Modifying The Microstructures And Mechanical Properties Of Zr And Zralloys By High-pressure Hightemperature Technology

Zr and its alloys are important materils that are widely used in nuclear and chemical industry due to their excellent physical and chemical properties,such as superior antiradiation performance,small neutron absorption cross-sectional,strong corrosion resistance,favorable oxidation resistance,etc.With the rapid development of science and technology,the application of Zr alloys in aerospace,biomedical applications are gradually increasing to deal with more demanding application environment.In this thesis,we choose Zr and its alloy as model system to study the effects of high-pressue and high-temperature(HPHT)treatments on the phase transformation,microstructure evolution and mechanical properties.The relationship between phase composition,microstructure and mechanical properties were investigated systematically.Based on the first-principles calculations,the strengthening and deformation mechanism of ? phase were studied,which provides a theoretical basis for the subsequent design of Zr alloys for excellent properties.Pure Zr metal with ?? martensite and high-pressure ? phase with high strength and high toughness were successfully prepared through reasonable regulation of HPHT processing conditions.The yield strength,tensile strength and elongation were ~616 MPa,~734 MPa and 14.9%,respectively.The strength was about twice that of the as-received Zr.Pure Ti metal with needle-like martensite was obtained througth HPHT treating,the yield strength,tensile strength and elongation were 455.5 MPa,495.2 MPa and 36.2%,respectively,and the strength was increased by 53% compared with coarse crystal Ti.The enhancement of strength came from the formation of fine needle-like ?? martensite and ? phase due to HPHT treating.Studies have shown that high pressure can promote the formation of fine needlelike ?? martensite and high-pressure ? phase by suppressing the diffusion of atoms.Ti-30Zr-5Al-3V(TZ-30)alloy was treated at 2 GPa and various temperatures,? phase region and various pressure,the effect of HPHT treatment on the phase composition,microstructure and mechanical properties of TZ-30 alloy was studied.The experimental results showed that TZ-30 alloy with good mechanical properties could be obtained by HPHT treating at 2 GPa pressure and 1073 K,the tensile strength and elongation of the TZ-30 alloy are 1387 MPa and 7.9%,respectively.Microstructure analysis showed that the strengthening of the alloy was derived from the formation of fine needle-like martensite containing a large number of dislocations and stacks.With increasing treatment pressure,the content of ?? martensite within TZ-30 alloy increased.High pressure that shows a significant effect on promoting ?? martensite formation in TZ-30 alloy.The sample treated at 5 GPa was full of ?? martensite.It was concluded that the ?? martensite in the TZ-30 alloy was retained in the metastable state,the dislocations,faults and the microstructure defects within the alloy had the effect of pinning the metastable phase.The tensile results showed the stress-strain curves were double yielding shape,which were caused by the stress induced ????? phase transition.The Young?s modulus of TZ-30 alloy decreased with increasing the ?? martensite content,and lowerer to only 34 GPa for the sample with pure ?? martensite.Based on our experimental data and literature data,the linear relationship between the Young's modulus of ?? type Ti(Zr)alloys and the average valence electron number of the alloys was proposed.The results shadow lights on the development of novel low-modulus medical Zr(Ti)alloys.The strengthening mechanism of ?-Zr was studied by the first principles calculations.The results showed that the shear modulus for ?-Zr was 1.37 times larger than that of ?-Zr.The weakest slip system of ?-Zr is(10(?)0)[0001] and its ideal shear strength is 3.25 GPa,which is 1.62 times that of the weakest slip system(10(?)0)[(?)2(?)0] of ?-Zr.The enhanced shear strength of ?-Zr was derived from the more reasonable electron local configuration in its chemical bonds,leading to an increasing in the ability to resist elastoplastic deformation.Based on the von Mises criterion for the uniform plastic deformation of the polycrystals,we proposed that the most likely deformation mechanism in the polycrystalline ?-Zr is the prismatic slip {10(?)0}[0001] or {11(?)0}[0001] + pyramidal slip {10(?)(?)}<(?)2(?)0>.