ساخت نانو كامپوزيت هاي Nb-AI/AI2O3 به روش آلياژ سازي مكانيكي و بررسي ترموديناميك و سينتيك تشكيل آنها

چكيده انگليسي :

Fabrication of Nb Al Al2O3Nanocomposites by Mechanical Alloying and Thermodynamic and Kinetic Studies on Their Formation Hossein Mostaan Email address Hossein mostaan@gmail com Date of Submission Department of Materials Engineering IsfahanUniversity of Technology Isfahan 84156 83111 Iran Degree M Sc Language Farsi Superviso s Name Fathollah Karimzadeh Associate Professor karimzadeh f@cc iut ac ir Mohammad Hasan Abbasi Professor Abbasi32@cc iut ac ir Abstract Niobium aluminides are of interest as candidates for high temperature structural materials due to their high melting point high elastic modulus and low densities compared with Ni based superalloys However despite their attractive features their applications are limited by two shortcomings inadequate fracture thoghness at room temperature and a relatively unsatisfactory oxidation resistance at temperatures above 700 C So for optimization of room temperature fracture thoghness and high temperature strength microstructural modifications are required For these reasons it has been recognized that intermetallic matrix composites may be appropriate materials for structural applications replacing in some cases intermetallics This research was conductedto synthesize Nb Al Al2O3 nanocomposites by mechanical alloying MA Nb75Al25 and Nb25Al75 powder mixtures were first MA d in order to synthesize Nb3Al and NbAl3 intermetallic compounds and then A thermodynamic analysis of the process was then performed using semi empirical theory of Miedema to calculate the driving force for phase evolution In the next step Nb Al Al2O3 nanocomposites were synthesized by utilizing the displacement reactions of Al and Nb2O5 during mechanical alloying The reaction mechanism and activation energy E for aluminothermic reaction between Al and Nb2O5 were investigated under nonisothermal conditions using X ray diffractometry XRD and differential thermal analysis DTA Five isoconversional methods of Kissinger Akahira Sunose KAS Flynn Wall Ozawa FWO Tang Starink and Friedman FR were used to determine the values of E on a conversion fraction of 0 5 and its variation with degree of conversion It was found that MA of Nb25Al75 powder mixture led to the formation of Nb Al solid solution and subsequent formation of DO22 NbAl3 intermetallic compound and no amorphous phase was formed after longer milling times The results showed that MA of Nb75Al25 for 8h led to the formation of amorphous phase and Nb3Al intermetallic was not formed during milling Thermodynamic analysis showed that there is a driving force for Nb Al binary to form solid solution for all compositions while amorphous phase was stable in the range of It was also found from the results that the reduction of Nb2O5 by Al in Nb2O5 49 wt Al powder mixture took place in an explosive mode and after 450 min of mechanical alloying leading to the formation of a microstructure consisting of NbAl3 matrix and nanocrystalline Al2O3 particles TEM observations confirmed that Al2O3 particles were nanosized The results also showed that heating the as blended powder mixture of Nb2O5 and Al led to the formation of NbAl3 Al2O3 composite through two exothermic reactions A thermite reaction at 839 C followed by the synthesis of NbAl3 intermetallic at 944 C The activation energies for thermite reaction and intermetallic formation were obtained 420 12 and 290 15kj mol 1 respectively using KAS method The mechanical activation of Nb2O5 and Al powder for 5h lowered the formation temperature of NbAl3 Al2O3 from 944 C to 625 C Mechanical activation also resulted in the simultaneous occurrence of the two reactions which led to the formation of NbAl3 Al2O3 composite The activation energy for the formation of NbAl3 Al2O3 composite from MA d powder was obtained 325 7 kj mol 1 using kAS method which shows that mechanical activation of Nb2O5 and Al leads to decrease in activation energy for NbAl3 Al2O3 formation Keywords Intermetallic compound Nanocomposite Mechanical alloying Mechanochem

مستعان، حسين

ساخت نانو كامپوزيت هاي Nb-AI/AI2O3 به روش آلياژ سازي مكانيكي و بررسي ترموديناميك و سينتيك تشكيل آنها

تركيبات بين فلزي , واكنش مكانوشيميايي , انرژي اكتيواسيون