Shape Memory Alloys (Part II): Classification, Production and Application

Shape memory alloys (SMAs) have been extensively investigated because of their unique shape memory behaviour, i.e. their ability to recover their original shape they had before deformation. Shape memory effect is related to the thermoelastic martensitic transformation. Austenite to martensite phase transformation can be obtained by mechanical (loading) and thermal methods (heating and cooling). Depending on thermomechanical conditions, SMAs demonstrate several thermomechanical phenomena, such as pseudoelasticity, superelasticity, shape memory effect (one-way and two-way) and rubber-like behaviour. Numerous alloys show shape memory effect (NiTi-based alloys, Cu-based alloys, Fe-based alloys etc.). Nitinol (NiTi) is the most popular and the most commonly used SMA due to its superior thermomechanical and thermoelectrical properties. NiTi alloys have greater shape memory strain and excellent corrosion resistance compared to Cu – based alloys. However, they are very costly. On the other hand, copper-based alloys (CuZn and CuAl based alloys) are much less expensive, easier to manufacture and have a wider range of potential transformation temperatures. The characteristic transformation temperatures of martensitic transformation of CuAlNi alloys can lie between −200 and 200 °C, and these temperatures depend on Al and Ni content. Among the Cu – based SMAs, the most frequently applied are CuZnAl and CuAlNi alloys. Although CuZnAl alloys with better mechanical properties are the most popular among the Cu-based SMAs, they lack sufficient thermal stability, while CuAlNi shape memory alloys, in spite of their better thermal stability, have found only limited applications due to insufficient formability owing to the brittle γ₂ precipitates. The most important disadvantage of polycrystalline CuAlNi alloys is a small reversible deformation (one-way shape memory effect: up to 4 %; two-way shape memory effect: only approximately 1.5 %) due to intergranular breakdown at low stress levels. The technologies for production of shape memory alloys are induction melting, vacuum melting, vacuum arc melting, following hot and cold working (forging, rolling, wire drawing). In addition, rapid solidification methods, like melt spinning and continuous casting have been developed. These methods are characterized by high cooling rates. High cooling rates allow very short time for diffusion processes and may lead to extremely fine microstructure, better homogeneity etc. SMAs have found applications in many areas due to their thermomechanical and thermoelectrical properties (biomedical applications, engineering industry, electrical industry). In this paper, a review of shape memory alloys, properties and applications of mentioned materials is presented

Ischaemic stroke and combined oral contraceptives: Results of an international, multicentre, case-control study

Background The association between use of oral contraceptives (OCs) and cerebral infarction was established in studies from northern Europe and the USA during the 1960s and 1970s. Since then, the constituents of hormonal OCs have changed and now contain lower doses of oestrogen and progestagen. Current recommendations restrict OC use to younger women who do not have other risk factors for cardiovascular disease, in this international study we assessed the risk of CC-associated first stroke in women from Europe and other countries throughout Vie world.Methods In this hospital-based, case-control study, we assessed the risk of ischaemic stroke in association with current use of combined OCs in 697 cases, aged 20-44 years, and 1962 age-matched hospital controls in 21 centres in Africa, Asia, Europe, and Latin America. The diagnosis of ischaemic stroke was almost exclusively based on computed tomography (CT), magnetic resonance imaging (MRI), or cerebral angiography carried out within 3 weeks of the clinical event. Ail cases and controls were interviewed while in hospital with the same questionnaire, which included information on medical and personal history, details of lifetime contraceptive use, and blood-pressure measurements before the most recent episode of OC use.Findings The overall odds ratio of ischaemic stroke was 2 . 99 (95% CI 1 . 65-5 . 40) in Europe and 2 . 93 (2 . 15-4 . 00) in the non-European (developing) countries, Odds ratios were lower in younger women and those who did not smoke, and less than 2 in women who did not have hypertension and who reported that their blood pressure had been checked before the current episode of OC use. By contrast, among current OC users with a history of hypertension, the odds ratio was 10 . 7 (2 . 04-56 . 6) in Europe and 14 . 5 (5 . 36-39 . 0) in the developing countries. In Europe, the odds ratio associated with current use of low-dose OCs (<50 mu g oestrogen) was 1 . 53 (0 . 71-3 . 31), whereas for higher-dose preparations it was 5 . 30 (2 . 56-11 . 0). In the developing countries, there was no significant difference between overall estimates of risk associated with use of low-dose or higher-dose OCs (3 . 26 [2 . 19-4 . 86] vs 2 . 71 [1 . 75-4 . 19]), This differential effect of dose in Europe and the developing countries is likely to be due to different levels of other risk factors among users of low-dose and higher-dose OCs in the two groups of countries. There was no significant increase in odds ratios with increasing duration of OC use among current users; odds ratios were not significantly increased after cessation of OC use.Interpretation The incidence of ischaemic stroke is low in women of reproductive age and any risk attributable to OC use is small. The risk can be further reduced if users are younger than 35 years, do not smoke, do not have a history of hypertension, and have blood pressure measured before the start of QC use. In such women OC preparations with low oestrogen doses may be associated with even lower risk.WHO, UNDP UNFPA WHO WORLD BANK SPECIAL PROGRAMME RES D, CH-1211 GENEVA 27, SWITZERLANDESCOLA PAULISTA MED, DEPT PREVENT MED, SAO PAULO, BRAZILUNIV CHILE, ESCUELA SALUD PUBL, SANTIAGO, CHILEESCUELA MED, DEPT SALUD PUBL, VALPARAISO, CHILENATL RES INST FAMILY PLANNING, BEIJING, PEOPLES R CHINASICHUAN FAMILY PLANNING RES INST, CHENGDU, PEOPLES R CHINASHANGHAI INST PLANNED PARENTHOOD RES, SHANGHAI, PEOPLES R CHINAUNIV VALLE, FAC SALUD, CALI, COLOMBIAUNIV OXFORD, DEPT PUBL HLTH & PRIMARY CARE, OXFORD OX3 7LF, ENGLANDZENTRUM EPIDEMIOL & GESUNDHEITSFORSCH, BERLIN, GERMANYCHINESE UNIV HONG KONG, DEPT COMMUNITY MED, HONG KONG, HONG KONGALBERT SZENT GYORGYI MED UNIV, DEPT OBSTET & GYNAECOL, SZEGED, HUNGARYUNIV INDONESIA, FAC MED, JAKARTA, INDONESIAUNIV W INDIES, TROP METAB RES UNIT, KINGSTON 7, JAMAICAKENYA GOVT MED RES CTR, NAIROBI, KENYAGRP INTERUNIV MEXICANO INVEST EPIDEMIOL SALUD REP, DURANGO, MEXICOUNIV LJUBLJANA, INST PUBL HLTH, LJUBLJANA, SLOVENIACHULALONGKORN HOSP, DEPT OBSTET & GYNAECOL, BANGKOK, THAILANDSIRIRAJ HOSP, SIRIRAJ FAMILY HLTH RES CTR, BANGKOK, THAILANDUNIV BELGRADE, SCH MED, BELGRADE, YUGOSLAVIAUNIV LUSAKA, TEACHING HOSP, LUSAKA, ZAMBIAUNIV ZIMBABWE, DEPT MED, HARARE, ZIMBABWEUCL, LONDON MED SCH, DEPT EPIDEMIOL & PUBL HLTH, LONDON, ENGLANDKAISER PERMANENTE, PASADENA, CA USANIH, BETHESDA, MD 20892 USAUNIV AARHUS, DANISH EPIDEMIOL SCI CTR, AARHUS, DENMARKUNIV LONDON LONDON SCH HYG & TROP MED, LONDON WC1E 7HT, ENGLANDESCOLA PAULISTA MED, DEPT PREVENT MED, SAO PAULO, BRAZILWeb of Scienc