Applied Physics 2019: Physical aspects of shape memory effect and reversibility in shape memory alloys - Osman Adiguzel - Firat University

Osman Adiguzel

Shape memory alloys take place in a class of smart materials by exhibiting a peculiar property called shape memory effect. This property is characterized by the recoverability of two certain shapes of material at different temperatures. These materials are often called smart materials due to the functionality and their capacity of responding to changes in the environment. Shape memory materials are used as shape memory devices in many interdisciplinary fields such as medicine, bioengineering, metallurgy, building industry and many engineering fields. Shape memory effect is performed thermally by heating and cooling after first cooling and stressing treatments, and this behaviour is called thermo elasticity. Shape memory effect is based on a solid state phase transition, martensitic transformation, and this transformation is characterized by changes in the crystal structure of the material. Shape memory effect is result of successive thermally and stress induced martensitic transformations. Shape memory alloys exhibit thermo elasticity and super elasticity by means of deformation in low temperature product phase and high temperature parent phase region, respectively. Super elasticity is another characteristic of shape memory alloys and performed by stressing and releasing the material in parent phase region.

Loading and unloading paths are different in stress strain diagram and cycling loop reveals energy dissipation. The strain energy is stored after releasing and these alloys are mainly used as deformation absorbent materials in control of civil structures subjected to seismic events, due to the absorbance of strain energy during any disaster or earthquake. Thermal induced martensitic transformation is first order lattice distorting phase transformations and thermally occurs as martensite variants with cooperative movements of atoms by means of lattice invariant shear. Lattice invariant shears occur in two opposite directions, <110 > -type directions on the {110} type planes of austenite matrix which is basal plane of martensite. Thermal induced martensite occurs as twinned martensite and the twinned structures turn into the det winned structures by means of stress induced martensitic transformation by stressing the material in the martensitic condition. Copper based alloys exhibit this property in metastable phase region, which has bcc-based structures at high temperature parent phase field. Lattice invariant shear and twinning is not uniform in copper based ternary alloys and gives rise to the formation of complex layered structures, depending on the stacking sequences on the close packed planes of the ordered parent phase lattice. In the present contribution, X-ray diffraction and transmission electron microscopy (TEM) studies were carried out on two copper based CuAlMn and CuZnAl alloys. X-ray diffraction profiles and electron diffraction patterns reveal that both alloys exhibit super lattice reflections inherited from parent phase due to the displaces character of martensitic transformation. X-ray diffraction grams taken in a long time interval show that diffraction angles and intensities of diffraction peaks change with the aging duration at room temperature. In particular, some of the successive peak pairs providing a special relation between Miller indices come close each other. This result refers to the rearrangement of atoms in diffusive manner. Cirrhotic patients admitted to medical care units (ICU) still have poor outcomes. Some current ICU prognostic models [Acute Physiology and Chronic Health Evaluation (APACHE), Organ System Failure (OSF) and Sequential Organ Failure Assessment (SOFA)] were wont to stratify cirrhotics into risk categories, but few cirrhotics were included within the original model development. Liver-specific scores [Child-Turcotte-Pugh (CTP) and model for end-stage disease (MELD)] might be useful during this setting to gauge whether ICU prognostic models perform better compared with liver-disease specific ones in cirrhotics admitted to ICU. We performed a structured literature review identifying clinical studies that specialize in prognosis and risk factors for mortality in adult cirrhotics admitted to ICU. We found 21 studies (five solely handling gastrointestinal bleeding) published during the last 20 years (54-420 patients in each).

APACHE II and III, SOFA and OSF had better discrimination for properly predicting death compared with the CTP score. The MELD score was evaluated only in one study and had good predictive accuracy [receiver operator characteristic (ROC) curve: 0.81). Organ dysfunction models (OSF, SOFA) were superior compared with APACHE II and III (ROC curve: range 0.83-0.94 vs. 0.66-0.88 respectively). Cardiovascular, liver and renal system dysfunction was more frequently independently related to mortality. General-ICU models had better performance in cirrhotic populations compared with CTP score; OSF and SOFA had the simplest predictive ability. Further prospective and validation studies are needed.

This work is partly presented at 6^th International Conference on Theoretical and Applied Physics May on 16-17, 2019 held at Rome, Italy