Investigando los mecanismos fisicos de plasticidad en la aleacion de memoria CuZr durante el mecanizado asistido por vibracion eliptica

Abstract

[ES] El campo de ultra-precision machining ha estado ganando relevancia en la fabricacion de components para los sectores de la electronica, optica y medica. Dos factores principales que afectan el mecanizado de los materiales son los parametros de el mecanizado y las propiedades fisicas de el material. Ciertos materiales como el acero reforzado o superlaeaciones de nickel estan clasificados como materiales de dificil mecanizado pero innovaciones en el campo de mecanizado han permitido desarrollar una tecnica definida como mecanizado assistido por vibracion eliptica. Esta tecnica permite mecanizar materiales dificiles de procesar. La aleacion de memoria CuZr es un material dificil de procesar y aunque el mecanizado asistido por vibracion eliptica se ha implementado en un numero de metales, aun no se ha implementado en la aleacion CuZr. Por consiguiente, el objetivo de el trabajo se divide en dos partes: El primer objetivo es obtener las propiedades mecanicas de la aleacion de memoria CuZr utilizando Dinamica Molecular, y el segundo objetivo es explorar los mecanismos de remocion de material a nanoescala en la aleacion CuZr durante el mecanizado asistido por vibracion eliptica. Los resultados de el trabajo permitiran realizar pautas generales en el diseño microestructural de la aleacion de memoria CuZr.

[EN] The field of ultra-precision machining has gained significant importance in the manufacture of components for the electronic, optical and medical industry. Two crucial factors that play a key role in the machinability of materials arethe machining parametersand the material’s physical properties. Certain materials such as hardened steel or nickel-based superalloys are difficult-to-machine but innovations in the field of precision machining have developed a technique known as elliptical vibration assisted machining, which enables to improve the machinability of these materials. CuZr high-temperature shape memory alloy is categorized as a difficult-to-cut material and although EVAM has been applied to a wide range of metals it hasn’t yet been studied in CuZrHTSMA. In this context, the purpose of this thesis is twofold: On the one hand, to characterise the mechanical properties of CuZr SMA using Molecular Dynamics and, on the other hand, to explore the nanoscale mechanism of material removal of CuZr shape memory alloy (SMA) during elliptical vibration assisted machining (EVAM). The conclusions of this thesis can be summarized as follows.To characterise the mechanical properties of Cu50Zr50, Cu2Zrand Cu5Zr, atensile and shear test were carried out using MD. Tensile test was done with crystal orientation and direction of tensile pulling as <010>. The results showed that Cu50Zr50and Cu2Zr exhibited a phase transformation (pseudoelasticity) during loading. However, Cu5Zr showed dislocation nucleation as the main plastic deformation mechanism followed by fracture. Shear tests were done in the same phases with crystal orientation and direction of shear pulling as <100>. Interestingly, the shear test results showed no phase transformation for Cu50Zr50and Cu2Zr but the Cu5Zr composition did show phase transformation during loading. It is important to highlight that all three phases of CuZr binary alloy that we have tested showed a different plastic response during the tensile test and the shear test.

As far as machining is concerned, we observed indications that EVAM shows improved machinability compared with conventional machining. Although cutting forces were lower in EVAM, the stresses on the workpiece were slightly higher and both techniques showed the same mechanism of plasticity during machining. Neither dislocation nucleation or martensitic transformation was exhibited in either of the two machining techniques and instead, amorphisation was observed as the main plastic deformation mechanism in both cases. Interestingly, amorphisation has been previously observed by Saitoh and Kubota (2010) during loading NiTi SMA [1]; however, it didn’t show up in every crystal orientation confirming that NiTi shows significant changes in response to loading in different lattice directions.One of the main outcomes from this thesis is that CuZr SMA exhibitsdifferent modes of plastic deformation; namely amorphisation, dislocation nucleation and martensitic transformation during loading. The governing mechanism that arises during loading highly depends in the lattice direction in which the load is being applied. These findings can potentially enable reliable predictions and provide guidelines of the microstructural design of CuZr SMA systems.