_STP_637-1977.pdf

CYCLIC STRESS-STRAIN AND PLASTIC DEFORMATION ASPECTS OF FATIGUE CRACK GROWTH A symposium sponsored by ASTM Committee E-9 on Fatigue AMERICAN SOCIETY FOR TESTING AND MATERIALS St.Louis,Mo.,2-8 May 1976 ASTM SPECIAL TECHNICAL PUBLICATION 637 L.F.Impellizzeri,symposium chairman*b 1 1 1 1 10 10 lO lO*1 10 REVERSALS 10=lO 10 10=FIG.3Cyclic hardening curves for AI-4 Cu alloy aged at 250 C for 5 h so as to produce a uniform dispersion of large 8 plates.Cold-working reduces the 6 plate spacing and raises the flow stress.control,the microstructure contained Guinier-Preston I(GPI)zones only.In another,containing 6.3Cu,undissolved,equiaxed 6 particles 5 to 10 fim in diameter were distributed in a matrix,containing GPI zones.As shown in Fig.4,the aluminum alloy containing GPI zones only first hardened and subsequently softened just like the alloy studied by Calabrese and Laird 12.The Al-6.3Cu alloy hardened and subsequently softened only at large and intermediate strains.However,at low strains,softening was not ob-served.The interpretation is that,at high strains,the GPI zones were cut sufficiently to disorder the structure and the alloy softened.At low strains,the large 6 particles homogenized the strain and thus prevented the locali-zation of the strain required to soften the structure in the active bands.Also shown in Fig.4 are cyclic hardening curves for 2024-T4.Consistent with the work of Endo and Morrow 26,no softening was observed 16.Com-mercial alloys contain more inclusions and dispersed phases than binary A1-6.3Cu alloy aiid can be expected,therefore,to be more effective in multi-plying dislocations and in homogenizing the strain.The role of these parti-cles in cyclic deformation is given extended treatment in the following section.It is possible that,at really low strains,where lives are greater than lO*cycles,strain locali-zation may occur on a scale smaller than that of the inter-spacing,in which case,softening might very well occur.Copyright by ASTM Intl(all rights reserved);Mon Dec 21 11:24:29 EST 2015Downloaded/printed byUniversity of Washington(University of Washington)pursuant to License Agreement.No further reproductions authorized.LAIRD ON ALUMINUM ALLOYS 9 Behavior of Complex Alloys Experimental Details Since aluminum-zinc-magnesium alloys have not been studied as exten-sively as commercial alloys more closely related to binary aluminum-copper,aluminum-zinc-magnesium has been chosen for study here.Another reason for the choice relates to the rather poor crack propagation behavior shown by these alloys as compared to that of 2024 and related alloys i.The vari-ables selected in this study of CSSR are:(a)types of dispersed phases,b)initial dislocation content of the material in relation to the hardening parti-cles and(c)the nature of the loading(Fig.1).A discussion of the nature of the dispersed phases which commonly occur in commercial aluminum alloys is necessary in order to clarify the specific choices of material.The literature FIG.4Cyclic response curves for the aluminum alloys indicated,during plastic-strain-controlled cycling.The peak stress refers to the stress amplitude at the tensile reversal.Courtesy of Fine and Santner 16.Copyright by ASTM Intl(all rights reserved);Mon Dec 21 11:24:29 EST 2015Downloaded/printed byUniversity of Washington(University of Washington)pursuant to License Agreement.No further reproductions authorized.10 FATIGUE CRACK GROWTH which deals with these phases is extensive,but a good reference which ex-plains the role of the particles in fracture can be found in the review article by Kaufman 27.The largest particles in aluminum alloys are termed the constituent par-ticles.They are generally greater than 1/xm in diameter and form by eutectic decomposition during ingot solidification.Since they consist of insoluble particles such as Al7Cu2Fe,Mg2Si,or(Fe,Mn)Al,they cannot be taken into solid solution during fabrication.Sometimes,the relatively soluble CuAl2 or CuAl2Mg also occur as constituent particles.A second grouping of particles(in the 0.03 to 0.5/um range),called dispersoids,consist of Ali2Mg2Cr or Al2oCu2Mn formed by solid-state precipitation,and is also difficult to dissolve.During fabrication,these particles suppress recrystalli-zation and hmit the growth of grains.The third and finest set of particles consists of the age-hardening precipitates of major alloying elements,GP zones,which impede dislocation motion,and lead to the optimum combina-tions of strength and toughness.It is apparent from the chemistry of the particles that the best way of eliminating the constituent particles(which fracture easily in unidirec-tional deformation and also in fatigue 27 is to reduce the iron and silicon content of the alloy.The control of the dispersoids is more difficult;one means of reducing their volume fraction is by eliminating chromium,but this makes certain stages of the processing,particularly grain control,difficuh.Accordingly,the alloys*listed in Table 1 along with th