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Crystallization Kinetics and Formation of Nano Crystals in Al-Ni-Fe-MM System and its Effect on Mechanical Properties

Mansouri Hassan Abadi, Mehdi | 2014

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 46107 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Varahram, Naser; Simchi, Abdolreza
  7. Abstract:
  8. In the present work, primary crystallization mechanism, glass forming ability, thermal stability and formation of nano Al crystals and their effects on the mechanical behavior were investigated in various Al amorphous alloy containing misch metal including Al90-xNi10-yFeyMMx (x=2-10%at, y=0-10 %at) alloys. In order to elaborate the Cu effect on the primary crystallization mechanism Al86Ni10-xCuxMM4(x=0-2/5) alloy were prepared. The results showed that in Al90-xNi10MMx (x=2-10 %at) alloying system variation of the misch metal from 2 to 10 atomic present results in increase in glass forming ability. The crystallization onset temperature also increases from 405 K in Al88Ni10MM2 to 630 K in Al80Ni10MM10 alloy. Replacing of Ni by Fe in Al90-xNi10-yFeyMMx (x=2-10%at, y=0-10 %at) alloys cause to increase in the thermal stability of the prepared alloy. On the other hand, replacing of Ni by Fe up to 2.5 atomic present results to increase in the glass forming ability due to increase in the number of the alloying components. Further increases in the Fe cause to decrease in the glass forming ability according to positive heat of mixing between Fe and rare earth elements. The results showed that the primary crystallization was the dominant crystallization event in the first crystallization step, up to 4 atomic present misch metal in Al90-xNi10MMx (x=2-10 %at) alloys. Increase in misch metal content due to strong interaction between rare earth and Ni results to decrease in the number of quench-in nuclei in the amorphous matrix causes to retard the primary crystallization in higher misch metal content. TEM results showed that fcc-Al nano particles in the Al88Ni10MM2 are spherical with average diameter of 10-15 nm. Increase in the misch metal results to increase in the diameter of fcc-Al nano particles to 50 nm with irregular shape in Al88Ni10MM2 alloy. The results reveal that increase in mish metal cause to reduce in number of quench-in nuclei results to increase in quench-in nuclei distance. Such increase lets further growth of fcc-Al nano particles in Al86Ni10MM4 alloy results to larger fcc-Al nanoparticles. The crystallization mechanism was investigated in non-isothermal condition with constant heating rate ranging from 5 to 60 K/min utilizing DSC. The primary crystallization activation energy of Al88Ni10MM2 was 140 kJ/mole that increased to 272 kJ/mole in Al86Ni10MM4 alloy due to increase in Al-RE atomic pairs. The results of the differential is-conversional method of Friedman showed that the activation energy is depending on the crystallization progression. Three regions can be distinguished. At the early stage of both alloys the lower value of the activation energy could be related to the quench-in nuclei. Growth of the fcc-Al nano particles results in increase of solute in the remained amorphous matrix causes to increase in the growth activation energy. In final stage, soft impingement of the growing nano crystals results to sharp increase in the activation energy. Nano indentation results revealed that increase in the misch metal according to increase of solute in amorphous matrix causes to increase in hardness of the prepared alloy. On the other hand formation of the fcc-Al nano particles due to their restriction against the shear band movement causes to significance increase in hardness.
    Replacing of Ni by Cu results to reduction in crystallization onset temperature. Presence of the Cu on the other hand also causes to reduction in the fcc-Al nano particles due to clustering of the Cu in the amorphous matrix. Clustering of the Cu in the matrix results to increase in the free Al atom in the matrix and causes to decrease in the fcc-Al nano particles distance that according to the happening of the soft impingement the further growth of the fcc-Al particle will retard significantly
  9. Keywords:
  10. Rapid Solidification ; Aluminum ; Thermal Stability ; Kinetics Study ; Mischmetal ; Amorphous Alloys ; Primary Crystalization

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