TiNi alloys are well known and the most widely used materials having the shape-memory effect (SME) and superelasticity. They  are applied in various areas of engineering and also in medicine as a material for manufacture of instruments and implants.
The studies conducted at IMAM USATU have demonstrated that structure refinement to the nano- and ultrafine-grained (UFG) size by severe plastic deformation (SPD) processing allows for a significant enhancement of the mechanical properties and the functional characteristics of the shape-memory effect in TiNi alloys.
We have experience in producing UFG and nanocrystalline (NC) TiNi alloys by various SPD techniques:
– High-pressure torsion (HPT)
– Equal-channel angular pressing (ECAP)
– ECAP combined with subsequent rolling and annealing
–  ECAP-Conform (a process with good industrial prospects)
Using ECAP processing, we have produced bulk samples ( 20 mm, length up to 100 mm) of TiNi alloys in the UFG state (with a grain size down to 250 nm). As a result, the strength  в  reaches 1400 MPa,  and the dislocation yield stress  y  is increased up to  1300 MPa  (depending on the alloy composition),  which is noticeably higher than the corresponding parameters in the initial coarse-grained state where  в  and  y  are around 1000 MPa and  600 MPa,  respectively.  At the  same time, the ductility of UFG  TiNi alloys is retained at a reasonably high level  –  up to  40%.  As a result of ECAP procesing, the following shape-memory effect characteristics have been achieved in TiNi alloys: reactive stress rmax– up to 1000 MPa, recoverable strain  ‘ r max–up to 9 %.
HPT processing of TiNi alloys leads to their amorphization, and subsequent controlled annealing leads to the formation of an NC structure with a grain size Dg of 20 nm. The dimensions of the HPT-processed samples reache  20 mm, and a thickness of 1 mm. After HPT processing, the strength в and the dislocation yield stress y exceed 2000 МPa. It has been shown that at a grain size of 20 nm and above, martensitic transformation takes place in NC TiNi alloys.
It has been shown that processing by ECAP combined with subsequent rolling or annealing enables fabrication of amorphicized a nd NC samples –  strips of  TiNi rmaxcan reach 1400 MPa.
We also have some experience and are further developing studies on the fabrication of rods from TiNi alloys with a UFG and NC structure through processing by ECAP-Conform, a process that has good industrial prospects, combined with subsequent rolling or drawing (on the basis of this approach, t he production of long-length rods from nanostructured Ti for medical implants was organized earlier in Ufa).
We have experience in the processing of shape-memory alloys with different compositions:
– Aging and non-aging TiNi alloys with the temperatures of martensitic transformation from –100 to +100С
– TiNiFe alloys with the temperatures of martensitic transformation below  –100С
– TiNiNb alloys with a wide hysteresis of martensitic transformation

Functional and mechanical properties of TiNi alloys in various states

Type of treatment  Structure and grain size σВ, MPa σy,
MPa		 *εr,1max,% σrmax,MPA
Initial state (as-quenched) 60 µm 1000 540 5 380
Low-temperature mechanical treatment* Substructure 1 – 0.5 µm 1200 900 7 720
ECAP UFG austenite, 300 nm up to 1400 1000 9.2 800
HPT + annealing 50 nm >2100 1900 8 1400

*S.D.Prokoshkin, I.Yu. Khmelevskaya, S.V. Dobatkin, TMS, 2006

Pilot products  have been developed at the National Institute of Science and Technology MISiS (Moscow), made from UFG TiNiFe alloys produced at IPAM USATU –  in particular, a device for demountable thermomechanical joint of parts and structure elements (see the figure below)

Basic characteristics of the coupling

Material
 Shearing force of the parts with
respect to the coupling at 20°С, N		 Specific circumferential swaging force (characterizes the load-bearing capacity), N/mm
Conventional 7800 550
UFG 16100 1140

Main publications:

1.  Pushin  V.G.,  Stolyarov  V.V.,  Valiev  R.Z.,  Kourov  N.I.,  Kuranova  N.N.,  Prokofiev  E.A.,  Yurchenko  L.I.  Features  of  structure  and  phase  transformation  in  shape memory TiNi-based alloys after severe plastic deformation. Ann. Chim. Sci. Mat., 2002, 27 (3), P. 77 – 88.
2.  Stolyarov V.V., Prokof’ev E.A.,  Valiev R.Z., Prokoshkin S.D., Dobatkin S.B., Trubitsyna I.B., Khmelevskaya I.Y., Pushin V.G.  Structural features, mechanical properties and the shape-memory effect in TiNi alloys subjected to equal-channel angular pressing  // The Physics of Metals and Metallography.  –  2005.  –  Vol. 100. – No. 6. – P. 608 – 618.
3.  Prokoshkin S.D., Khmelevskaya I.Yu., Dobatkin S.V., Trubitsyna I.B., Tatyanin E.V., Stolyarov V.V., Prokofiev E.A. Alloy composition, deformation temperature, pressure and post-deformation annealing effects in severely deformed Ti-Ni based shape memory alloys // Acta Materialia. – 2005. – V. 53. – Issue 9. – P. 2703 – 2714.
4.  R. Valiev, D. Gunderov, E. Prokofiev, V. Pushin, Y. Zhu. Nanostructuring of a TiNi alloy by SPD processing for advanced properties. Materials Transactions, Vol. 49, No.1 (2008) pp. 97-101.
5.  D. Gunderov, A. Lukyanov, E. Prokofiev, A. Kilmametov, V. Pushin, R. Valiev. Mechanical properties and martensitic transformations in nanocrystalline Ti49.4Ni50.6alloy produced by high-pressure torsion. Materials Science and Engineering: A, Vol. 503, Issues 1-2 (2009), pp. 75-77.
6.  Valiev R.Z., Gunderov D.V., Lukyanov A.V., Prokofiev E.A., Kuranova N.N., Makarov V.V., Pushin V.G., Uksusnikov A.N. Study of the heat treatment influence on the formation of nanostructured states in bulk titanium nickelide alloys subjected to severe plastic deformation  //Bulletin of the Russian Academy of Sciences: Physics, 2009, Vol. 73, No. 11, P. 1519–1521.
7.  E.A. Prokofiev, J.A. Burow, E.J. Payton, R. Zarnetta, J. Frenzel, D.V. Gunderov, R.Z. Valiev and G. Eggeler,  Suppression of Ni4Ti3  Precipitation by Grain Size Refinement in Ni-Rich NiTi Shape Memory Alloys, Advanced Engineering Materials, Vol. 12, 8, pp.747–753.
8.  Gunderov Dmitry, Lukyanov Aleksandr, Prokofiev Egor, Churakova Anna, Pushin Vladimir, Prokoshkin Sergey, Stolyarov Vladimir,  Valiev Ruslan,  Microstructure and mechanical properties of the SPD-processed TiNi alloys, Materials Science Forum Vols. 738-739 (2013) pp 486-490.
9.  Tong Y.X., Chen F., Guo B., Tian B., Li L., Zheng Y.F., Gunderov D.V., Valiev R.Z.  Superplasticity and its stability of an ultrafine-grained  Ti49.2Ni50.8  shapememory alloy  processed by equal channel angular presing, Materials Science and Engineering: A, 2013, Vol. 587, pp. 61-64. IF 2,1.
10.P.C. Jiang, Y.F. Zheng, Y.X. Tong, F. Chen, B. Tian, L. Li, Dmitry V. Gunderov, Ruslan Z. Valiev,  Transformation hysteresis and shape memory effect of  anultrafine-grained TiNiNb shape memory alloy. Intermetallics 54 (2014) 133-135 IF 1,8.