Issue 21

C. Maletta et alii, Frattura ed Integrità Strutturale, 21 (2012) 5-12; DOI: 10.3221/IGF-ESIS.21.01 6 transformation (TMT); the latter is a diffusionless phase transition which can be activated either by temperature (thermally-induced martensitic transformation, TIM) or by applied stress (stress-induced martensitic transformation, SIM) [1]. As a result of these microstructural changes, NiTi alloys show high recovery capabilities (up to a maximum deformation of 12%), by either raising the temperature of the material above the characteristic transition temperatures (SME) or by removing the mechanical load (PE). However, despite the increasing interest and the efforts of many researchers to understand these unusual mechanisms, the use of NiTi alloys is currently limited to high-value applications (e.g. medical devices, MEMS, etc.), due to high raw material and manufacturing costs, the latter resulting from the need to control precisely the processing parameters since the functional and mechanical properties of NiTi alloys are significantly affected by the thermo-mechanical loading history experienced during manufacturing. The design of NiTi based components also needs accurate knowledge of the mechanical and functional response of the material, as well as how this evolves during subsequent thermo-mechanical processes. In addition, as most NiTi components are characterized by complex shape and small size scale (e.g. endovascular stents, micro-surgery devices, MEMS etc.) their properties cannot be directly obtained from the bulk raw material. Thus the use of non-destructive techniques to analyze the mechanical and functional properties of small volumes of material is essential. Among the techniques available, nanoindentation is widely used to measure mechanical properties [2], such as hardness, elastic modulus, scratch resistance, creep, etc., of small volumes of materials with negligible damage to the surface. However, despite the aforementioned advantages, various difficulties arise in analyzing the mechanical properties of SMAs from the indentation response, due to micro-structural changes, such as phase transition and martensite variant re-orientation. In fact, the latter is expected to play a significant role in the indentation response of SMAs, as this takes place in the indentation region due to the presence of highly localized stresses. As a consequence, well known contact mechanics theories for conventional metals cannot be directly applied to SMAs and work has been carried out, in recent years, to understand better the effects of microstructural transitions on the indentation response of both thin films [3-8] and bulk specimens [9-15]. These studies revealed marked effects of material composition, as well as the thermo-mechanical treatments carried out during material processing, on the indentation response of SMAs. In particular, both the mechanical and thermal recovery mechanisms of nanoindents have been analyzed in order to study the pseudoelastic and shape memory capabilities of the alloys, respectively. Furthermore, the effects of the test temperature on the indentation response of a pseudoelastic alloy have been analyzed [11] by numerical simulations. In addition, a method to estimate the phase transformation stresses of a pseudoelastic alloy has been proposed in [13], based on comparing the indentation response of the SMA with that of a conventional elastic material. Finally, cyclic instrumented indentation was carried out in [14] so as to capture the stress-induced phase transition mechanisms from the experimentally measured load-displacement curves. However, notwithstanding the encouraging results obtained recently, considerable research needs to be carried out to elucidate the relationship between the indentation response of SMAs and their mechanical and functional properties. In this study a commercial pseudoelastic NiTi alloy (Type S, Memory Metalle, Germany) has been analyzed by indentation tests and finite element analysis. In particular, indents have been made at room temperature using a spherical diamond indenter and indentation loads in the range 50-500 mN, in order to promote a large stress-induced transformation zone in the indentation region and, consequently, to avoid local effects due to microstructural variations. Experimentally measured force-displacement curves have been analyzed to obtain information on the pseudoelastic response of the alloy. Furthermore, Finite Element (FE) simulations were developed, by using a special constitutive model for SMAs implemented in a commercial FE software code, to study the microstructural mechanisms occurring during indentation. The FE models have been used to analyze the stress induced transformation zone in the indentation region and systematic analyses have been carried out to understand better the relationship between the nanoindentation response and the typical thermo-mechanical parameters of SMAs. Finally, the FE model has been used to analyze the effects of temperature and transformation stresses, calculated from the Clausius-Clapeyron relationship, on the indentation response of the alloy. M ATERIAL AND METHODS Material commercial pseudoelastic NiTi sheet (Type S, Memory metalle, Germany), with a nominal chemical composition of 50.8 at.% Ni-49.2 at.% Ti and thickness t = 1.5 mm, has been used in this investigation. It was supplied in the flat annealed condition. The raw material was first analyzed by Differential Scanning Calorimetry (DSC) and standard tensile tests in order to determine the main thermo-mechanical parameters of the alloy. Fig. 1.a illustrates the A