Effects Of Cooling Rates And Following Aging Process On The Microstructures And Precipitation Behavior Of Zr-Sn-Nb Alloy

In present study,a new Zr-Sn-Nb alloy?Zr-1.0Sn-0.3Nb-0.3Fe-0.1Cu?were used to study the influence of cooling rate on the microstructure,second phase particles?SPPs?,and the evolution of SPPs in specimen cooled at different rates during aging were also investigated.A suite of modern characterization methods including transmission electron microscopy?TEM?,scanning transmission electron microscopy?STEM?,electron channeling contrast?ECC?,opitical microscopy?OM?were jointly used.The main conclusions obtained by present work are as following:?1?With the cooling rates droping,the microstructure evolves in a sequence as:tiny needle-like martensites?water-cooling??Basket-weave widmanstatten structure?Parallel-plate widmanstatten structure?Lenticular structure.The average?-plate width increases as the cooling rate increases.Moreover,there are some{101?2}tensile twins being spoted in furnace-cooled specimen.?2?Two previously unidentified Zr-?Fe,Cu?phases were detected and their crystal structures along with the lattice parameters have been revealed using electron diffraction experiments.One phase?named as I phase?is body-centered tetragonal?bct?with lattice parameters a=b=6.49?,c=5.37?,the possible space groups have also been disgussed to be among I4cm?110?,I4c2?120?and I4/mcm?140?.According to the HAADF images,the reason for its formation has been attributed to the addition of Cu,and no structural or chemical change has been perceived through the aging process.Another phase?named as H phase?possesses a simple hexagonal structure and the lattice paremeters are a=7.87?,c=3.33?.Unlike I phase,H phase has only been spotted in some aged specimens cooled at rather large rates?water-cooling and liquid-nitrogen cooling?,as the aging prolongs H phase gradually disappeared thus it's assumed to be a metastable phase.?3?The evolution of SPPs as aging proceed in specimen cooled at different rates are as following:1)Water-cooling:?+??Zr₃?Fe,Cu?,H phase?Zr₃?Fe,Cu?,H phase,Zr?Nb,Fe?₂?Zr₃?Fe,Cu?,I phase,Zr?Nb,Fe?₂?Zr₃?Fe,Cu?,I phase,Zr?Nb,Fe?₂2)Liquid-nitrogen cooled:??riched in Nb?,I phase?I phase,H phase,Zr?Nb,Fe?₂?I phase,H phase,Zr?Nb,Fe?₂?I phase,H phase,Zr?Nb,Fe?₂?I phase,Zr?Nb,Fe?₂3)Air-cooling:??riched in Nb?,I phase?I phase,Zr?Nb,Fe?₂?I phase,Zr?Nb,Fe?₂?I phase,Zr?Nb,Fe?₂?I phase,Zr?Nb,Fe?₂4)Furnace-cooled:??riched in Nb?,I phase,Zr?Nb,Fe?₂???riched in Nb?,I phase,Zr?Nb,Fe?₂???riched in Nb?,I phase,Zr?Nb,Fe?₂???riched in Nb?,I phase,Zr?Nb,Fe?₂?I phase,Zr?Nb,Fe?₂?4?A phenomenon associated with the precipitation of SPPs at dislocations has been found in several specimens as the aging proceeds,and the movement of dislocations due to the aging process has led to the formation of dislocation nets and tangles near SPPs and grain boundaries.Nb concentration or Nb-containing SPPs?Zr?Nb,Fe?₂?have also been observed near many Zr-?Fe,Cu?particles,and the reason accounting for this phenomenon might be related to the Nb depeletion during the growth of Zr-?Fe,Cu?particles.