Chemical Component Design, Microstructure And Mechanical Properties Of Zr₃Al-based Alloys

The selection of structural parts for the space is mainly high-strength steels or titanium alloys for years. However, titanium alloys and steels are not optimal aerospace structural materials due to their low hardness and high density respectively. Zr and Zr alloys have excellent corrosion resistance, low density and good wear resistance. But the low strength of the available Zr alloys limited their application as aerospace structural materials. Current studies on Zr alloys mainly focus on solution strengthening and amorphous strengthening. Few reports have examined the potential of using Zr-based intermetallics alloy as structural material. Zr-based intermetallic compound alloys have high hardness, high strength and anomalous yield behavior that strength increased with the temperature increased as well as many other excellent mechanical, physical and chemical properties. Zr-based intermetallics alloys will become a new type of aerospace structural material if their room temperature ductilities can be further improved. This dissertation is aiming to develop new type aerospace materials by means of chemical composition design and optimization, thermal deformation and technology for heating processing. A series of Zr₃Al-based alloys with high room temperature ductility and high strength are prepared. Innate relevance of chemical composition, technology, microstructure and mechanical properties are explored in this dissertation.A series of Zr-x Al binary alloys with different Al contents are prepared by a vacuum non-consumable electro arc furnace. Then Nb alloying and B micro-alloying are designed and optimized for Zr-x Al alloys again. All the alloys have been hot deformed and heat treated by different processes. The experiment results show that phase composition of Zr-Al binary alloys is Zr2 Al and α-Zr by a vacuum non-consumable electro arc furnace. Zr₃Al-based alloys can be obtained by annealing to operate the peritectoid reaction βZr+Zr2Al?Zr₃Al. Mechanical property of Zr₃Al-based alloys depends on the phase composition and the grain size. The ductility of Zr₃Al-based alloys is almost zero when there exists Zr2 Al phase in the phase composition due to its high brittleness. The tensile fracture morphology is brittle fracture. Ductility of Zr-Al binary alloys is higher when its phase composition is Zr3 Al and α-Zr. The tensile fracture morphology is ductile fracture. Zr3 Al phase contents increases with the Al content increases, tensile strength increases and elongation decreases meanwhile. As the Al contents of Zr₃Al-based alloys increases from 6wt% to 8wt%, the temperature of peritectoid reaction increases from 750 to 850℃ ℃. As the annealing temperature of Zr-6Al decreases from 850 to 750℃ ℃, grain size decreases from 400μm to 150μm, tensile strength increases from 589 MPa to 664 MPa, elongation increases a little. Thermal deformation can improve significantly mechanical property of Zr₃Al-based alloys. Tensile strength and elongation of the rolled Zr-6Al alloy after annealed at 750 is 777 MPa and ℃ 8.8%, respectively.6 wt% Nb doped Zr-6Al alloy can retain a part of β phase with body centered cubic structure to room temperature, whereas 9wt% Nb doped Zr-6Al alloy can completely inhibite the peritectoid reaction βZr+Zr2Al?Zr₃Al. It suggests that a certain amount of Nb doped Zr₃Al-based alloys is beneficial for retaining a part of high temperature phase β-Zr with higher ductility to room temperature. However, for purpose of completing peritectoid reaction, the combined effect with increasing annealing temperature or prolonging annealing time increase the grain size, resulting in the decreased mechanical properties of Zr₃Al-based alloys. As a consequence, mechanical properties of Zr₃Al-based alloys can not be improved by means of large amount quantity of Nb doped.Boron can dramatically refine grains of Zr₃Al-based alloys. Doping 0.1Wt% B in Zr-6Al alloy annealed at 750 for 4 hours, grain size decreases ℃from 395μm to 23μm, tensile strength increases from 777 MPa to 1093 MPa, elongation increases from 8.8% to 15.2%. The aim of obtaining Zr₃Al-based alloys with good mechanical properties is achieved.Zr₃Al-based alloys with dimensions of φ38×260mm are prepared by a vacuum induction melting furnace and thermal deformation. The second phase of Zr-Al alloys transforms from rod to particle through thermal deformation. Tensile strength and elongation of Zr-6Al-0.1B alloys annealed at 750 for 4 hours℃ is 1173 MPa and 17.6%, respectively. Therefore, raw materials of Zr₃Al-based alloys used for fabricating smaller size aerospace structural components are obtained.