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Study Of The Thermal Stability,Mechanical And Corrosion Properties Of Cu-Zr Based Bulk Metallic Glasses And Their Composites

Posted on:2014-07-22Degree:DoctorType:Dissertation
Country:ChinaCandidate:G Z MaFull Text:PDF
GTID:1361330488499839Subject:Materials Processing Engineering
Abstract/Summary:PDF Full Text Request
Cu-Zr-Al bulk amorphous alloys have become an attractive advanced material with much potential applications due to their unique physical,chemical and mechanical properties.However,this alloy system still has many unsolved issues which need to be done in further research.In this dissertation,the Cu46Zr46Al8 bulk amorphous alloy was selected and then treated by mechanical deformation and micro-alloying.The effect of rolling and element addition on glass formation ability(GFA),thermal ability,mechanical and corrosion properties of this based alloy was investigated systematically.At last,the Cu46Zr46Al8 bulk amorphous composite was subjected to cryogenic treatment(CT).The effect and its mechanism of CT on phase transformation,mechanical and corrosion properties of the composite were discussed.The as-cast Cu46Zr46Al8 bulk amorphous alloy has good GFA and high thermal ability.The supercooled region ?Tx,reduced glass transition temperature Trg and the parameter y of this alloy are 81K,0.608 and 0.421,respectively;the activation energies of the onset of the crystallization(Ex)and the crystallization peak(Ep)are 355kJ/mol and 403kJ/mol.In addition,the crystallization of this alloy proceeds through two reactions.The primary crystallization mechanism is one-dimensional,two-dimensional and three-dimensional growth.As the crystalline volume fraction is increasing,the nucleation rate increases as well,which indicates that the crystallization behavior of this alloy is controlled by the growth of the primary phase and the nucleation as well as the growth of the second phase.Furthermore,the gradually increasing activation energy suggests that the thermal stability of this alloy is enhanced.Rolling deformation and element addition reduce the thermal stability of the as-cast Cu46Zr46Al8 bulk amorphous alloy.With the increase of the deformation strain and element content,the activation energy reduces correspondingly.When the deformation strain increases from 0 to 28%,the activation energies of Ex and Ep are reduced to 251kJ/mol and 250kJ/mol,respectively.When the Ti content increases from 0 to 8at.%,the supercooled region ?Tx reduces from 81K to 36K,while the reduced glass transition temperature Trg and the parameter y of this alloy reduce from 0.608 to 0.566 and from 0.421 to 0.381,respectively.In addition,with the increase of the Ti content,the crystallization behavior evolves from one stage to multi-stages process.The change of the corresponding activation energies reduces,but not linearly.Rolling deformation and element addition could significantly improve the mechanical property of the as-cast Cu46Zr46Alg bulk amorphous alloy at room temperature.When the deformation strain increases from 0 to 28%,the compressive fracture stress and the plastic strain of the alloy increase from 1754MPa to 2237MPa and from 0 to 11.9%,respectively.The compressive fracture stress and the plastic stain of the quaternary Cu-Zr-Al-Ti amorphous alloy system do not increase linearly with the Ti content increases.When the Ti content reaches to 3.2at.%,the alloy has the largest plastic strain in compression of around 12%and a fracture strength up to 1950MPa.The high density of shear bands and the enhanced interactions between the shear bands lead to the superior plasticity of this bulk amorphous alloy.Furthermore,the shear bands formation of the rolled samples is co-dominated by adiabatic temperature rising and local dilatation mechanism,while the shear band formation of the samples with Ti addition is mainly controlled by local dilatation mechanism.Corrosion measurements results of the as-cast Cu46Zr46Al8 bulk amorphous alloy show that the active dissolution behavior appeares in NaCl aqueous solution,while the passivation phenomenon happenes in the NaOH solution.Immersion treatment reduces the anti-corrosion property of the as-cast Cu46Zr46Al8 bulk amorphous alloy.After being immersed for 24h,the corrosion current density increases and the electrochemical impedance spectroscopy spectrum of this alloy changes from single capacitive arc to multi-arcs which consist of capacitive arc at high frequency,capacitive arc at medium frequency and diffusion impedance at low frequency.The addition of Ti element improves the anti-corrosion property of the as-cast Cu462r46Al8 bulk amorphous alloy in different solutions.Comparing with the based bulk amorphous alloy,the corrosion current density of the quaternary Cu-Zr-Al-Ti amorphous alloy system reduces and its corrosion potential improves in the NaCl solution.Meanwhile,the passivation phenomenon happenes in the NaOH solution,which shows a wide passivation range and low maintaining passivity current density.In addition,the electrochemical impedance spectroscopy spectrum of the quaternary amorphous alloys in lmol/L NaOH solution are all composed of single capacitive arc,which indicats that the oxides on the face of these alloys have a protective effect after the Ti element addition.Cryogenic treatment(CT)refines the CuZr crystalline phase and induces the martensitic transformation in the Cu46Zr46Al8 bulk amorphous composite.After CT,the size of the CuZr phase is reduced from about 3?m to 2?m,and the primary CuZr phase with B2 structure partially transformes to martensite with a monoclinic B19'structure.The micro structure modification caused by CT contributes to the improvement of the mechanical property of this composite.Comparing with the as-cast samples,the micro hardness and the compressive fracture stress of the samples after CT for 72h reach to 574Hv and 1759MPa,which increase about 18.55%and 37.5%,respectively.However,due to the formation of martensite phase,CT accelerates the fracture morphology transition from the ductile to the brittle and reduces the anti-corrosion property of the as-cast Cu46Zr46Al8 bulk amorphous composite in different solutions.The martensitic transformation in this amorphous composite induced by CT indicates that CT may provide a new approach for designing the "Martensite-glass".
Keywords/Search Tags:bulk amorphous alloy, rolling deformation, micro-alloying, thermal stability, mechanical property, corrosion property, cryogenic treatment, martensitic transformation
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