Constitutive description of creep behavior of Mg-4 Al-1Ca alloy

Creep behavior of an advanced magnesium alloy AX41 (4 wt.% Al, 1 wt.% Ca, Mg balanced) was investigated in temperature interval from 343 to 673 K and stresses from 2 to 200 MPa. Compressive creep experiments with stepwise loading were used in order to obtain stress dependence of the creep rate in interval from 10~9 to 10~3 iT 1 fo r a given temperature. All stress dependences can be well described by the Garofalo sinh relationship with natural exponent n = 5. An analysis ofthe parameters of this relationship has shown that lattice diffusion controls creep at all experimental conditions. While climb-controlled creep mechanism is decisive at lower stresses and higher temperatures, glide-controlled mechanisms act at higher stresses and lower temperatures. A typical power-law breakdown is observed at intermediate stresses and temperatures.Выполнены исследования ползучести оптимизированного магниевого сплава AX41 (4 вес.% Al, 1 вес.% Ca, Mg - баланс) в температурном диапазоне 343...673 K и уровнях напряжений 2...200 МПа. Были проведены испытания на ползучесть при ступенчатом нагружении сжимающей нагрузкой с целью определения зависимости скорости ползучести от напряжения в диапазоне 10-9...10-3 с-1 для данной температуры. Все вышеуказанные зависимости хорошо описывались уравнением Барафоло (типа sinh) с показателем степени n = 5. Анализ параметров этого уравнения показал, что ползучесть во всех проведенных экспериментах определяется диффузией решетки. При низких уровнях напряжений и высоких температурах определяющим является механизм восхождения, а при высоких уровнях напряжений и более низких температурах - механизм скольжения. В промежуточном диапазоне напряжений и температур наблюдается характерное отклонение экспериментальных точек от степенной зависимости

Precise Characterization and Multiobjective Optimization of Low Noise Amplifiers

Although practically all function blocks of the satellite navigation receivers are realized using the CMOS digital integrated circuits, it is appropriate to create a separate low noise antenna preamplifier based on a low noise pHEMT. Such an RF front end can be strongly optimized to attain a suitable tradeoff between the noise figure and transducer power gain. Further, as all the four principal navigation systems (GPS, GLONASS, Galileo, and COMPASS) work in similar frequency bands (roughly from 1.1 to 1.7 GHz), it is reasonable to create the low noise preamplifier for all of them. In the paper, a sophisticated method of the amplifier design is suggested based on multiobjective optimization. A substantial improvement of a standard optimization method is also outlined to satisfy a uniform coverage of Pareto front. Moreover, for enhancing efficiency of many times repeated solutions of large linear systems during the optimization, a new modification of the Markowitz criterion is suggested compatible with fast modes of the LU factorization. Extraordinary attention was also given to the accuracy of modeling. First, an extraction of pHEMT model parameters was performed including its noise part, and several models were compared. The extraction was carried out by an original identification procedure based on a combination of metaheuristic and direct methods. Second, the equations of the passive elements (including transmission lines and T-splitters) were carefully defined using frequency dispersion of their parameters as Q, ESR, etc. Third, an optimal selection of the operating point and essential passive elements was performed using the improved optimization method. Finally, the s-parameters and noise figure of the amplifier were measured, and stability and third-order intermodulation products were also checked

Constitutive description of creep behavior of Mg-4 Al-1Ca alloy

Creep behavior of an advanced magnesium alloy AX41 (4 wt.% Al, 1 wt.% Ca, Mg balanced) was investigated in temperature interval from 343 to 673 K and stresses from 2 to 200 MPa. Compressive creep experiments with stepwise loading were used in order to obtain stress dependence of the creep rate in interval from 10~9 to 10~3 iT 1 fo r a given temperature. All stress dependences can be well described by the Garofalo sinh relationship with natural exponent n = 5. An analysis ofthe parameters of this relationship has shown that lattice diffusion controls creep at all experimental conditions. While climb-controlled creep mechanism is decisive at lower stresses and higher temperatures, glide-controlled mechanisms act at higher stresses and lower temperatures. A typical power-law breakdown is observed at intermediate stresses and temperatures.Выполнены исследования ползучести оптимизированного магниевого сплава AX41 (4 вес.% Al, 1 вес.% Ca, Mg - баланс) в температурном диапазоне 343...673 K и уровнях напряжений 2...200 МПа. Были проведены испытания на ползучесть при ступенчатом нагружении сжимающей нагрузкой с целью определения зависимости скорости ползучести от напряжения в диапазоне 10-9...10-3 с-1 для данной температуры. Все вышеуказанные зависимости хорошо описывались уравнением Барафоло (типа sinh) с показателем степени n = 5. Анализ параметров этого уравнения показал, что ползучесть во всех проведенных экспериментах определяется диффузией решетки. При низких уровнях напряжений и высоких температурах определяющим является механизм восхождения, а при высоких уровнях напряжений и более низких температурах - механизм скольжения. В промежуточном диапазоне напряжений и температур наблюдается характерное отклонение экспериментальных точек от степенной зависимости

Compressive creep of Fe3Al-type iron aluminide with Zr additions

High-temperature creep ofa Fe3Al-type iron aluminide alloyed by zirconium was studied in the temperature range 873-1073 K. The alloy contained (wt.%) 31.5% Al, 3.5% Cr, 0.25% Zr, 0.19% C (Fe balance). It was tested in two states: (i) as received after hot rolling and (ii) heat treated (1423 K/ 2 h/air). Creep tests were performed in compression at constant load with stepwise loading: in each step, the load was changed to a new value after steady state creep rate had been established. Stress exponent and activation energy of the creep rate were determined and possible creep mechanisms were discussed in terms ofthe threshold stress concept. A rapidfall ofthe stress exponent and ofthe threshold stress with the increasing temperature indicates that creep is impeded by the presence of precipitates only at temperature 873 K. The results were compared with the results oflong-term creep tests in tension performed recently on the same alloy. Изучена высокотемпературная ползучестьИзучена высокотемпературная ползучесть алюминида железа типа Fe3Al, легирован­ного цирконием в диапазоне температур 873...1073 К. Сплав содержал (ат.%) 31,5 Al, 3,5 Сг, 0,25 Zг, 0,19 С (остальное Fе). Испытания проводили в двух состояниях: в состоя­нии поставки после горячей прокатки и после термообработки (1423 К/2 ч/воздух). Испытания на ползучесть выполняли при постоянной сжимающей нагрузке при сту­пенчатом нагружении: на каждой ступени использовали нагрузку другой величины после того, как наступала стадия установив­шейся скорости ползучести. Определена экс­понента напряжения и энергия активации для скорости ползучести, обсуждены возмож­ные механизмы ползучести с позиций кон­цепции порогового напряжения. Резкое умень­шение экспоненты напряжения и порогового напряжения при увеличении температуры свидетельствует о том, что наличие вторич­ных фаз снижает скорость ползучести толь­ко при температуре 873 К. Полученные ре­зультаты сравнивали с данными длительных испытаний на ползучесть при растяжении, выполненных на том же сплаве

Estimation of anisotropy of mechanical properties in Mg alloys by means of compressive creep tests

A detailed knowledge of dependence of mechanical properties on orientation in materials prepared by directional processes may present an important factor influencing the design of construction parts. Toward this end, the compressive creep testing of short specimens may be useful. Three different magnesium-based materials were subjected to this testing: (i) pure magnesium, (ii) magnesium matrix composite reinforced with 10 vol.% of titanium, and (iii) magnesium alloy WE54. All three materials were prepared through a powder metallurgical route with final hot extrusion. The specimensfor creep tests were cut in such a way that their longitudinal axis (i.e., the direction of compressive creep stress) and the axis of extruded bar contained a predestined angle. Two extreme cases can be observed: In pure Mg and in Mg-Ti composite, the dependence of the creep rate is very sensitive to the orientation especially at small inclinations from extrusion axis. The greatest creep resistance is observed in specimens with stress axis parallel to the extrusion axis, the lowest at declinationsfrom 45 to 90°. On the other hand, in WE54 no orientation dependence was observed. Possible explanations of the behaviour based on microstructural observations are discussed.Глубокое понимание зависимости механи­ческих свойств от ориентации волокон в материалах, полученных методами направ­ленного воздействия, может быть важным фактором в создании структурных элемен­тов. Исходя из этого, испытания коротких образцов на ползучесть при сжатии могут дать полезную информацию. Испытания проводили на трех различных материалах на основе магния: чистый магний; композит с магниевой матрицей, упрочненный 10 об.% титана; магниевый сплав WES4. Все три материала были получены методами порош­ковой металлургии и горячей экструзии. Образцы вырезали таким образом, чтобы их продольная ось (т.е. направление напряже­ния ползучести при сжатии) и ось экстру­дированного образца имели заданный угол. Для чистого магния и Mg-Ti композита за­висимость скорости ползучести существен­ но зависит от ориентации, особенно при не­ большом отклонении от оси экструзии. Наи­большее сопротивление ползучести имели образцы с осью напряжений, параллельной оси экструзии, наименьшее - при отклонении на 45°-90°. В сплаве WES4 не наблю­далось зависимости от ориентации. Подоб­ное поведение может быть связано с микро­структурой материала

Harnessing person-generated health data to accelerate patient-centered outcomes research: the Crohn’s and Colitis Foundation of America PCORnet Patient Powered Research Network (CCFA Partners)

The Crohn’s and Colitis Foundation of America Partners Patient-Powered Research Network (PPRN) seeks to advance and accelerate comparative effectiveness and translational research in inflammatory bowel diseases (IBDs). Our IBD-focused PCORnet PPRN has been designed to overcome the major obstacles that have limited patient-centered outcomes research in IBD by providing the technical infrastructure, patient governance, and patient-driven functionality needed to: 1) identify, prioritize, and undertake a patient-centered research agenda through sharing person-generated health data; 2) develop and test patient and provider-focused tools that utilize individual patient data to improve health behaviors and inform health care decisions and, ultimately, outcomes; and 3) rapidly disseminate new knowledge to patients, enabling them to improve their health. The Crohn’s and Colitis Foundation of America Partners PPRN has fostered the development of a community of citizen scientists in IBD; created a portal that will recruit, retain, and engage members and encourage partnerships with external scientists; and produced an efficient infrastructure for identifying, screening, and contacting network members for participation in research

Use of small specimen creep data in component life management: a review

Small specimen creep testing techniques are novel mechanical test techniques that have been developed over the past 25 years. They mainly include the sub-size uniaxial test, the small punch creep test, the impression creep test, the small ring creep test and the two-bar creep test. This paper outlines the current methods in practice for data interpretation as well as the state-of-the-art procedures for conducting the tests. Case studies for the use of impression creep testing and material strength ranking of creep resistant steels are reviewed along with the requirement for the standardisation of the impression creep test method. A database of small specimen creep testing is required to prove the validity of the tests

The formation of human populations in South and Central Asia

By sequencing 523 ancient humans, we show that the primary source of ancestry in modern South Asians is a prehistoric genetic gradient between people related to early hunter-gatherers of Iran and Southeast Asia. After the Indus Valley Civilization’s decline, its people mixed with individuals in the southeast to form one of the two main ancestral populations of South Asia, whose direct descendants live in southern India. Simultaneously, they mixed with descendants of Steppe pastoralists who, starting around 4000 years ago, spread via Central Asia to form the other main ancestral population. The Steppe ancestry in South Asia has the same profile as that in Bronze Age Eastern Europe, tracking a movement of people that affected both regions and that likely spread the distinctive features shared between Indo-Iranian and Balto-Slavic languages