Glass Forming Ability And Crystallization Behavior Of Al-Ni-Re Alloys

Due to their high specific strength,good ductility,superior corrosion resistance,and other desirable properties,aluminum-based amorphous alloys are highly potential in application and have drawn much attention in recent years.However,up to now,only a few compositions have been reported to form amorphous samples with a diameter up to 1 mm.Thus,how to improve the glass forming ability(GFA)of Al-based amorphous alloys is a significant issue.Meanwhile,glass formation is a result that the amorphous phase won the competition with crystalline.The crystallization behavior of amorphous alloys more or less reflects the solidification process.It is also of significance to investigate the crystallization behavior of Al-based amorphous alloys for deeper understanding the glass forming ability.In this dissertation,taking Al-Ni-La alloys as the base alloys,the influences of the mutual-substitution between the atomically similar structured La and Gd and the Si addition on the glass forming ability and crystallization behavior have been systematically investigated by using optical microscopy(OM),X-ray diffractometry(XRD),differential scanning calorimeter(DSC),high-solution transmission electron microscopy(HRTEM),scanning electron microscope(SEM)and 3D atom probe(3DAP).The mutual-substitution between La and Gd was performed in the optimal ternary glass forming alloy,Al85.5Ni9.5La(Gd)5,and six neighboring compositions(Al85Ni10La(Gd)5,Al85.5Ni10La(Gd)4.5,Al86Ni9.5La(Gd)4.5,Al86Ni9La(Gd)5,Al85.5Ni9La(Gd)5.5,Al85Ni9.5La(Gd)5.5).10 at.%or 20 at.%La was substituted by the same amount of Gd in each step of mutual substitution for each alloy,and vice versa.It was found that the GFA becomes poor and poor as one rare earth element is substituted by the other one for the Al85.5Ni9.5La(Gd)5 base alloy,but the GFA can be enhanced by the mutual substitution,before the optimal substitution content is reached for the six neighboring base alloys of Al85.5Ni9.5La(Gd)5.The function of GFA as the substitution content shows a M-shape trend.It is demonstrated that the atomic packing efficiency in the solute-centered clusters dominates the variation of GFA in these alloys.0.2,0.5,1.0,1.5 and 2.0 atomic percentages of Si was added to the optimal glass forming alloy,Al85.5Ni9.5La5,and four neighboring compositions Al86Ni9La5,A186Ni9.5La4.5,Al86Ni10La4 and Al86Ni10.5La3.5 whose Al contents keep constant to investigate the variation in GFA.It was found that the GFA of the optimal ternary glass former is also deteriorated due to the minor Si addition but the GFAs of other four base alloys can be improved more or less by a proper Si minor addition.Interestingly,the worse the GFA of the base alloy,the large the best Si addition,and the higher the GFA increment.The crystallization products of both the first and second exothermic peaks are fcc-Al during isothermal annealing of(Al86Ni9La5)98Si2 amorphous alloy.The interval between the second and third crystallization peaks increases with the heating rate increasing in the order of 5,10,20,40 and 80 K/min.Based on the Kissinger theory on phase transformation kinetics,the crystallization peak temperature is revealed to be a function of activation energy.The larger the activation energy,the smaller the increase in crystallization peak temperature with the increasing heating rate.Thus,the temperature interval between two crystallization peaks increases if the activation energy of the latter one is smaller,as observed in the second and third crystallization peaks of(Al86Ni9La5)98Si2 alloy.The reseason for the precipitation process of fcc-Al to be separated into two steps during isochronal heating is that the growth of fcc-Al crystals is retarded as amount of La and Si elements enrich around the growng fcc-Al grains.The mixing enthalpy between Si and La is as large as-73 kJ/mol,much more negative than with the other main components,indicating that Si and La atoms tend to bond each other.As a result,the capacity for La to diffuse significantly decreases as many La and Si atoms are rejected by the growing fcc-Al crystals in the amorphous matrix.The growth of fcc-Al can be activated again only when the temperature is highe enough.