بررسي تاثير چيدمان اتمي و تخلخل بر رفتار شبه الاستيك آلياژ حافظه دار NiAl تك كريستال با استفاده از روش شبيه سازي ديناميك مولكولي

چكيده انگليسي :

138 Abstract The properties of the porous shape memory materials can be a combination of the properties of the porousmaterials and the shape memory alloys Indeed achieving the advantages of these two types of materialssimultaneously and staying away from their disadvantages requires structural design and optimization One of theimportant properties of memory alloys is the pseudo elastic property which as a mechanical quantity can benefit ordamage from the presence of porosity Although typically increase the amount of porosity can decrease themechanical properties of the material one of the ways to compensate for this can be to change the size of theporosity Therefore in this study it was tried to evaluate the effect of porosity size to optimize the properties ofporous shape memory alloys Therefore molecular dynamics simulation was performed in three separate steps Initially the properties of the ordered Ni Al single crystal were investigated Then in the second step disorderedsingle Ni Al crystals with structural disordering of 0 8 13 14 18 and 22 were simulated In the final step theporous samples with 5 10 and 15 porosity were created from single Ni Al crystal with 18 structuraldisordering In each porosity percentage three samples with 2 16 and 250 pores were made and evaluated by themethod of molecular dynamics simulation The simulations were performed using the Lammps software The datawere then processed using Ovito software and Python codes The interatomic potential for simulating the sampleswas of the type EAM presented by Pan and Mishin In these simulations the desired samples were loaded in the 100 crystallographic direction and unloaded at a strain of 0 24 The main results of this study can be divided intothree parts 1 The stress strain curve obtained from the simulation prior to the failure of the specimen was observedto consist of five regions four with positive slopes and one with a negative slope The loading stress strain curvewas according to unloading stress strain curve During the loading path the bcc phase transforms to the fcc phase Anon vibrational mathematical model was developed based on mathematical analysis to explain the behavior of thespecimen during mechanical loading The results obtained have shown that stress on the sixth neighboring atomicshell could result in a negative stiffness that forms a zone with a negative slope in the relevant stress strain curve Itwas found that the potential function can have the most influence on the mechanical behavior of the material 2 Inthe second part of the molecular dynamics simulation results it was found that the structural disordering causes achange in the phase transformation mechanism and consequently a change in the pseudo elastic behavior of the Ni Al single crystal In fact structural disordering leads to formation of the hcp metastable phase The existence of thisphase leads to the formation of a hysteresis loop in the stress strain curve i e Structural disordering variations from0 to 22 can create a hysteresis loop and as the value increased the samples have a multiphase structure that is incontrast to the experimental procedure In other words with the excessive disordering 22 the initial phase ofhcp and fcc are observed in the primary structure Due to the different percentages of hcp metastable phase duringthe two loading and unloading paths the mechanism of phase transforms in these two paths was not identical Thisphenomenon is leading to the formation of a hysteresis loop pseudo elastic behavior in the stress strain curve of aNi Al single crystal Atomic configuration and structural defects can be considered as one of the most importantfactors in the pseudo elastic behavior of shape memory alloys The sample with 18 disordering had a largehysteresis loop and also it was single phase at the start of the loading so it was used to fabricate porous shape memory material samples 3 Based on the molecular dynamics simulation results of Ni Al porous shape memorymat

عليزاده، جواد

بررسي تاثير چيدمان اتمي و تخلخل بر رفتار شبه الاستيك آلياژ حافظه دار NiAl تك كريستال با استفاده از روش شبيه سازي ديناميك مولكولي

شبيه سازي ديناميك مولكولي , شبه الاستيك , بي نظمي , حافظه دار , متخلخل , NiAl