X-ray diffraction and dilation studies have shown that alloys near the stoichiometric TiNi composition undergo transformation into the related phases Ti2Ni and TiNi3 at low temperatures. The main factors controlling these phase transformations are alloy composition, temperature, and mode of plastic deformation. In plastic deformation, tensile or compressive stressing produced separate and unlike decomposition phases; this finding was dramatically demonstrated by unique temperature-sensitive dimensional changes in plastically deformed specimens. Changes of large magnitude in vibration damping have also been noted and appear related to variations in the phase equilibria of the system.

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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.

Through an investigation of the transport and related thermodynamic properties of TiNi at and around its ``martensitic'' transition temperature, the existence of a critical range extending over a 60°C interval is established. Within this critical range, the phase transition is second order. Irreversibility of various properties within the critical range is interpreted in terms of irreversible shear movement of atoms. On the basis of transport data, the band structure of TiNi is inferred to be a single or ``nearly'' single positive band within the temperature range investigated. The postulate that some of the valence electrons undergo a ``covalent''→``conduction'' electronic transformation in the course of the second-order transition is consistent in large measure with experimental data.

Some unique physical property changes related with the TiNi martensitic transition, such as ``memory'' effect, have been known for some time. In this letter we report some additional observations which are of interest and are related to the TiNi transition.