the crystal structure of TiNi has been determined in the temperature range −70° to 900°C. Contrary to what has been assumed from previous work based on the powder pattern methods, the TiNi crystal structure is not a simple CsCl type. Rather, it has an a0=9Å superlattice and an a0=3Å sublattice with 54 atoms per unit cell complex structure.
The 9Å superlattice undergoes, at about 166°C, a ``martensitic'' pseudo order‐disorder transition which is not accompanied by a crystallographic transformation. Through the understanding of this unique transition the apparent contradicting observations made on TiNi by various past investigators can now be reconciled and the unusual physical properties associated with the alloy are explained qualitatively.
REFERENCES
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J. R. Long, E. T. Hayes, D. C. Root, and C. E. Armantrout, U.S. Bur. Mines Rept. Invest. p. 4463 (1949).
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W. J. Buehler and R. E. Wiley, Naval Ordnance Laboratory Rept. 61‐75 (August 1961).
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M. V.
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The wide range of transition temperature is due both to the sluggishness of the diffusion process and to the nonuniform heating of the “single‐crystal wire”.
26.
The terminology utilized here is that of Guinier (Ref. 27).
27.
A. Guinier, X‐ray Diffraction in Crystals, Imperfect Crystals and Amorphous Bodies (W. H. Freeman and Company, San Francisco, California, 1963).
28.
The reasoning is similar to that given for cleavage planes.
29.
Over‐all, the x‐ray will “see” a scattering power of for the (0,0,0) lattice and for the () lattice. This, therefore, gives rise to a 3‐Å sublattice of the CsCl type.
30.
This is indicated by the length of the streaks appearing in Fig. 3(a).
31.
M. J. Buerger, Vector Space (Jonh Wiley & Sons, Inc., New York, 1959), p. 310.
32.
Since the super‐reflections in Fig. 3(b) are weak and show about the same intensities, the traces are unweighted.
33.
The interatomic distances of Ti and Ni in their metallic states are 2.89 and 2.49 Å, respectively, the metallic interatomic distance of Ti‐Ni should therefore approximate 2.69 Å.
34.
A wire specimen was used because preparation of powder specimens is impossible with this alloy.
35.
The agreement is good, considering the fact that the “powder pattern” was obtained from a wire specimen which presumably is highly strained and is preferentially oriented.
36.
From the heat capacity of the TiNi alloy (0.077 cal ) the latent heat of the 40 °C transition is estimated to be 5.78 cal/g.
37.
B. Chalmers, Physical Metallurgy (John Wiley & Sons, Inc., New York, 1959), p. 358.
38.
The fact that this temperature coincides with that obtained from the damping curve (Fig. 1) suggests that the as‐cast bars (which were used to obtain the curve) must be relatively free of internal strain as are the single crystals; therefore, the damping curve can be taken as an approximation to the transition curve.
39.
R. C. Wiley and C. E. Sutton, “Stresses Associated with Structural Transformations in 55.4 wt% Nitinol,” TN‐6232 (24 January 1964).
40.
For further discussion on this subject, see F. S. Bowles and C. S. Barrett, Progress in Metal Physics (Pergamon Press, Inc., New York, 1952), Vol. 3;
L. Kaufman and M. Cohen, ibid., Vol. 7;
B. Chalmers, Physical Metallurgy (John Wiley & Sons, Inc., New York, 1959);
B. A. Bilby and J. W. Christian, “The Mechanism of Phase Transformations in Metals,” Institute of Monograph and Report Series, No. 18, p. 121 (1955).
41.
As far as the authors are aware, this is the highest latent heat ever observed in a diffusionless noncrystallographic transformation.
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J. M. Ziman, Electrons in Metals (Taylor or Francis Ltd., Red Lion Court, Fleet Street, London, 1963), p. 15.
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F. E. Wang, “The Mechanical Properties as a Function of Temperature and Free Electron Concentration in Stoichiometric TiNi, TiCo, and TiFe Alloys” presented at the International Conference on Fracture, Sendai, Japan, 12–17 September 1965.
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