D0I:10.13374/j.issn1001-053x.1989.06.028 第11卷刘G期 北京科技大学学报 Vol.11 No.6 1989年11月 Journal of University of Science and Technology Beijing Nov.1989 Role of Mg on Structure and Mechanical Properties in Alloy 718' Xu Zhichao(徐志超)，Xie Xishan(谢锡善)， QuBo(屈波)，Chen Guoliang(陈国良)· Radavich John F·'· ABSTRACT:The role of Mg in alloy,718 has been systematically investigated. Mg raises not only high temperature tensile and stress-rupture ductilities but also increases considerably smooth and notch stress-rupture life.Mg containing alloy 718M is free :f stress-rupture notch sensitivity.Mg improves creep and fatigue interaction properties (LCF er cyclic stress rupture)at any grain size.The basic role of Mg is equilibrium segregation at grain boundaries which helps to change continuous grain beundary 8-Nia Nb morp'ology to discrete globular form which has a retardalion effect on intergranular fracture.Mg promotes the change from intergranular to transgranular fracture mode. KEY WORDS:superalloy,creep and fatigue,mechanical properties For the past several decades,alloy 718 continues to be used in gas turbines in greater volume and for many applications.High performance demands and high quality requirements expecially in disk application,have required material homo- genity,grain size control,and high mechanical properties (such as LCF or cyclic stress rupture)at operating conditions In the early 70's Couts et al.t1 studicd the effect of Mg (from 1-350 ppm) on mechanical properties of alloy 718 and showed stress rupture ductility impro- vement in the range of Mg content from 30 to 200 ppm,but little data was pre- sented in the lower content range (up to 100 ppm)of Mg.In 1971 Muzyka et al.t21 showed beneficial stress rupture ductility improvement at 30 ppm Mg in alloy 718.Recently,Moyert31 in his extra low carbon alloy 718 study showed a remarkable stress rupture ductility and life improvement with a small addition of Mg (13-19 ppm).However,the true effects of Mg have not been fully understood. Manuscript Reccived October 14,1989 ..Dept.of Material Science and Engineering ...School of Materials Engineering Purduc University 560
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A systematic study of Mg effect in nickel-and iron-base superalloys has been conducted in China for a long timet).Our previous studiests showed opti- mum small addition of Mg (less than 100 ppm)not only can increase stress rupture ducitlity but also prolong stress rupture life.The beneficial effect of Mg in alloy 718 can be still maintained even after 5000 h long time exposure at 650C.For further understanding the role of Mg in wrought alloy 718,especially for disk application,an investigation of Mg and grain size effects on structure and mecha- nical properties,especially on stress rupture notch sensitivity and cyclic stress ru- pture or LCF was undertaken. 1 Materials and Experimental Procedure Two 79 kg heats of alloy 718 containing 4 ppm (Mg free)and 59 ppm (Mg containing)were VIM melted.Chemical composition and alloy designation are listed in Table 1. Table 1.Alloy chemical compositions (wt%) Alloy C Mn Si P S Cr Fe Mo Al Ti Nb B Mg 7180.0570.040.230.0060.00419.1018.242.950.681.014.980,00540.0004 718M0.0520.040,230.0050.00419.0418.102.950.671.004.980.00580.0059 Alloy ingots were partially homogenized at 1150C for 6 h and then forged to produce different grain size experimental disks (d 200mmx 45mm).Typical structure of coarse,fine and mixed grains are shown in Fig.1. b 1004用 Fig.1 Typical grain structure of experimental disks with different ASTM grain sizes (after heat treatment) (A)coarse grain,(B)duplex grain,(C)fine gran Samples were cut from the rim of disks with different grains size and given the ASM559°C heat treatment,i.e.950°C/1h/AC+720°C/8h/FC50°C/1h-→620°C/ 8h/AC mechanical property samples were tensile tested at 650C,smooth and notched bar stress rupture tested at 650C/686 MPa,cyclic stress rupture tested at 650C/686 MPa with different holding times or LCF tested. 561
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Structural characterization techniques included optical,SEM and TEM micro- scopy,fractography,Auger analysis and X-ray analysis of extracted residues. 2 Experimental Results 2.1 Mechanlcal Properties In order to study systematically the grain size and Mg effects on mechanical properties,different grain size disks of Mg free (718)and Mg containing (718M) alloys were made.Different forging procedures were used to produce various amounts of recrystallized and unrecrystalized grains especially in the mixed grain disks.After the ASM 559C heat treatment,grain sizes of the experimental disks of the two alloys varied from ASTM 3 to ASTM 10.The mixed grain disks of alloy 718 and 718 M displayed a necklace structure of ASTM 7-8 fine grains surrounded by ASTM 3-4 coarse grains (see Fig.1). Results of 650C tensile tests on all grain size disks showed that Mg can grea- tly increase ductility but had little effect on ultimate strength,which is only incre- ased by grain refining (see Fig.2). Similar to that seen in tensile tests,Mg can remarkably increase the 650C stress rupture ductility as shown in Fig.3.It should be noted that Mg not only can increase smooth S/R life but also increase notch S/R life considerably even in mixed grain samples of alloy 718M.Smooth bar S/R testes of mixed grain samples from Mg free alloy 718 disk show only 109 h /4.7%elongation,but 176 h/20.2%elongation from Mg containing 718M disk.Smooth bar S/R life decre- ases where notch S/R life increases with finer grain size in Mg free alloy 718. It is clear from Fig.2 that Mg free alloy 718 will be susceptible to S/R notch sensitivity when grain size is coarser than ASTM 5.A positive advantage of alloy 718M is that Mg increases the notch S/R life remarkably;consequently,Mg contai- ning alloy 718M is not susceptible to S/R notch sensitivity even at coarse grain and mixed grain conditions.This should be of great benefit for forging of disks. High temperature LCF or cyclic stress rupture characters are the most impo- rtant mechanical properties for gas turbine disk application.A study of the grain size and Mg effects on cyclic stress rupture life with different holding times (5, 180,1800 s)at maximum stress of 686 MPa/650C.Fig.4 shows that Mg really improves cyclic stress rupture (namely stress controlled LCF with dwelling time) properties at fatigue and creep interaction conditions,representative of disk service conditions. 2.2 Microstructure Analyses Microchemical phase analysis results show that the amount of main strength- 562
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ening phase of y'and y phases is not affected by Mg addition or grain size in alloy 718 and 718M as shown in Fig.5.Mg free alloy 718 or Mg containing alloy 718M both contains approximately 14%+y"strengthening phase,independent of grain size.However,8-NisNb phase precipitated at grain boundaries increases with grain refinement and amount of Mg.Consequently,the amount of 8-NisNb is much higher in fine grain alloy 718M as compared to Mg free alloy 718. 1300 8 ●18 35 0711t 1200 人L 30 1100 500 400 1000 20 300 30 200 20 10 10 10 3 0 10 12 0 6 81012 hST】grain size ASTM grain size Fig.2 Grain size and Mg effect Fig.3 Grain size and Mg effect on on 650C tensile properties stress rupture life and elon- gation at 650C,686 MPa 718 1.2 2:5n 800 Q718M M 2.0 600 1.5 5 3.4 2 1.0 9 400 0.5 se[o/ 5.6 0 200 16 0 4 10 2 6 1012 ASTM grain size ASTM grain size Fig.4 Grain size and Mg effect on cyclic Fig.5 Grain size and Mg effect stress rupture life with different on the amount of y and holding times at maximum stress 8-NiaNb of686MPa,650°C 1,2-5s:3,4-1805;5,6-18005 Mg addition to alloy 718 increases not only 8-NisNb amount but also changes its morphology from plate-like form to globular and discrete form as shown in 563
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Fig.7(A and D).Quantitative analysis on the amount of grain boundary 8-NisNb shows that concentration coefficient of 8-NiaNb at grain boundaries (number of particles/wt%8 in certain area)increases with grain refinement because of the increment of total grain boundaries.Mg addition can raise 8-NisNb concentration coefficient at grain boundaries to a higher level in alloy 718M than that in alloy 718 (see Fig.6).Thus,much smaller and more particles of 6-NisNb phase appear in Mg containing alloy 718M with fine grain structure. 1000 718 800 718M 600 Fig.6 Grain size and Mg effect on 兰 uonenuaouoD 400 concentration coefficient of 8-NisNb at grain boundaries 200 81012 ASTM Grain size 10n Fig.7 Mg effect on grain boundary behavior of alloy 718M (a,b,c) and 718 (d,e,f) a,d-grain boundary NigNb behavior, b,c-grain boundary crack behavior, c,f-intergranular fracture bchavior at 65 C,686MPa 2.3 Fractography Observation Optical microscopy observation on longitudinal sections of stress rupture samples shows extended elongated grain structure of Mg containing alloy 718M with grain boundary cavities because of high stress rupture ductility (see Fig.7).In 564
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