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National Repository of Grey Literature 3 records found  Search took 0.00 seconds. 
Heat treatment conditions and their influence on precipitation of Ni4Ti3 phase in Ni-rich NiTi shape memory alloys
 Dlouhý, Antonín ; Bojda, Ondřej ; Fiala, J.
The present work deals with a quantitative analysis of Ni4Ti3 precipitates after solution annealing (850°C/30min) and ageing (450°C/1hod) in air or inert argon atmosphere. The attention is given not only to the size of precipitates in the vicinity of grain boundaries and in the grain volume and to the corresponding Ni4Ti3 volume fractions, but also to subsequent martensitic transformations B2-(R)-B19’. Quantitative analysis of transmission electron microscopy micrographs is based on the automatic image processing by means of the Adaptive Contrast Control programme. Martensitic transformations were documented using differential scanning calorimetry. Results indicate a homogeneous precipitation of Ni4Ti3 with the heterogeneous distribution of crystallographic particle variants. A possible influence of the heat treatment environment on Ni4Ti3 precipitation and martensitic transformations is discussed in this work.
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Redistribuce napětí a její vliv na precipitaci v mikrostruktuře
 Holec, David ; Dlouhý, Antonín
The present study considers two mechanisms that can contribute to the local redistribution of stresses in the microstructure of engineering alloys. The first model takes into account the anisotropy of thermal expansion coefficients and predicts a stress state generated close to a boundary between two differently oriented grains after a temperature change. The second model addresses a stress distribution in a polycrystalline aggregate characterized by anisotropic elastic constants and subjected to the isothermal aging under the assistance of external stress. The influence of local stress states on the selective precipitation of the Ni4Ti3 crystallographic variants in the microstructure of the NiTi shape memory alloy is demonstrated.
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Stress-free and stress-assisted aging of NiTi shape memory alloys
 Dlouhý, Antonín ; Bojda, Ondřej ; Fiala, J.
The present work investigates coherent Ni4Ti3 precipitates in a Ti-50,7at%Ni shape memory alloy after different thermo-mechanical treatments. The study is based on the quantitative TEM (transmission electron microscopy) image analysis using the ACC (Adaptive Contrast Control) programme. The attention is given to a number, size and volume fraction of individual crystallographic variants of the Ni4Ti3 precipitates. Preliminary results suggest that, after common heat treatments, the precipitation of Ni4Ti3 particles is inhomogeneous not only as far as the density of particles in the original B2 grains is concerned but also with respect to the distribution of the crystallographic particle variants. The possible influence of the heterogeneous Ni4Ti3 precipitation on the characteristics of the martensitic transformations B2-R-B19’ is discussed.
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