Formation And Transformation Of Quasicrystal In Mg-Zn-Gd Alloy

Quasicrystal phase is a hard phase and it has a strong interface capability with the matrix.So quasicrystal phase could often be used as the strong phase to refine the grain size and improve the overall performance of alloys.The investigation of Mg-Zn-Gd quasicrystal alloys is still in its infancy.Mg-Zn-Gd quasicrystal phase is a new type of quasicrystal phase in Mg-Zn-RE quasicrystal alloy.It is of great significance on clarifying the heredity of icosahedral short-range order?ISRO?structure in icosahedral quasicrystal phase.Because Mg-Zn-Gd alloys have a large negative enthalpy of mixing,they can easily form metastable structures,such as quasicrystal structure and amorphous structure.The study of the structural transformation process of metastable phase is conducive to analysing the formation mechanism of icosahedral quasicrystal phase in Mg-Zn-Gd alloys.Quasicrystal and amorphous alloys have a pseudo-energy gap at the Fermi level,thus they have a negative temperature coefficient of resistivity.The structural changes of the quasicrystal and amorphous alloys were during annealing process in order to analyse the hereditary of the ISRO structure.Mg_70-xZn₃₀Gd_x?x = 3,4 and 5 at.%?quasicrystal and amorphous alloys were prepared by the conventional solidification and rapid solidification techniques.The thermodynamic characteristics of quasicrystal and amorphous phases were characterized by differential scanning calorimetry?DSC?.The change rules of electrical resistivity with the increase of temperatures were investigated using the direct current four-probe technique to analyze the structural transformation of quasicrystal and amorphous alloys during annealing at constant pressure.The structure information of the alloys was detected by scanning electron microscopy?SEM?,X-ray diffraction?XRD?and transmission electron microscopy?TEM?.In addition,two interesting aspects were investigated,including the relationship between the quasicrystal phase and the ISRO structure in the melt and the effect of quasicrystalline master alloy on the microstructure and properties of AZ91 alloys.The results showed that the Mg-Zn-Gd alloys with quasicrystal phase can be obtained by the conventional solidification method.When the melt of holding time was 21 min,the alloy was composed by the Mg3?Gd,Zn?phase,?-Mg phase,Mg7Zn3 phase and a small amount of Mg-Zn phase.When the melt of holding time was 41 min,the Mg3?Gd,Zn?phase was changed to quasicrystal phase.When the melt of holding time were 61 and 81 min,the quasicrystal phase was transformed into the Mg3?Gd,Zn?phase.The Mg-Zn-Gd quasicrystal phase was consisted of typical 3-f and 5-f rotational symmetry axes in the icosahedron quasicrystals.When the content of Gd was 4 at.% and the melt of holding time was 41 min,the content of quasicrystalline phase was larger than that of others,reaching 46.51 %.Mg_70-xZn₃₀Gd_x?x = 3,4 and 5 at.%?amorphous alloys were prepared by high vacuum single roll quenching.Speeds of copper roller were 25.00,12.50 and 8.33 m/s,respectively.With the increase of Gd content,the critical cooling rate of amorphous gradually decreased,and the formation ability of amorphous gradually increased.The glass transition temperature of the Mg65Zn30Gd5 amorphous alloy at the 1000 r/min is 132°C and the crystallization temperature is 150°C.The structural transformation of quasicrystal alloy annealed at 410 and 467°C were measured by resistivity method.In the first heating process,the quasicrystal alloy had a negative temperature coefficient of resistivity.In the subsequent,the quasicrystal alloy showed positive temperature coefficient of resistivity.After 5 cycles annealing of 410°C,the columnar phase was formed in the alloy.After 3 cycles annealing of 467°C,a large amount of the Mg3?Gd,Zn?phase was formed in the alloy.Amorphous alloy annealing process appeared three major crystallization stages.There was a negative temperature coefficient of resistivity during the crystallization in the Mg66Zn30Gd4 amorphous alloy.When annealing at 230°C,the quasicrystal phase was precipitated in the amorphous alloy.When annealed at 304°C,a large amount of Mg3?Gd,Zn?phase and Mg7Zn3 phase were formed in the amorphous alloy.The coordination numbers of ordered clusters of amorphous alloys and quasicrystal alloys were 12.According to the heredity of alloy melt structure and the results of XRD,EDS and SEM of Mg-Zn-Gd quasicrystal/AZ91 alloy,Mg-Zn-Gd quasicrystal/AZ91 alloy was prepared by external addition method.When the amount of quasicrystal alloy was 0-6 wt.%,the grain size of the alloy decreased gradually.When the amount of quasicrystal alloy was 8-16 wt.%,the grain size of the alloy gradually increased,the ? phase changed into a continuous network distribution,and promoted the formation of large quasicrystal phase particles.When the amount of quasicrystal alloy was 6 wt.%,the tensile strength of the Mg-Zn-Gd quasicrystal/AZ91 alloy was 187 MPa which is larger than that of others,and the elongation was 6.5 %.