High Strength And Ductility Nickel Aluminum Bronze Alloy:Fabrication,Microstructure And Mechanical Behavior

Due to the remarkable mechanical properties,excellent corrosion resistance and distinctive microbial resistance,nickel aluminum bronze（NAB）alloy is regarded as an indispensable structural material for marine engineering.In this paper,for optimizing of microstructure and developing mechanical properties,processing methods such as heat treatment and heavy deformation rolling are conducted to NAB alloys,and series of alloys with ultrahigh strength and high ductility have been prepared.The mapping relationship of processing,microstructure and mechanical properties is established.The precipitation,deformation and fracture behavior of alloys with ultrafine grains are systematically studied,and the contribution of strengthening mechanisms in alloys with ultrahigh strength is revealed.The strengthening mechanism and generation mechanisms of annealing twins,an important strengthening microstructure in the alloy,are also systematically investigated.A stepwise heat treatment process involving quenching,aging and tempering is proposed to achieve ultrahigh strength in NAB alloys without plastic deformation,retaining considerable ductility.The aging treatment on the martensite induce high-density nano precipitates.Their pinning effect avoid the generation of coarse lamella tempering structures,thereby obtaining uniform and ultrafine tempering structures.The nano precipitates also act as nucleation points of k phase in the martensitic tempering transformation,so that the precipitation size after tempering can be kept at the nanometer scale.The high strength of alloy is mainly attribute to the ultrafine lamella tempering structure and nano precipitation,and the improvement of toughness and plasticity is mainly due to the avoidance of brittle fracture at the boundary of coarse lamella tempering structures,so that the plasticity is improved.Based on the deep understanding of the mechanisms of phase transformation through the research of heat treatment process,the process of warm rolling in steels is conducted on NAB alloys to prepare ultrahigh strength alloy in this paper.The investigation results on the rolling process shows that the preferred rolling process is as follows:solution treating in a singleβ-phase region before rolling and air cooling,then continuous rolling with few passes at the temperature below the recrystallization temperature of theβ-phase and the precipitation temperature of the nano-k₄ phase.The final rolling temperature is suggested to be around the eutectoid transformation.After rolling with a reduction ratio of 90%,the average grain size ofαphase can be refined to about 300 nm,and the tensile strength of the alloy can be increased to more than 1300 MPa.The ultrahigh strength alloys obtained by heavy warm rolling are annealed at different temperatures from 200 to 900℃.Series of ultrafine-grained alloys with high strength,ductility and corrosion resistance can be obtained.The microstructure,mechanical behavior and fracture behavior of the samples annealing at different temperatures are studied,and the mapping relationship of processing,microstructure and properties is established.The investigation on the precipitation behavior of ultrafine-grained alloys shows that the current classification system of k phases based on as-cast alloys needs to be further optimized in ultrafine-grained alloys.High-density nano annealing twins are introduced into the alloy by short-time medium-temperature annealing after heavy warm rolling,which makes the samples have ultrahigh strength,good plasticity and excellent work hardening ability.Defects such as high-level deformation energy storage,high-density dislocations,and dispersed ultrafine k-phases brought about by rolling are important factors for the generation of high-density twins.The study of the generation mechanism of annealing twins shows that the ultrafine precipitation in ultrafine grains has a significant promoting effect on the generation of twins.