Research On Effects Of Micro-alloying Sb/Sn/Cd In Mg-Cu-Gd-based Bulk Amorphous Alloys

Mg-based bulk amorphous alloys which have attracted increasing attention have been researchedin this paper. Aiming at proposing the new criterion of glass forming ability and improving the glassforming ability and plasticity of Mg-Cu-Gd-based amorphous alloys (MCGBAA), the reliability ofnew criterion was researched and validated in this paper, then the MCGBAA were prepared by the arcmelting furnace with suction casting equipment which was designed and fabricated independently.The effects and mechanism of minor additions of Sb, Sn or Cd which have the positive mixing heatwith the main constitute Cu on the glass forming ability, continuous heating and isothermalcrystallization kinetics, mechanical properties and corrosion behaviors of MCGBAA wereinvestigated by means of XRD, SEM, OM, TEM, DSC, nano-indenter and electrochemicalworkstation in this thesis. Furthermore, the influence mechanism of electroless plating copper onplasticity of amorphous alloys was analyzed in detail.Based on the relationship between glass formation and thermal characteristic temperatures, thenew criterion of glass forming ability1c(c=(TxTg)/2/(Tg+Tl)) has been proposed from the overallconsideration of crystallization resistance as well as the liquid stability of amorphous alloys, andverified in various of amorphous alloy systems. The results show that cexhibits higher reliabilitythan other criterions. In the MCGBAA of this thesis, the new criterion calso exhibits betterreliability with Zcthan other criterions, R2achieves76.79%.The arc melting furnace with suction casting equipment designed and fabricated independentlyexhibits excellent protective effect and agitation role for the molten bath and obtains bettermold-filling results by local pressurization and vacuum suction. The temperature field of coolingcopper mold has been simulated by the finite element software and the mold size has been optimized.The cylindrical and conical samples of MCGBAA with uniform composition, well mold-filling andwithout oxide inclusion and shrinkage porosity have been prepared successfully by this equipment.The glass forming ability of Mg-Cu-Gd-based amorphous alloys are improved by the minoradditions of single Sb, Sn and Cd. The critical size of glassy alloys increases at first and thendecreases as the content of minor additions increasing, while the best glass forming ability is gainedby Cd-containing alloys, the maximum size of the optimum composition (Mg₆₁Cu₂₈Gd₁₁)₉₈Cd₂isabout two times that of the monolithic alloy. The atomic size grads of alloy system could be modifiedby the minor additions, and this will increase the liquid viscosity and block the atom diffusion and decrease the undercooling degree of melting alloy (Tg=Tm-Tg), the formation of major Mg2Cu phasewill be then restrained, this could increase the tendency of glass formation. The addition with highersolid solubility in the matrix alloy exhibits the best improvement effect on the glass formation. Butexcessive additions will result in the precipitation of GdMg5phase and decrease the glass formingability obviously.The minor single additions of Sb, Sn and Cd could delay the crystallization process ofMCGBAA, while the Cd-containing alloy exhibits the best crystallization-resistance ability. Theincubation time of isothermal crystallization, activation energies of continuous and isothermalcrystallization, as well as Avrami exponent begin to increase and then decrease with the increasing ofCd content. For the optimum (Mg₆₁Cu₂₈Gd₁₁)₉₈Cd₂glassy alloy, the activation energies of continuouscrystallization, Eg,Exand Ep, as well as the activation energy of isothermal crystallization, are30.9%,63.6%,65.8%, and50%, respectively, higher than the monolithic alloy. The Avrami exponents of(Mg61Cu28Gd11)98Cd2glassy alloy at469K-479K are all larger than2.5, indicating the grain nucleationand growth of the glassy alloy is diffusion-controlled with an increasing nucleation rate.In the isothermal annealing experiments at469K, The crystallinity of monolithic alloy and(Mg61Cu28Gd11)98Cd2glassy alloy keeps increasing with the annealing time coursing, while thecrystallization rate increases at first and then decreases. The precipitation of Mg2Cu phase of theCd-containing glassy alloy is obviously delayed. Minor additions which have the positive mixing heatwith the main constitute Cu could block the path of Mg and Cu atoms diffusion, and promote thecombination of Mg and Gd atoms to form the competing phase and suppress the nucleation of majorMg2Cu phase.The compression fracture strength and plasticity of Mg61Cu28Gd11glassy alloys have beenimproved by the minor addition of single Sb, Sn and Cd, while the Cd-containing glassy alloys exhibitthe best mechanical properties. The strength, plasticity, elastic modulus, hardness and indentationcreep-resistance of MCGBAA increase at first and then decline with the increase of Cd content, andexhibit a peak as the Cd content achieves2at.%. The fracture strength of (Mg61Cu28Gd11)98Cd2amorphous alloy is increased by15.9%as compared with the monolithic alloy, while its plasticityincreases from0to0.45%. Moreover, the apparent plastic flow is found in the fracture surface of(Mg61Cu28Gd11)98Cd2glassy alloy. The improvement of compression strength is related to the increaseof structural stability during the deformation of glassy alloys with the minor addition of Cd.Additionally, the Cd addition could cause chemical micro-inhomogeneity and then lead to the localphase separation in the compression, which will further retard the propagation of shear bands and promote the generation of multiple shear bands, improving the plasticity of glassy alloys.The plasticity of (Mg₆₁Cu₂₈Gd₁₁)₉₈Cd₂amorphous alloy with Cu plating keeps increasing as theplating thickness rising, and the plastic strain of the glassy alloy with the maximum Cu platingthickness about100m increases to1.76%, approximately four times that of the glassy alloy withoutplating, and the shear bands and branches in the deformed side surface also increase considerably. Inthe compression, the internal surface of copper layer imposes three-dimensional compressive stress onthe outside surface of the amorphous alloys, and this will increase the difficulty of the crack formationand propagation. When subjected to the equal axial compressive force, the thinner the copper layer,the larger the three-dimensional deformation. The local defects will further accelerate the fracture anddelamination of the layer and result in the crack formation in that the local amorphous alloy surface isin the plane stress state.The corrosion-resistance properties of MCGBAA could be both improved in the alkaline andacid solutions by minor addition of Cd. The corrosive potential is ennobled, while the corrosioncurrent and rate both decrease with the increase of Cd content. The breakdown of passive film in thealkaline solutions is also delayed by the addition of Cd. Based on the theory of "Point Defect Model",the increase of oxygen-ions diffusion concentration in the passive film of Cd-containing alloy wouldaccelerate the formation and growth of passive film. In the acid solutions, the reduction deposition ofboth Cu2+and Cd2+in the surface could further reduce the corrosion process and improve thecorrosion-resistance behaviors of glassy alloys.