**1. Introduction**

Multilayered composites have attracted much attention in engineering design as a promising technique to develop a novel combination of physical and mechanical properties acquired from the individual characteristics of the incorporated materials [1–5]. Bimetallic shape memory composites are among the most widely investigated class of composites, offering better shape memory properties for the design of new decomplex applications [6].

Shape memory alloys are stimulus-responsive materials with two universal properties: superelasticity and shape memory effect [7,8]. The occurrence of martensite-to-austenite and austenite-to-martensite transitions gives rise to shape memory and superelastic responses [9,10]. Among these alloys, Ni-Ti SMAs are among the most interesting thermo-responsive SMAs that are capable of exhibiting reliable shape memory characteristics, in addition to presenting high ductility and

strength [11]. Research on Ni-Ti shape memory alloys has been revived by controlling the "size-e ffect". Grain size reduction to the nano range greatly increases recovery stress [12]. The nanocrystalline (NC) or ultrafinegrained (UFG) microstructure significantly enhances the mechanical and shape memory characteristics in comparison with the coarse grained alloy of the same composition [13]. One way to refine the microstructure of the fabricated bulk UFG and nanostructured SMAs is severe plastic deformation (SPD) processing [14]. Most metal-matrix composites are obtained by accumulative roll bonding, a variant of the SPD technique, which also makes it possible to obtain multiple layers [15]. In addition, bimetallic "Ni-Ti/Ni-Ti" shape memory composites obtained by welding present large recoverable strain on heating and cooling [16], and are potential candidates for use as thermomechanical actuators [7,9,17]. In earlier work, the major problem of the formation of a thick brittle intermetallic layer was encountered, in particular in high temperature bonding processes [18]. Recent developments have led to the use of high pressure torsion (HPT) as an appropriate severe plastic deformation technique in the manufacturing of bimetallic composites [18].

The aim of our research is to study the structure and phase transformations of smart Ni-Ti multilayered composites obtained using the HSHPT technique. This process combines HPT and friction stir processing, and is capable of fabricating UFG discs of about 40 mm in diameter from di fferent types of alloys [19–21]. In addition, a more dependable method to fabricate bulk UFG metallic composites is HSHPT. We fabricated an "Ni-Ti/Ni-Ti" composite with 2 to 32 multilayered discs. The HSHPT process helps achieve very good bonding, high-quality interfaces with no intermetallic layers and ultrafine-grained microstructure from individual Ni-Ti layers.
