the metallography of impact fatigue

8/13/2019 The Metallography of Impact Fatigue

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Purdue University

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International Compressor Engineering Conference School of Mechanical Engineering

1978

e Metallography of Impact FatigueT. O. Smith

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Smith, T. O., "e Metallography of Impact Fatigue" (1978).International Compressor Engineering Conference. Paper 253.hp://docs.lib.purdue.edu/icec/253
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can be seen, The subgra in boundaries are usuallya ttached to these par t ic le s . The s tructure in th evast majority of the im pac te d samples was thesame as in the unstrain ed reg ions. However, in

Figv .re 1 a)An eleatronmiarog ra ph sh owing the subgra in st rua-ture of the uns trained region of the alumini umS 117ple

Fig ure 1 b)An eleatronmic ro graph showing a region o increas-ed dis loaation densi ty in the viainity ofa sea ondphase partiale in the im pacte dregion of thealuminium s0117plesome localized regio ns some effe c ts of impactfatigue could be see n as shown in Fig ure l b ) . This shows a g rea t ly increased density of ta ngledd is-locations in the v ic in ity of a la rge se cond phasepa r t ic le . Acomparison of th i s f igure withFigure1 a),; reveals a s ign if ican tly increased d is -loca tiondens ity in th e impa ct ed materia l in loca l-ised reg io ns. This was espec ia lly evident in theregions ne ar in te rm e ta ll ic par t ic le s in the sample sex amined. Thus, themeta ll ogra phy of th e alumin iumimpact fa tigue samples ind icates local ized def orm-a tion in the region of so me la rge in termetall ic par t ic le s . The deformation i s very inhomQgeneouslydis tr ibu ted and the grea tbulkof thematerialappears to have undergone l i t t l e or no defo rm ati on.

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S NDVIK5R6

Figure 2a) shows th e microstructureof theun-stra ined regions of the samples. The s truc ture i s typ ical of a well annealed aus ten it ic s tee lwith alowdisloca t ion dens itybein gpresen t. Most of theareas of sampleswhich had been subjecte d to impactfatigue had a sim ilrstructure to th is withnos igns ofdeformatio n. However, in someareas ofsom e sample s th e very locl izeddamage depicted in Figure 2b) and c) was found . Nopos it ive connec-t ionbetween these bands and any element of th em icrostruc tu re could be established . The band s arere la ted to the crystall ographyof the grains andfollow 111) planes, and so change direc tionwhencrossin g a grainboundary . Figure 2 c) shows apossible associat ionof bands with a grain bounda-ry pa r t ic le . Within the in tense defo rm ati on bandsthe disloca tion struc tu res are complex and so merecovery appears to have occurred resu lt ing in theformation of ne tworks as shown in Figure 2a).

Fig ure 2 a)An el ec tronmicr ogra ph revealing the microstructureof the unstrained region ofa SANDVIK 5R60 sample .

F ig ure 2 b)An electron micr ograph sh owing an in tense defo rm-ation band in the im pacted region ofa SANDVIK 5R60sample .


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Fig ure 2a)neleatronmiarog ra ph show ing the poss ib le assoaia-ti onofa de formation band witha la rge grainboundar y partiale inS NDVIK5R60.The impact fa t igue o f SR6 appears to r e su l t in very in tense lo ca liz ed deforma ti on . o po s i t iv e evidence o f the a s so c ia t ion o f these in tense bandso f deformation w ith any element o f the micro-s t ruc tu re was ob ta ined .N itin o l N it ino l was chosen fo r the inves tiga tion because ofi t s ex tremely h igh damping capaci ty . Dampingcapaci ty had been id e n t i f ie d in th e inves tiga tio n of a range of valve s te e l s as an importan tproper ty determin in g impact fa t igue re s is tan ce 1). twasproposed th a t mater ia ls of high damping capac ity su ffe red le s s ob lique impacts due to th e damping o f the to rs io na l and f lexu ra l v ib ra tions an d sosu ffered lower s t re s s lev e ls uringimpact. Un-fo r tuna te ly the d if f ic u l ty in es tab lish ing abso-lu te values o f the impact in ten s i ty prec luded an yassessment o f the r e la t iv e re s is tan ce o f N it in o lto impact fa t igue .

Figure J a}bright fieldeleat ronmia ro graph showin g theasso-iationof a defo rmation band with a seaond-p haseart ia le in impaatfa tiguednitinol.

Figure 3 b)Thecorrespond ingdark fieldelect ronmiaro gra phshowing the presenae ofmartensite inthe deformationband in impactfatiguednitinol.

Figure J a)nelec tronmig ro graph showing dislocation arrays associatedwith la rge seo ond-p hase particles_inimpaat fatiguedniti noZ.Figure 3a) and b) a re a b r ig h t f ie ld -da rk f ie ld p a i r o f mic ro graphswith the dark f ie ld imagefo rm ed from a martens ite re f le c t io n showing a lo c a l ized deformation bandwhi ch appears to have in t e r acted with or o r ig ina ted a t a second phase pa r t i c le . F igure 3c) shows another example of lo ca lized deformation in the fo rm of d is loca tio n arrays whi chare ty p ica l o f au s ten ite whi ch has reve r ted fr ommartensi te 2). Again as in the o th e r m a te r ia ls ex am ined, the deformation was extremely loca l ized and the major ity o f the sample appeared to be un-affec ted by the impact fa t igue .SANDVIK l2R l0In th is m ate r ia l most a reas o f th e fo i l s examin edshowed no evidence of deform ati on and the impactfa tig u e damage was confin ed to narrow and i so la te d bands. Figure 4 a) i s a lowmagn if ica tion mic ro-graph of a deformation band in impacted 12Rl0 . Thec rys ta l lo g raph ic natu re of the band can be seen in

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the d ire c t iona l changeswhen tcrosses a g ra in boundary. Figures 4 b) an d c) show in tense deformation bands in which th e deformation has t r a n s fo rmed some of the au s ten i te to martens ite .

Figure 4a)An electronmic rograph showing a deformati on bandcrossing several gra in s in im pacted SANDVIK 12R10

Figure 4 b)An electronmicrograph sh owing a deformation bandin impactfa tigued SANDVIK 12R10o convincing evidence o f a c le a r assoc ia t ion of

carb ide p a r t ic le s with th e in i t i a t io n of deform-a t ion bands could be es tab lished as th e .o r ig in ofthe bands was never in th e viewable area o f thef o i l

SANDVIK 2 C

Figu re s 5 a) , b) an d c) aremicrographs o f th e s t ru c tu re of impact-fatigued 2 Cvalve s te e l . Themic ro s truc tu re o f t h is mate ria l con s is ts oftemperedmarten s ite and r e la t iv e ly la rge carb idep a r t ic le s and i s very comple x. This complexitymakes td i f f i c u l t to be sure of th e e f fe c ts ofimpact fa t igue . Howeve r, Figure S b) and c) show a higher than average d is lo ca tion dens ity in the v ic in i ty o f carb ide p a r t ic le s . As such, themicrographs are no t un lik e those obta ined fo r

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aluminium.

Figu:re 4c)An electronmicrograph showing a deformation bandin impact-fatiguedSANDVIK 12R10.

Figure 5a)An electronmicrograph shahling the typical structure ofSANDVIK 20C valve steel.

Figure Sb)An electronmicrograph showing the microstructureofimpact-fatiguedSANDVIK 20C


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the metallography of impact fatigue

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