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Abstract: Phase transformational shakedown of a structure refers to a status that plastic strains cease developing after a finite number of loading cycles, and subsequently the structure undergoes only elastic deformation and alternating phase transformations with limited magnitudes. Due to the intrinsic complexity in the constitutive relations of shape memory alloys (SMA), there is as yet a lack of effective methods for modeling the mechanical responses of SMA structures, especially when they develop both phase transformation and plastic deformation. This paper is devoted to present an algorithm for analyzing shakedown of SMA structures subjected to cyclic or varying loads within specified domains. Based on the phase transformation and plastic yield criteria of von Mises-type and their associated flow rules, a simplified three-dimensional phenomenological constitutive model is first formulated accounting for different regimes of elastic–plastic deformation and phase transformation. Different responses possible for SMA bodies exposed to varying loads are discussed. The classical Melan shakedown theorem is extended to determine a lower bound of loads for transformational shakedown of SMA bodies without necessity of a step-by-step analysis along the loading history. Finally, a simple example is given to illustrate the application of the present theory as well as some basic features of shakedown of SMA structures. It is interesting to find that phase transformation may either increase or decrease the load-bearing capacity of a structure, depending upon its constitutive relations, geometries and the loading mode. [Copyright &y& Elsevier]

Abstract: The excellent microwear performance of nano-grained superelastic nickel titanium (NiTi) polycrystalline shape memory alloy (SMA) is reported in this paper. The microwear test was conducted at temperatures ranging from 22 to 120°C by a Hysitron triboindenter. The results showed that the NiTi SMA has superior microwear resistance compared to traditional tribo-materials such as stainless steel AISI 304 and that the material exhibits unusual hardness dependence of wear within certain temperature regimes. With the increase in temperature from 22 to 120°C, wear resistance was found to decrease anomalously with an increase in hardness. Further investigation and analysis confirmed that the stress-induced phase transition during contact and wear play an essential role in the material''s high wear resistance. It is demonstrated through contact mechanics analysis that the increase of hardness with temperature was mainly due to the increase in the phase transition stress. The observed applied threshold load that corresponds to the onset of the plastic deformation in the contact area was strongly influenced by the phase transition process at the tip region. For the investigated superelastic NiTi, the temperature-dependent interplay between reversible phase transition and irreversible plastic yielding plays a key role in the temperature dependence of the wear performance and is responsible for the observed apparent unusual hardness–wear relationships. [Copyright &y& Elsevier]

Deformation behaviors of the gradient nanostructured superelastic NiTi shape memory alloy strip with grain size ranging from 20 to 175 nm along the tensile axis are investigated. Compared with the commercial NiTi of uniform grain size, the gradient nanostructure leads to the increase of transformation stress as well as the morphology change, from localized deformation to homogeneous one. However, the behaviors in local different grain size regions are independent and maintain the grain-size dependent characteristics. [ABSTRACT FROM AUTHOR]

Microscale plastic deformation mechanisms of nanocrystalline NiTi micropillars with an average grain size (GS) of 65 nm are investigated by uniaxial compression. Cuboidal NiTi micropillars with uniform cross-sectional area along the height are manufactured by focused ion beam. It is found that the plastic deformation of the micropillars under compression is realized by localized shear band formation with large strain bursts, leading to significant amorphization and GS reduction from 65 nm to as small as 11 nm in the shear band. The experiment indicates that microfabrication by severe plastic deformation could be a promising manufacturing technology. [ABSTRACT FROM AUTHOR]

NiTi shape memory alloys offen subject to bending deformation as common functional materials. Laser shock peening (LSP) was utilized to fabricate a gradient layer to improve the bending mechanical behavior of a coarse-grained NiTi plate. A surface gradient layer with a thickness of approximately 100 µm was created in a 1.38 mm thick coarse-grained NiTi plate. The LSPed surface contained nanocrystals, dislocations, and amorphous, while the interior layer contained a substantial number of deformation twins, dislocations, and precipitates. The nanohardness of the surface treated with LSP can reach 10 GPa. The results of a three-point bending experiment indicate that LSP treatment increases the bending resistance of the NiTi plate. This study demonstrated that laser shock peening is an effective method for increasing the deformation resistance of a coarse-grained NiTi by means of surface gradient structures, residual stress, and precipitates. • Laser shock peening (LSP) creates surface gradient structure and compressive residual stress on a coarse-grained NiTi. • Achieves 10 GPa surface hardness. • Finds LSP induce deformation twins and grain refinement. • Demonstrates LSP enhances 3-point bending resistance. [ABSTRACT FROM AUTHOR]

Abstract: Fretting wear tests of a NiTi shape memory alloy plate against a GCr15 steel ball (NiTi/GCr15) were performed on a horizontal servo-hydraulic fretting test machine at various temperatures. As a comparison, fretting wear tests of a GCr15 steel plate against a GCr15 steel ball were also conducted under the same experimental conditions. The fretting behavior of GCr15 and NiTi alloy was studied by analyzing both the frictional logs (variation of the tangential force F t and displacement D with the number of fretting cycles N) and fretting scar features. The experiments showed that NiTi exhibits much better fretting behavior than GCr15 steel even with a relatively low hardness at room temperature. With an increase in temperature from 22 to 200°C, while the fretting behavior of GCr15 is almost temperature independent, the range of the partial slip regime of NiTi/GCr15 becomes small and the fretting wear resistance of NiTi is found to anomalously decrease with an increase in nominal hardness. It was demonstrated that the unique temperature dependent phase transition properties of NiTi play a key role in its fretting behavior. The stress-induced phase transition not only greatly improves the elastic accommodation ability of NiTi through the large recoverable deformation, but also increases its fretting wear resistance through the transformation shielding effect. With the increasing temperature, the elastic accommodation ability of NiTi decreases not only because of the increase of austenite elastic modulus and phase transition stress but also the plastic deformation of austenite, which in turn causes a small range of the partial slip regime in the NiTi/GCr15 pair. As the temperature increases, the decrease in fretting wear resistance is attributed to the decrease in the elastic accommodation ability of NiTi and the plastic yield of austenite before phase transition at high temperatures. [Copyright &y& Elsevier]