Microstructures And Mechanical Properties Of CuZr-based Amorphous Alloys After Annealing

B2-CuZr/CuZr-based amorphous composites are considered to be one of the most promising amorphous materials for engineering applications at present,but the uneven distribution and the agglomeration of B2-CuZr precipitates in the castings restrict the further improvement of the properties.The method of bulk metallic glass crystallization by annealing is a new way to prepare amorphous composites for avoiding the"die wall chilling effect"of copper mold casting and making the B2-CuZr precipitation phase more evenly distributed in the cross section.Consequently,this paper selects high-aluminum Cu-Zr-Al amorphous alloys,and obtains bulk amorphous alloy samples for annealing crystallization by studying the microstructures and glass forming ability of high-Al content alloys.On this basis,through analyzing the annealing microstructures and properties of bulk metallic glass samples,the crystallization behavior in annealing and the effects of as-cast microstructures and Ta addition are explored.The results will provide an experimental basis for the preparation of B2-CuZr/CuZr-based amorphous composites by annealing crystallization.Firstly,a high Al content(Cu_0.5Zr_0.5)_100-xAl_x（x=6,7,8,9,10at.%）alloys are selected to carry out experimental research.The results show that an appropriate Al addition reduces the critical cooling rate and improves the alloy’s GFA.With the increase of aluminum content,the GFA of the alloys increases first and then decreases;when x=8at.%,the alloy has the best GFA and thermal stability,andφ4mm Cu₄₆Zr₄₆Al₈amorphous specimens are prepared at the same time;when x<8 at.%,the alloys form the amorphous composites with single B2-CuZr precipitation,and with the increase of aluminum content,the B2-CuZr precipitation phase decreases;when x>8at.%,the alloys have the competing precipitation phase Al₂Zr,corresponding to the decline of compressive properties.Secondly,theφ4mm Cu₄₆Zr₄₆Al₈amorphous samples are selected to be annealed at 405°C（below T_gtemperature）for 4-32 h and at 430-450°C（above T_gtemperature）for 4h respectively.The results show that the samples annealed at the temperature below T_gare still amorphous,but with the annealing time prolonging,the amorphous short-range order range increases,and the compressive properties show a trend of increase first and then decrease.When annealing at 440℃for 4h above T_gtemperature,the amorphous samples begin to crystallize and precipitate Cu₁₀Zr₇and CuZr₂phases;at 450℃for 4h,the precipitated Cu₁₀Zr₇and CuZr₂phases increase significantly,and the compressive properties of the annealed amorphous samples drop sharply accordingly.Further the crystallization behavior ofφ4mm Cu₄₇Zr₄₇Al₆and(Cu_0.47Zr_0.47Al_0.06)_99.5Ta_0.5as-cast B2-CuZr/amorphous composites in annealing at405℃for 2-8h is investigated,the results show that both samples crystallize obviously,and with the prolongation of annealing time,the B2-CuZr precipitation phase increases,and the compressive properties show a trend of increase first and then decrease;both samples annealed for 4h have the best compressive properties,the compressive fracture strength is 1861Mpa and 1881Mpa,respectively,and the compressive plasticity is respectively 0.39%and 0.43%;too long annealing time leads to the eutectoid decomposition of the pre-eutectoid B2-CuZr into Cu₁₀Zr₇and CuZr₂phases,and the corresponding decrease of the compressive properties.Then,theφ4mm Cu₄₇Zr₄₇Al₆amorphous samples prepared by low-temperature suction casting,φ4mm Cu₄₇Zr₄₇Al₆and(Cu_0.47Zr_0.47Al_0.06)_99.5Ta_0.5as-cast B2-CuZr/amorphous composites are tested at 405℃for 2-8h annealing treatment,the effects of the as-cast microstructures and Ta addition on the annealing crystallization of the alloys are studied;the results confirm that the initial as-cast microstructures are an important factor determining the crystallization behavior of amorphous alloy samples in annealing at below the T_gtemperature,and the as-cast alloys containing crystal precipitation phase are beneficial to the annealing crystallization and the in-situ precipitation of the crystal phase;trace Ta addition promotes the precipitation of B2-CuZr crystal phase in Cu₄₇Zr₄₇Al₆alloy during annealing crystallization,the mechanism is that the co-lattice crystal structure between Ta and B2-CuZr promotes the nucleation of B2-CuZr in amorphous and a trace amount of Ta reduces the thermal stability of the alloy.In summary,high-alumina CuZrAl amorphous samples of appropriate aluminum content have high thermal stability,and the samples are difficult to crystallize below T_gtemperature annealing;above T_gtemperature annealing,mainly precipitation phases are Cu₁₀Zr₇and CuZr₂,and it is difficult to obtain single B2-CuZr precipitation phase on the amorphous matrix by annealing.It is a feasible process for preparing bulk B2-CuZr/amorphous composites that as-cast amorphous composites with a small amount of B2-CuZr precipitation are annealed below the T_gtemperature.