Fizikai Tudományok Doktori Iskola

Állandó link (URI) ehhez a gyűjteményhez

https://hdl.handle.net/2437/2270

Természettudományi Kar

Fizikai Tudományok Doktori Iskola
(vezető: Dr. Kun Ferenc)

Természettudományi és Informatikai Doktori Tanács

D57

Doktori programok:

  • Atom- és molekulafizika
    (programvezető: Dr. Vibók Ágnes)
  • Magfizika
    (programvezető: Dr. Fülöp Zsolt)
  • Szilárdtestfizika és anyagtudomány
    (programvezető: Dr. Kökényesi Sándor Jenő)
  • Fizikai módszerek interdiszciplináris alkalmazásokban
    (programvezető: Dr. Kun Ferenc)
  • Részecskefizika
    (programvezető: Dr. Trócsányi Zoltán)

Böngészés

Fizikai Tudományok Doktori Iskola Szerző szerinti böngészés "Beke, Dezső"

Megjelenítve 1 - 1 (Összesen 1)
  • Nincs kép
    TételSzabadon hozzáférhető
    Investigation of noisy character of thermally and stress induced phase transformations by acoustic emission in Ni49Fe18Ga27Co6 shape memory single crystals
    Kamel, Sarah Mahmoud Mohamed; Beke, Dezső; Kamel, Sarah Mahmoud Mohamed; Fizikai tudományok doktori iskola; Természettudományi és Technológiai Kar
    Shape Memory Alloys, SMAs, form a class of smart materials that can remember their original shape after deforming under an external stimulus, like stress, thermal, magnetic or electric (𝝈 ,𝑻,𝔹,𝔼) fields. The properties of SMAs arise from the martensitic transformation, MT, between two solid phases in a diffusionless manner or from the rearrangement of martensite variants. During structural changes (SCs), SMAs usually absorb/release energy by undergoing a reversible hysteretic shape change. The hysteresis is the key characteristics of many properties in SMAs including: super-plasticity, super-elasticity, rubber-like behavior, elasto-caloric and magneto-caloric effects. These properties have made them important in many technical applications including medical implants, sensors, robotic muscles, electric and thermal actuators. The stress induced reversible transformation (super-elasticity) is one of the most frequently used property of shape memory alloys (two-way shape memory effect). Increasing the stress (loading) in austenitic state, at a certain stress level, the transformation to martensite starts and takes place until the transformation is finished, which results in a plateau with positive slope on the stress-strain curves, and the unloading leads to a closed hysteresis loop. Below a certain temperature, after removing the stress, the sample remains in martensitic state and turns back to austenite only by thermally induced reverse transformation (i.e. by increasing the temperature). In some cases it was observed that, instead of having a smooth stress-strain plateau, stress jumps on the loading and unloading stress strain curves appeared and, at the thermally induced recovery, had a burst-like transformation, by about 2-4 orders of magnitude faster transition than the normal thermally induced one and often accompanied with jumping of the sample and an audible click was herd. Understandingof these phenomena is still far not complete and for better understanding of the dynamics of these behaviors, Ni49Fe18Ga27Co6 single crystals were selected as a new type of promising ferromagnetic and high temperature SMAs. These, as compared to the well studied Ni2MnGa alloy, are less brittle and by changing the Co content the mechanical and magnetic properties can be finely tuned. Structural changes (SCs) in SMAs proceed through a sequence of small discontinuous jumps between local stable states: this due to the discontinuous nucleation events and to intermittent motion of A/M interfaces and/or twin boundaries. Under slowly changing external driving field, these usually lead to emission of different acoustic, thermal or (in case of ferromagnetic SMAs) magnetic, noise signals. The emitted noise signals consists of the so called avalanches. It is well-known that temporal shapes of avalanches, 𝑉(𝑡) (𝑉 is the detected voltage signal, t is the time), have self-similar behavior. It was theoretically predicted that the normalized 𝑉(𝑡)function (e.g. dividing both the values of V and t by 𝑆^(1/2) , where S is the area of the avalanche), averaged for fixed S, should be the same, for the same avalanche mechanisms. However, there are experimental evidences that the average temporal shape of avalanches do not scale completely in a universal way. Furthermore, self-similarity of the crackling noise implies scaling relations between avalanche parameters: amplitude, A, duration, D, size, 𝑆 = ∫ 𝑉(𝑡)𝑑𝑡, energy, E∝ ∫ (𝑉(𝑡)^2) 𝑑𝑡, were E ∝ 𝐴^( 2𝛾−1/ 𝛾−1)and S ∝ 𝐴^( 𝛾−1) . According to the main field theory, MFT, 𝛾 = 2. In addition to the problem of universal scaling of the temporal avalanche shapes, the above exponents obtained from AE experiments contradicted to the theoretical predictions leading to the so-called AE enigma (e.g. the power relation between E and A led to E ∝ 𝐴^2 instead of E ∝ 𝐴^3, predicted from the MFT. Thus, part of my theses is devoted to this problem.