Phy­si­cal­ly based mo­del­ling and si­mu­la­ti­on of the me­cha­ni­cal be­ha­viour of me­tal­lic thin film sys­tems and fine grai­ned sur­faces under cy­clic loading - FASS

Pro­ject sum­ma­ry

In this pro­ject we aim at phy­si­cal­ly based mo­del­ling and si­mu­la­ti­on sup­por­ted by quan­ti­fied cha­rac­te­ri­sa­ti­on of the me­cha­ni­cal be­ha­viour of mi­cro­samp­les (po­ly­crystal­li­ne me­tal­lic thin film sys­tems and mi­cro­pil­lars) under cy­clic loads. In­ves­ti­ga­ti­on of the phy­si­cal me­cha­nisms of fa­ti­gue in these sys­tems is mo­ti­va­ted by the fol­lo­wing con­side­ra­ti­ons: (i) Early sta­ges of fa­ti­gue failu­re in bulk sys­tems (in­cep­ti­on and stage-​I pro­pa­ga­ti­on of fa­ti­gue cracks) are go­ver­ned by near-​surface phe­no­me­na and can be stron­gly in­flu­en­ced by sur­face tre­at­ment. To un­der­stand fa­ti­gue th­res­hold and in­iti­al crack-​propagation, it is ne­ces­sa­ry to model the in­ter­play of sur­faces and in­ter­faces (grain bounda­ries, GBs) with fatigue-​induced dis­lo­ca­ti­on pat­terns and cracks. (ii) Samp­les on the mi­cro­me­ter scale are amen­able to full si­mu­la­ti­on of mi­crost­ruc­tu­ral pro­ces­ses. This al­lows us to di­rect­ly va­li­da­te mo­dels by com­pa­ring with ‘tailo­red’ ex­pe­ri­ments. At the same time, lar­ger samp­les in this scale range al­rea­dy ex­hi­bit bulk-​like be­ha­viour. (iii) Fa­ti­gue is a mul­tis­ca­le phe­no­me­non in­vol­ving pro­ces­ses from the ato­mic to the con­ti­nu­um scale but, de­spi­te its huge tech­no­lo­gi­cal im­por­tance, has ra­re­ly been ad­dres­sed from a com­pre­hen­si­ve mul­tis­ca­le mo­del­ling point of view. In si­mu­la­ti­ons, the phy­sics of fa­ti­gue still poses im­portant chal­len­ges as ma­te­ri­al be­ha­viour is go­ver­ned by slip lo­ca­li­sa­ti­on and dis­lo­ca­ti­on pat­ter­ning phe­no­me­na which can­not be pre­dic­ted by stan­dard con­ti­nu­um or ato­mistic ap­proa­ches. Present-​day dis­cre­te dis­lo­ca­ti­on dy­na­mics (DDD) for the first time pro­vi­des a phy­si­cal­ly based model of emer­gent dis­lo­ca­ti­on pat­ter­ning, but can­not ac­cess the large cu­mu­la­ti­ve strains as­so­cia­ted with failu­re under cy­clic loads. We over­co­me this li­mi­ta­ti­on by ex­ploi­ting re­cent­ly de­ve­lo­ped coarse-​graining me­thods that map DDD onto con­ti­nu­um dis­lo­ca­ti­on dy­na­mics (CDD) si­mu­la­ti­ons which re­p­re­sent the same dy­na­mics in a con­ti­nu­um frame­work. These can be ca­li­bra­ted and va­li­da­ted by re­fe­rence to the DDD mo­dels but ex­tend to lar­ger spa­ti­al and tem­po­ral sca­les. Plasti­ci­ty si­mu­la­ti­ons are com­bi­ned with ato­mistic si­mu­la­ti­ons that give ac­cess to dis­lo­ca­ti­on nu­clea­ti­on at sur­faces and in­ter­faces and crack nu­clea­ti­on at sur­face he­te­ro­gen­ei­ties or stress con­cen­tra­ti­ons. The ul­ti­ma­te goal of the pro­ject is to pro­vi­de phy­si­cal founda­ti­ons for compu-​tational de­sign of fa­ti­gue re­sistant near-​surface mi­crost­ruc­tu­res. To this end we de­ve­lop a pre­dic­ti­ve mul­tis­ca­le frame­work for the evo­lu­ti­on of dis­lo­ca­ti­on sys­tems in­ter­ac­ting under cy­clic loads with sur­faces, cracks and GBs, and eva­lua­te da­ma­ge and failu­re pro­per­ties. Si­mu­la­ti­ons will be pa­ra­me­teri­zed and va­li­da­ted by re­fe­rence to ex­pe­ri­ments car­ried out on model sys­tems, using tech­ni­ques which pro­vi­de in­for­ma­ti­on on mi­crost­ruc­tu­ral pro­ces­ses on sca­les from na­no­me­ters up to sever­al mi­cro­me­ters. In­ves­ti­ga­ted sys­tems in­clu­de po­ly­crystal­li­ne thin films and sin­gle/bic­rys­tal mi­cro­pil­lars of va­ry­ing size and grain ori­en­ta­ti­on. Cy­clic plasti­ci­ty of these sys­tems will be stu­di­ed using ten­si­le and com­pres­si­on tes­ting. Per­for­ming selec­ted ex­pe­ri­ments in situ in a scan­ning elec­tron mi­cro­scope will fa­ci­li­ta­te di­rect ob­ser­va­ti­on of de­for­ma­ti­on and failu­re pat­terns. On even smal­ler sca­les, in-​situ trans­mis­si­on elec­tron mi­cro­scopy will di­rect­ly vi­sua­li­se dis­lo­ca­ti­on mi­crost­ruc­tu­re evo­lu­ti­on under load, dis­lo­ca­ti­on nu­clea­ti­on and in­ter­ac­tions at the in­ter­faces. The pro­po­sed ef­fort will re­sult in a si­mu­la­ti­on frame­work for mo­del­ling the phy­si­cal pro­ces­ses be­hind a vast range of tech­no­lo­gi­cal pro­blems in­clu­ding the en­han­ce­ment of fa­ti­gue re­sis­tance by sur­face tre­at­ment, and fa­ti­gue of mi­crosca­le com­po­n­ents. While work on any of these spe­ci­fic pro­blems is bey­ond the scope of the pro­ject, we aim at es­tab­li­shing and ex­pan­ding our con­tacts with po­ten­ti­al ‘users’ at an early stage in order to fa­ci­li­ta­te fu­ture ap­p­li­ca­ti­on and dis­se­mi­na­ti­on of our re­se­arch fin­dings.