一种含4.2%Re单晶镍基合金在蠕变期间的组织演化与变形特征
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Available online at\^n^n^f.sciencedirect.com ScienceDi rect EI s踟ER Press Trans.NonferrousMet.Soc.China 21f201 I1 I 532一l 537 Transactions of Nonferrous Metals Society of China www tnmsc.Cn Microstructure evolution and deformation features of single crystal nickel—based superalloy containing 4.2%Re during creep TIAN Su—gui,LIANG Fu—shun,LI A—nan,LI Jing-jing,QIAN Ben-jiang School of Materials Science and Engineering,Shenyang University of Technology,Shenyang 1 1 0870,China Received 18 October 2010;accepted 19 January 201 l Abstraet:By means of microstructure observation and measurement of creep properties,the high temperature creep behaviors of a single cwsta1 nickel—based superalloy containing Re were investigated.Results show that the single crysta1 nickel—based superalloy containing 4.2%Re possesses a better creep resistance at high temperature.After being crept up t0 fracture,the various morphologies are displayed in the diierent areas of fthe sample.and the phase is transforuled into the rafted structure along the direction verticai t0 the applied stress axis in the regions far from the fracture.But the coarsening and twisting extents of the rafted y phase increase in the regions near the fracture。which is attributed to the occurrence of the larger plastic deformation.In the 1ater stage of creep。the deformation mechanism of the alloy is that the dislocations wim『0 1 11 and『01 1]trace features shear into the rafted y phase.The main/secondarv slipping dislocations are altemately activated to twist the rafted 7 phase up to the occurrence of creep fracture.which is thought to be the fracture mechanism of the alloy during creep. Key words:single crystal nickel—based superalloy;Re:creep:microstmcture evolution;deformation mechanism sphere-structure【9],which decreases only slightly the l Intr0ducti0n Single crystal nickel based superalloys have been widely used because of their high volume fraction of y strengthening phase and good high temperature mechanical and creep properties,to make the blade parts creep properties of alloy at high temperature.However,a few 1iteratures reported the microstructure evolution feature and creep behaviors of Re。containing single crystal nickel—based superalloy at high temperature. In the present work.the creep behaviors of 4.2% Re--containing single crystal nickel-・based superalloy were investigated by microstructure observation and creep curves measuring of the alloy in the range of the of the advanced aeroengine『卜2].With the increasing of service performance,such as the power of aeroengine and thermal eficifency.higher service temperature is required to increase the working e ciency of the aeroengine.Thus,it is very important to improve high applied stress and temperature, and deformation mechanism of the alloy during creep was briefly discussed. temperature properties of superalloys[3-41.Adding refractory elements,Re and W,can improve the high 2 Experimental temperature properties of superalloys[5-6】because about 80%element Re is mainly distributed in the 7 matrix to form the atomic group with short—range The single crystal nickel—based superalloy with[00 1] orientation has been produced with selecting crystal method in a vacuum directional solidiication furnace fordering structure of about severa1 nm in size.which may reduce the diffusion rates of the elements and directional coarsening of), phase during creep to improve the stability of the microstructure,and hinder the dislocation movement.Some literatures reported that the element Re may promote the precipitation of TCP under the condition of a high temperature gradient.The nominal chemical composition of the superalloy was Ni—Cr—Co—W.Mo—Al—Ta.4.2%Re(mass fraction).The heat treatment regimes of the single cwstal nickel—based superalloy bars were given as follows:(1 280。C,2 h phase[7-8],and the TCP phase precipitated in the superalloy with element Re may be transformed into the (1 320。C,4 h,AC)+(1 120。C,4 h,AC)+(870。C,24 h, AC1. Foundation item:Project(5057 1 070)supported by the National Natural Science Foundation of China Corresponding author:TIAN Su—gui;Tel:+86-24—25494089;E-mail:tiansugui2003@163.corn DOI:10.1016/S1003-6326(11)60892-3 TIAN Su—gui,et al/Trans.Nonferrous Met.Soc.China 21(201 1)1532—1537 After fully heat treatment,the bars of the alloy were 1533 of the alloy under the applied stress of 1 20]VIPa displays machined along the『00 1 1 orientation into the tensile creep samples with a cross—section Of 4.5 minx2.5 mm and gauge length Of 20 mm.The uniaxial constant 1oad tensile tests were conducted on a creep testing machine a shorter initial stage and longer steady state stage lasting for about 300 h.the creep lifetime of the alloy is measured to be 450.9 h.The strain rate of the alloy during steady state creep increases slightly as the applied stress enhances to 1 37 MPa.the creep lifetime of the alloy is measured to be about 200.6 h,and decreasing extent of the alloy lifetimes is about 55.4%.As the rGWT504 mode1).The creep curves of the alloy at diferent conditions were measured.The microstructures of the alloy at the different states were observed with SEM and TEM.The activation energy and stress exponent of the alloy during steady state creep were applied stress enhances to 1 50 MPa,the creep lifetime of the alloy decreases rapidly to 93.3 h.which indicates that the alloy during creep at 1 080。C possesses an obvious calculated according to the creep data. 3 Results and analysis 3.1 creep features of alloy The creep curves of the single crystal nickel—based superalloy containing 4.2%Re under different conditions were measured.and are shown in Fig.1.All Ofthe creep curves display the obvious feature of three stages,initial creep,steady state creep and accelerated creep stages. The creep curves of the alloy under the applied different stresses at 1 080。C are shown in Fig.1(a).It may be understood from Fig.1(a)that the creep feature Time/11 Fig.1 Creep curves of 4.2%Re-containing single crystal nickel-based superalloy under different conditions:(a)Applied different stresses at 1 080。C;(b)Applied srtess of 137 MPa at different temperatures sensibility to the applied stress. The creep curves of the alloy under the applied stress of 137 MPa at different temperatures were measrued,as shown in Fig.1(b),indicating that the strain rate of the alloy during the steady state creep at 1 060。C was measured to be 3.1x10 s~.the time of the alloy during steady state ereep lasts for about 240 h.and the creep 1iretime of hte alloy was measured to be 311.7 h. The strain rate of the alloy during the steady state creep was measured to be 5.8×10 s as the temperature enhanced to 1 080。C.the lifetime of the alloy was measured to be 200.6 h,decreasing extent of the alloy lifetimes is about 35.4%.With elevating the temperature tO 1 1 00。C.the creep lifetime of the alloy decreases to l13 h,indicating that the alloy during creep at 137 MPa displays a better resistance in the applied temperature range. 3.2 COilStitutive equation and relative parameters Transient strain is produced when load is applied at high temperature.Dislocations multiply rapidly and fill in me matrix channels between the cubica1)r phases, which corresponds to the initia1 strain of the creep 『10-1 1].As the creep goes on,the creep rate 1s reduced while dislocations pile up in the channels to produce the strain strengthening.At the same time,the slipping and climbing of dislocations OCCur under the action of thermal activation, which releases the stress concentration in the local region『l2一l3].And then the strain rate of the alloy during creep maintains constant ofr entering the steady.state stage when the equilibrium of the strain strengthening and recovery intenerating 1s obtained,and the strain rate of the alloy during steady state creep may be expressed by Dom law as follows [14]:  ̄SS=A exp(一 ) (1) where ss is the strain rate during the steady state creep; A is the constant related to material struc ̄tre; is hte applied stress; is the apparent stress exponent;R is the mole gas constant;T is the thermodynamics temperature; Q is the apparent active energy. In the ranges of the applied temperature and stress, 1536 TIAN Su.gui.et al/Trans.Nonferrous Met.Soc.China 21(2011)1532—1 537 understood from Fig.5(a)that the orientation of the rafted Y phase is still ve ̄ica1 to the applied stress axis and displays the slight twisted configuration,and the thickness ofthe rafted y phase is about 0.6 gm.Some dislocations are activated in the rafted y phase marked 2)After creeping for a Ionget time up to fracture, the various morphologies are displayed in dift ̄rent areas 0f the sample.The cubical y phase is trans formed into the sphere.1ike configuration in the no—strain regions. And the v phase is transformed into the rafted structure along the direction vertical to the applied stress axis in the regions of the applled tensile stress.But the coarsening and twisting extents of the rafted y phase in the region near the fracture increase,which is attributed to the 1arger plastic deforelation. by black arrow,and the rafted), phase displays a slight wisted and broken conftiguration due to the plastic deformation marked by A jn Fig.5(a). After the alloy creeping for 113 h up to fracture, microstructure in the region near the fracture is shown in Fig.5(by,and hte normal ofthe film is【100】orientation. It may be understood from Fig.5(by that the orientation of the rafted v phase is still veaical to the applied stress axis,and the twisted extent of the rafted phase increases.Some dislocation traces of『0 11]and[0 1 1] orientations with the feature of upright each other are activated within the twisted rafted Y phase marked by arrow.Thereinto,the direction of dislocations slipping is about 45。angle relative to the applied stress axis,which indicates that the slipping of the dislocation is activated along the direction wiht the maximum shearing stress during creep. It is considered by analysis that the deformation mechanism of the alloy in the later stage of creep is that the super.dislocations with[0 11】and【0 1 1]trace features shear into the rafted Y phase under the action of the maximum shearing stress.As creep goes on.the main slip system of the dislocations is firstly activated within the twisted rafted), phase,and then secondary slipping dislocation is activated.The alternative operation of the main/secondary slipping dislocations results in the occurrence of the rafted Y phase twist.And the twisted extent of the rafted y phase increases with the strain quantities of the alloy during creep,which makes the rafted Y phase transform into the irregular configuration marked by letter B in Fig.5(b、.The feature of the microstructure corresponds to the larger plastic deformation in the neck region.As creep goes on.the effective stress of the alloy during creep enhances and creep resistance of the alloy decreases.The alternative operation of the main/secondary slipping dislocations promotes the significant amount of dislocations shearing into the rafted y phase up to the occurrence of creep rfacture. 4 Conclusions l、The 4.2%Re—containing single crystal nickel— based superalloy possesses a better creep resistance in the temperature ranges of 1 060—1 100。C,but displays an obvious sensibility to the applied stress.In hte ranges of the applied temperature and stress,the apparent creep active energy of the alloy was measured to be Q=483.5 kJ/mo1 3、The deformation mechanism of the alloy n the 】ater stage of creep is that some dislocations with[0 l】] and f0 1 1 1 trace features shear into the rafted phase. The main/secondary slipping dislocations are altemately activated to twist the rafted y phase up to the occurrence of creep fracture,which is thought to be the fracture mechanism of the alloy during creep. 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