Study On The Design And Glass-forming Ability Of Al Based Amorphous Alloys

Al-based amorphous alloys have higher specific strength,corrosion resistance,wear resistance and high temperature resistance than traditional crystalline aluminum alloys,being one of the most prospective and potential materials.It is therefore very important to study the alloy systems and disclose their amorphous forming mechanisms for the development of Al-based amorphous alloys.Mechanical alloying has been demonstrated to be an effective technology to produce Al-based amorphous alloys.This method also has other advantages such as relatively simple equipment,low cost and easy operation etc.Mechanical alloying combined with powder metallurgy technique have been successful in preparing large sized block amorphous alloy materials or composite materials,which makes the industrial applications of Al-based amorphous alloys possible.In view of the key problems such as narrow supercooled liquid region and weak amorphous forming ability of Al-based amorphous alloys,the present dissertation analyzed and summarized the general rules and criteria for designing amorphous alloys based on the previous studies,and a series of new Al-based amorphous alloy systems have been designed through selective component doping or similar atom replacement methods.Then the compositions of these Al-based amorphous alloy systems were optimized,and the relevant characteristic transition temperatures were elevated according to the formation rules and mechanisms of amorphous alloys.It follows that several new Al-based amorphous alloy powders were fabricated with strong amorphous forming ability,high thermal stability and wide supercooled liquid region.These results provide some basis for designing and optimizing Al-based amorphous alloy systems as well as preparing block amorphous materials.The main conclusions are as follows.The parameter mixing entropy,?Smix,based on the ratio of atomic radius is proposed.By comparing the ?Smix with existing design theories and glass-forming ability of amorphous alloys,it is found that an Al-based amorphous alloy with a high ?Smix usually has large absolute values of negative mixing entropy and of the product of entropy and enthalpy.The mixing entropy has a good linear relationship with the glass-forming ability of amorphous alloys.Relatively large absolute value of mixing entropy is more favorable for enhancing the glass-forming ability.Accordingly,the value of mixing entropy can be used as a criterion for whether the alloy system can form amorphous state or not,and can be also used to assess the glass-forming ability of an Al-based alloys to a certain extent.An alloy was designed with the composition(in atomic percentage)of Al70Fe12.5V12.5Nb5,and its mechanical alloying process by ball milling was investigated.It is found that there appeared diffusive peaks in the XED spectra and annular diffusive halo in the selected area electron diffraction pattern of alloy powder milled for 6h,typical characteristics of vitrification matter.It is therefore suggested that the alloy has been fully amorphous.It is shown that,by energy spectrum and standard deviation analysis,the content of Fe is obviously higher than that of designation.This means that long-time ball milling would result in the increase of Fe content.There are two obvious crystallization exothermic peaks in the DSC curves of the amorphous alloy.With increasing the heating rate,the characteristic temperature increased,showing an obvious kinetic effect.When the heating rate was increased to 20K/min,the width of supercooled liquid region reached 117.2K.By examining the crystallization behavior of the alloy,it is found that the crystallization consists of two processes i.e.nucleation and growth.The nature of crystallization is a process dominated by diffusion and characterized by gradually decreased nucleation rate,in which the activation energy corresponding to the second crystallization process is significantly greater than that of the first process.Annealing at a temperature in the supercooled liquid region,for example,773K for 100 minutes,will produce crystalline phases in the amorphous alloy,which is related to the structural relaxation of amorphous alloy.An Al70Fe12.5V12.5Ni5 alloy was designed by substituting Nb by Ni that has the negative mixing enthalpy with Al,Fe and V.The alloy was completely amorphous after milled for 50h.Although the powder particle size is different,the sizes have a good normal distribution characteristics with the average diameter of 14.13 ?m.There are two obvious crystallization exothermic peaks in the DSC curves.Since the influence of heating on the glass transition is more obvious than on the kinetic effect of crystallization,the width of supercooled liquid region gradually decreased from 90.9K to 82.8K with the increase of heating rate.According to the continuous heating transformation curves,the time to maintain the amorphous structure at 400K can reach 1026 ks,indicating that Al70Fe12.5V12.5Ni5 amorphous alloy has good crystallization resistance and thermal stability.For the three alloy systems of Al70Fe12.5V12.5X5(X=Zr,Nb,Ni)obtained by varied ball milling times,the sizes of amorphous powder particles were gradually decreased with the ball milling time.Among them,the amorphous degree of Al70Fe12.5V12.5Zr5 alloy powder was the lowest,Al70Fe12.5V12.5Ni5 alloy has the strongest glass-forming ability,and Al70Fe12.5V12.5Nb5 alloy has the widest supercooled liquid phase region.It is showed that the width of supercooled liquid region does not mean strong or weak glass-forming ability of amorphous alloys.In terms of the theory of atomic radius ratio,the increase of mixing entropy is beneficial for enhancing the glass-forming ability of Al-based amorphous alloys if the effective atomic size radius ratio gives rise to the formation of local icosahedral atom clusters and the mixing enthalpy is negative.