Research On Martensite Phase Transformation And Thermodynamic Properties Of Co-Ni-al Shape Memory Alloys

Magnetically Controlled Shape Memory Alloys have attracted considerable attention as a new functional material in recent years. Co-Ni based alloy has good plasticity and processing performance, high magnetocrystalline anisotropy and saturation magnetization. In this thesis, Co-Ni-A1alloy’s equilibrium phase diagram, the isothermal section diagram in different temperature, and the regularity of solidification were used to find out the phase composition and morphology changes, the phase transition properties of alloys in different composition, different treatment and different preparation process. With rapid solidification technique to prepare Co-Ni-A1and Co-Ni-A1-X (X=Si/Sn) magnetic shape memory alloys(MSMA), the alloying and proper heat treatment process is supposed to realize microstructure optimization, improve magnetic properties and simplify the preparation process. The microstructure, morphology, phase structure were measured by optical microscopy(OM), X-ray diffraction (XRD), Scanning electron microscope (SEM) and Field emission scanning electron microscope (FE-SEM). The effect of alloying with Si and Sn, the characteristics of phase transformation after heat treatment were measured by differential scanning calorimetry (DSC). Miedema model and Chou model were used to calculate the thermodynamic formation enthalpy of Co38Ni34Al28-xSix alloys and forecast the effect of alloying and heat treatment of phase transformation. Vibrating Sample Magnetometer (VSM) and micro-hardness measurements were carried out to measure the saturation magnetization, curies temperature, which could be used to investigate the trend of magnetic anisotropy and saturation magnetization. In addition, this thesis studies the stress-induced martensitic transformation mechanism of Co-Ni-Al-Sn alloys. It is expected to establish the experiment foundation for producing good machining performance and excellent magnetic shape memory properties of Co-Ni based magnetic control shape memory alloys. The main results and conclusions are as follows:1. Co3gNi34Al28-xSix (x=0,1,3,5,7,9) alloys were prepared by rapid solidification technique. The microscopic structure of Co38Ni34Al28-xSix alloys are related to the content of Si. With the increased of Si content, the morphology changed from (3+y two phase to β+M two phase structure, to M+E two phase structure and finally to M single phase. As Si content in lat%to5at%, the volume fraction of eutectic phase structure increased as the Si content increased. When Si content continued to increase, the volume fraction of eutectic phase reduced gradually. To obtain a single martensitic phase microstructure, Si content was increased with the result of gradually reduced heat treatment temperature Adding of Si promotes the formation of the martensite phase and leads to the disappearance of γ phase, which is supposed to form a large magnetic strain.2. Co38Ni34Al23Si5and Co38Ni34Al19Si9alloys were prepared by melt spinning method. Comparing the free surface of two ribbon samples with different Si content, the free surface of5at%Si content is more homogeneous of than9at%Si. The matrix of both ribbons is consisted of martensitic phase and eutectic phase structure. The volume fraction of martensite phase of9at%Si content sample is larger than5at%Si content sample at free surface, while the5at%Si content sample contained more eutectic phase in free surface. Energy spectrum measurement results showed that, martensite region contained more Cobalt than eutectic region but less silicon and aluminium than eutectic region.3. DSC test results show that the martensitic transition temperature is sensitive to the composition. The martensitic transition temperature increased with the increase of Si content. The microstructure is martensite phase at room temperature as the content of Si increased, which is the main characteristic of magnetic control shape memory alloy. Higher quenching temperature enhanced the martensitic transformation temperature and martentitic reverse transformation temperature. Alloying with silicon also results in the decrease of quenching temperature to gain single martensitic phase matrix.4. The measurement of curies temperature (Tc) shows that adding of silicon cause the curies temperature increase. The curies temperature reached the highest values as Si content is5at%and then began to decrease as Si content continued to increase. For the ribbon sample, the curies temperature of5at%Si was higher than the9at%content one. The quenching temperature also influenced the curies temperature. The curies temperature increased as the quenching temperature increased, and it changes linearly with the quenching temperature when the Si content is the same. Miedema model and Chou model were used to calculate the enthalpy of formation of Co-Ni-Al-Si quaternary alloy. Calculated results show that the enthalpy of formation reaches to the most negative as silicon content of5at%～7at%, which indicates the crystal structure is the most stable at this composition range. Therefore, the substitution of aluminium with silicon with content of5at%～7at%to achieve the best performance.5. Test of saturation magnetization show that all of the Co38Ni34Al28-xSix alloys are soft magnetic properties. Quenching temperature and composition had great influence on the saturation magnetization of Co-Ni-based alloys. Saturation magnetization of Co38Ni34Al28-xSix alloys increased as the matrix was homogenized. For as-cast alloy, the saturation magnetization reduced as content of Si increased. After annealed a larger and larger proportion of martentitic phase emerged, then the saturation magnetization increased greatly. Higher temperature of heat treatment is needed to obtain a single phase matrix when content of Si was low.6. Magnetocrystalline anisotropy constant K1of Co-Ni-Al-Si quaternary alloy was calculated. The calculated result shows that the volume fraction of martensite phase increased as Si content increased or the heat treatment temperature increased, therefore the magnetocrystalline anisotropy K1increased.7. The measurement result of micro-hardness shows that:comparing with non-silicon alloy, the alloy with silicon content of lat%has a fine grain strengthening effect which result in the micro-hardness to increase. The micro-hardness increased as matrix changed from β single phase structure to M+E two phase structure gradually with the content of silicon increased. In addition, the micro-hardness improved as the heat treatment temperature increased.8. The phase transition behavior under stress at room temperature was investigated. Three groups of alloys showed obviously stress-induced martensitic transformation in0.2kg, lkg and2kg load at room temperature, in which the stress induced martensitic transformation behavior of Co3gNi34Al27Sn1alloy is the most obvious. The study also shows that adding of Sn caused the increase in enthalpy of reverse martensitic transformation. The stress induced martensite transition first produced in the middle area of the indentation diagonal and expanded gradually to the two diagonal regions as the load increasing. Different directions of martensite had been formed as the martensite expanded. The results show that alloying with Sn is supposed to improve the martensitic transformation temperature of Co38Ni34Al28alloy. It also makes the TMd temperarture close to room temperature, which results in the possibility to produce the stress induced martensitic transformation at room temperature.