The Microstructure and Mechanical Properties of Nanostructured Ni-based Alloy Produced by Heavy Rolling

Author:Sun Yan Le

Supervisor:dan ai dang


Degree Year:2018





Ni-based alloy is widely used in industry.With the advancement of industrial technology,the industry has put forward higher requirements for the comprehensive performance of Ni-based alloys,especially higher strength for many applications.Severe plastic deformation(SPD)is an important method to obtain nanostructured materials,since it could greatly enhance the strength of the material without adding addition alloying elements.It is hard to prepare bulk nanostructured Ni-based alloys by traditional SPD methods,such as equal-channel angular pressing,high pressure torsion.The method of synchronous-rolling is the most commonly used and most ideal method for industrial production of sheet metal.So far,few works reported the preparation of nanostructured Ni-based alloys produced by heavy synchronous-rolling.Accordingly,there is no systematic study about the microstructure evolution and mechanical behavior of cold-rolled nanostructured Ni-based alloy.In the present study,a new type of low-cost Ni-Fe-Cr alloy with compostion of Al1. Ti1.21.6Nb0.5Mn0.6W1.0Mo0.2Si0.1Zr0.02C0.03B0.004Fe2030Cr2025Nibal(wt.%)was selected and bulk nanostructured Ni-based alloys was produced by heavy synchronous-rolling at room temperature.The microstructure evolution and mechanical properties of the nanostructured Ni-based alloy during annealing were systematically investigated.The strengthening mechanism of nanostructured Ni-based alloy was revealed,while the the effect of theγ?precipitation on the thermal stability of nanostructured Ni-based alloy and the precipitate behavior ofγ?was investigated.Ni-based alloy with multi-scale structure that owing high strength and improved ductility could be produced by microstructure adjustment through annealing.In the present work,nanostructured Ni-based alloy with an average grain size of 50 nm was prepared by heavy cold-rolling at room temperature with 98.3%deformation reduction.The yield strength of cold-rolled nanostructured alloy is up to 1455 MPa,which is 4.7 times higher than the yield strength(253 MPa)of coarse-grained alloy before rolling.Quantitatively study was carried out on the contribution of each strengthening mechanisms to the yield strength of the cold-rolled(CRed)nanostructured Ni-based alloy with single-phase.It was found that the contribution of grain boundary strengthening and dislocation strengthening was 723.7 MPa and 602.7 MPa,respectively.The microstructure of cold-rolled Ni-based alloys with different deformation reduction were studied.It was found that when the rolling deformation was 44%,the original coarse grains of the matrix became twin-matrix interlamellar structure with width of several tens to several hundred.As the deformation reduction over 79%,ultra-fine grain or nano-grain appeared in the matix.The microstructure evolution and mechanical properties of the nanostructured alloy during annealing were systematically investigated.The different precipitation behavior ofγ?phase between coarse-grained Ni-based alloy and heavy cold-rolled nanostructured Ni-based alloy were studied.Since the sphericalγ?phase with uniform size was homogeneously distributed in the coarse-grained alloy,theγ?phase is preferentially precipitated along the grain boundaries and in the intragranular with high density of dislocations.Due to the fast diffusion of grain boundaries,the size of precipitates around grain boundaries is much larger and part of precipitates along the grain bounday are irregularly spherical.The strengthening contribution ofγ?phase is 683.3 MPa,which is the major contribution to the yield strength.The dislocation strengthening and grain boundary strengthening are 424.3 MPa and 461.9 MPa,respectively.The measured yield strength of sample annealed 1 h at 700℃is 1827 MPa,while the calculated yield strength is 1749.3 MPa,which is in good agreement with the measruement.Yield elongation phenomenon in Ni-based alloy was verified that correlated with the grain size and initial dislocation density.When the grain size ranges from 0.81 to 2.11μm,the yield elongation phenomenon appears in the tensile test curves,suggests highly dependency on the grain size.When a large amount of dislocation was firstly introduced into the sample with grain size of0.91μm by exerting plastic deformation,the yield elongation phenomenon disappeared.In turn,if the dislocations introduced by pre-deformation were eliminated by annealing,the yield elongation phenomenon appeared again,which confirmed the correlation between the yield elongation phenomenon and initial dislocation density.High thermal stability of nanostructured Ni-based alloy was investigated.It was found that the nanostructured Ni-based alloys owns high thermal stability,wiht thermal stability temperature of 700°C(0.62 Tm).When the nanostructured Ni-based alloys annealed at 700°C for 300 h,the average grain size is only180nm while the size ofγ?grows from31 nm to50 nm.The mechanism of high thermal stability of the nanostructured Ni-based alloy was investigated,and the long-term stability of matrix at high temperatures is attributed to the continuous“pinning”effect from theγ?phase.The large volume of grain boundaries and high density of dislocations in the cold-rolled nanostructured Ni-based alloy would accelerate the formation of precipitation which could exert a pinning effect on the grain boundaries and hinder the grain growth.Based on our previous work,we used two step heat treatment that firstly aged for 10 h at 600℃and secondly,annealed for 5 min at 950℃,aimed to obtain a sample with multi-scale structure that consisted of microsized grains,ultra-fine grians and nanosized precipitations.The yiled strength and ductility of the sample with multi-scale structure both are higher than the cold-rolled nanostructured alloys with 1455 MPa yield strength and 2%elongation.The yield strength of multi-scaled sample is 1534 MPa,while the elongation is up to 9.8%.The improvement of ductility is attributed to the coarse grains which could coordinate the deformation of the ultra-fine grains around and avoid the formation of micro-fracture in the early stage.The higher yield strength are due to the ultra-fine grains and nano-precipitates.