Study On The Microstructure Control And Mechanical Properties Of Inconel 718 Alloy

Inconel 718,a precipitation hardening superalloy,has high strength and good plasticity at room temperature.What's more,it still maintains good strength-plasticity coordination at 650 ?,and shows excellent creep property or stress-fracture life.Therefore,Inconel 718 is widely used in the manufacture of disk,shaft and fastener in aerospace,shipbuilding,energy and other industrial fields.The matrix of Inconel 718 alloy is ?-austenite and its precipitation phases include ? ?,? ' and ? phases.The properties of the alloy are closely related to the number,morphology,size and distribution of these precipitated phases in the microstructure.Among them,the regulation of ? is relatively important to control the grain size,toughness and notch sensitivity of this alloy,so it has been widely studied and discussed.Depending on the environment in which the alloy is used,appropriate processing techniques and pretreatments are required,and the conventional heat treatment method is solid solution plus twostage aging.However,as the service temperature limit of superalloys has been improved in aerospace and other fields,Inconel 718 alloy has gradually failed to meet market demand.At present,it has become an important means to control the precipitation behavior of phases in this alloy by changing the alloy composition to meet the increasingly stringent environmental requirements.For those problems,this paper systematically studied effects of microstructure changes on the properties of Inconel 718 alloy in a small temperature range(temperature range is about 40?).At same time,the dissolution characteristic of precipitates have been explored in modified Inconel 718 alloy.(1)Through tensile and impact tests at room temperature and stress fracture test at high temperature,the important factors that cause the anisotropy of mechanical properties of Inconel 718 alloy have been determined,including coarse grains deformed along the rolling direction,? phases and carbides aligned along a certain direction.With the increase of solution temperature(from 940? to 980?),the coarse grains deformed along the rolling direction disappeared,the ? phase dissolved,but the carbides were still arranged along a certain direction.The content and distribution of ? phases have no significant effect on the tensile strength at room temperature,but reduce the tensile plasticity and impact toughness at room temperature.For samples which have been treated at a relatively low solution temperature,the ? phases distributed along a certain direction significantly reduce its mechanical properties,and this distribution is the main cause of anisotropy.However,for those samples treated by higher solution temperature,the directional distribution of carbide is responsible for the significant anisotropy.However,what we should pay attention to is a change of the effect factor about anisotropy at high temperature.The directional distribution of carbides become the main factor affecting the anisotropy of Inconel 718 alloy,and a certain amount of ? phase is relatively important to increase alloy's high-temperature stress-rupture life,but the ? phase directional distribution characteristics have no obvious effect on the anisotropy of fracture plasticity.(2)For those Inconel 718 alloys with different Nb,Al and Ti contents,the microstructure and tensile mechanical properties of them after standard solution and double aging heat treatment were studied.The results show that when the values of Al / Ti and(Al + Ti)/ Nb do not change much,with the increase of Nb content,the percentage of ? phase is more,and it has better high temperature stability.High temperature solution can effectively prevent the growth of grains,and the stable ? phase ensure the fine grain strengthening of the alloy.When the ratio of Al / Ti and(Al + Ti)/ Nb is low,the contents of ? ? and ? ? decrease,and the morphology of the ? ? phase changes from lip-like to disc-like or sesame-like,in this case Nb element will mainly form ? phase.The results of mechanical properties show that when there is no change in the ratio of Al / Ti and(Al + Ti)/ Nb,the tensile strength is mainly affected by the grain size,while the ? phase content is less affected.However,When the ratio of Al / Ti to(Al + Ti)/ Nb is low,the tensile mechanical properties and micro-hardness decrease with the decrease of tensile strength due to the change of the quantity and morphology of the strengthening phases ?'' and ?'.(3)The precipitation and dissolution characteristics of ? phase in IN718-modified alloy and its influence on thermal deformation have been investigated.The variations of stress-strain curve meet the characteristic of some alloys with low stack fault energy.The peak stresses were found to decrease with increasing deformation temperature and decreasing strain rate.The fastest precipitation temperature of ? phase is 950 ° C,and it is completely dissolved at 1030 ° C.However,it is consistent with ? in preventing grain growth.The ? phase significantly reduced the activation energy of hot deformation,which the energy of as-received sample is 411.82 KJ/mol and the solution one is 377.6 KJ/mol.In the process of hot compression,the ? phase deflects and breaks,resulting in the grain breaking and accelerating the dynamic softening process.