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The Research On The Preparation, The Structure And The Properties Of Magnesium Based Amorphous Alloys

Posted on:2007-08-31Degree:DoctorType:Dissertation
Country:ChinaCandidate:X J WangFull Text:PDF
GTID:1101360212990360Subject:Materials Processing Engineering
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Magnesium is very important metallic material for modern industry. However, bulk metallic glasses, a new material, have unique structure and novel properties. Integrating these two sides into a magnesium based bulk metallic glass is the focus for many physical scientist and material specialist. The bulk metallic glasses also possess promising application.At first, the effect of milling time, milling speed, ball/powder weight ratio and process control agents on the GFA (glass forming ability) of five powder blend with three eutectic compositions and two compound compositions based on the Mg-Cu binary phase diagram was studied under mechanical alloying conditions. X-ray diffraction results show that compound composition may be vitrified easily. Milling time, speed and ball/powder ratio have a key influence on the structure transforming process and process control agents have a definite influence. The particle size of powder becomes smaller and smaller firstly after milling. Then magnesium atoms diffuse and melt into crystal lattice of copper. At last all the magnesium atoms melt into the crystal lattice and supersaturation solid solute is formed. If the solid solute gets more energy at this time, the crystal lattice will crash and metastable amorphous state will replace.MgCuY amorphous ribbons were prepared by melt spinning process and the amorphous structure was testified by X-ray diffraction analysis and differential scanning calorimeter analysis. The values of Tg, Tx, T1,Trg and ΔTx, which are the parameters to describe the GFA for bulk metallic glasses, are determined. The ratio of the crystallization heat and melting heat was presented as a new parameter—reduced glass transition enthalpy. DSC quantification analysis showed that this new parameter and old parameters gave consistent results when described the GFA of MgCuY amorphous ribbons. A new differential and extremum method was proposed to analysis DSC data and to determine characteristic temperature. Negative direction of y axis expresses exothermic in the DSC graph when determining onset crystallization temperature. While position direction of y axis expresses exothermic when determining Tg. If differentiating local data of DSC, the minimum value of the n multiple derivative is smaller than that of n-1 multiple derivative and it is closer to Tg. The minimum value of two times derivative is determined as Tg. The temperature is only and has enough precision. It is better than tangent method recommended byICTA (international confederation for thermal analysis).A new experimental instrument for measuring the brittleness of ribbons was designed and fabricated. It solved the measurement of brittleness successfully. Heat transfer during rapid solidification processing of a ribbon prepared by melt spinning can be approximately modeled by a one dimensional heat conduction equation. The temperature distribution and the cooling rate within the ribbon are determined by integration of the equation based on heat transfer principle. According to the integration function, the cooling rate is in inverse proportion to the square of the thickness of the ribbon. When molten liquid of a 50 μm thick magnesium ribbon solidifies, the calculated cooling rate at the free surface of the ribbon is up to 5.85×106K/s. This result agrees very well with other estimated values reported previously.The influence of fourth constituent, Al, Ni, La, Tb and Gd, addition on the GFA of MgCuY bulk metallic glasses was studied and corresponding quaternary alloys were prepared. The GFA of Mg60-xAlxCu30Y10 (x=0.3, 0.9, 2.1) bulk amorphous alloy will decrease while Al substituting Mg and the thermal stability and GFA of the amorphous alloy decreased at the same time. It is identified that the width of super cooled region ΔTx of Mg65Cu25-xNixY10 (x=1, 2, 3) will be enlarged with little Ni addition and the width will reach as much as 43.30K when x=1. The GFA will be increased with larger Ni addition and will reach up to 0.5892 when x=3. It indicates that a proper amount of Ni substituting Cu in the alloy will increase the GFA but decrease the thermal stability. La substituting Y have a significant influence on the thermal stability and GFA of the Mg65Cu25Y10-xLax (x=0, 0.35, 1, 2, 3, 4) bulk amorphous alloys. Bulk amorphous alloy has a largest glass-forming ability (Trg=0.5872) when x=2. It is found that some crystalline phases appear in the matrix of the amorphous alloys when x=3 and x=4, indicating that the GFA of these alloys decreases markedly. The GFA of Mg65Cu25Y10-xTbx (x=0, 2, 4, 6, 8, 10) amorphous alloys become more and more strong with the increment of Tb. The GFA of the Mg65Cu25Y10-xTbx amorphous alloy is the strongest when x=8, and then drops when x=10. It is helpful to the GFA of Mg65Cu25Y10-xGdx( x=0, 4, 10) alloy with larger Gd addition.The crystallization kinetics of amorphous alloy Mg65Cu15Ag10Y10 and Mg65Cu22Ni3Y10 were studied. The remarkable dynamics character exists in the crystallization process revealed by DSC curves. The incubation time is becoming shorter and heat flux is obviously enhancing with increasing temperature whenisothermal heating. And the exothermal peaks are significantly shifted to higher temperatures and heat flux is also obviously enhancing with increasing heating rate when continuous heating. The overall activation energy for crystallization of amorphous alloy Mg65Cu15Ag10Y10 are determined as 186.12, 184.40, and 180.86 KJ/mol for the heating rates used being 5, 10, 15, and 20 K/min, and 107.52, 109.95, and 110.15 KJ/mol for the heating rates used being 20, 40, 60, and 80 K/min, when using the Kissinger peak method, Ozawa peak method, and Ozawa's isoconversional method, respectively. The overall activation energy for crystallization of amorphous alloy Mg65Cu22Ni3Y10 are determined as 117.48, 125.47, and 114.29 KJ/mol when using the Kissinger peak method, Ozawa peak method, and Ozawa's isoconversional method, respectively. The nucleation-and-growth kinetics is manifested as a rule in the early stages of the crystallization. The Avrami exponent, n, is larger than 1.5 and less than 2.5 for amorphous alloy Mg65Cu15Ag10Y10 and Mg65Cu22Ni3Y10 during isothermal heating; suggesting that the diffusion controlled three-dimensional growth dominates in the growth of the nucleation.A statistical term, standard deviation of radii, is used for describing the confusion degree of dense random packing of hard spheres with different radii in multicomponent amorphous alloys. The parameter has clear physical meaning and coincides with our common understanding of confusion degree. The greater the RD (r) value of a glass is, the greater its confusion degree and the lower its Rc is. The parameter can describe the packing confusion exactly and may be used for constituent selection and composition selection for binary alloy after further investigation. But it is not fit for ternary alloy. For further explaining the deviation of radii caused by mixture enthalpy, redistribution of valence electron and other factors should be considered.
Keywords/Search Tags:amorphous alloy (metallic glass), BMGs (bulk metallic glasses or bulk amorphous alloy), MA (mechanical alloying), rapid solidification process, crystallization kinetics, confusion principle, GFA (glass forming ability)
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