| Bulk metallic glasses (BMGs) often display little obvious macroscopicplastic deformation before i nsta ntane ous fr act ure in uniaxial te nsion orcompr essi on at room temper ature, whi ch ha ve li mite d thei r applica t ions inindustrial engineer ing as key struct ural materials, thus t heir fractureme cha nis m and yi elding be havi ors beco me hot spots of curre nt a cade mi cresearch. This article is intended primarily for an in-depth analysi s of t h eyield/fr acture mechani s m as well as wide spre ad stre ng th asymmetry ofBM Gs. Through re searc hing yiel ding be havi or s of BM Gs and t heir yiel dcriteria to be followed, seeking the mechanism of plasti c defor mat ion andyield for BM Gs fr om t he ma crosc opi c mec ha ni c a l vie ws, and looking forways to improve their plastic deformation capability. The main conclusionsas follows:1. Thr ough comparative analysis of the yield criteria commonly used inBMGs fi eld, it is found that these yield criter ia c oul d all not describeeffectivel y the yiel d charact eristic of BMGs. Intermediate principal stresseffect and hydr ostat ic pressur e effect are i gnored i n Mohr-Coulomb (M-C)criterion. In t he meanti me, M-C criterion has seamed edges andsingularitiesis in the space of thr ee di mensi onal s tress (i.e. not a cont inuousfunction), therefore it is not suitable to t he numer ical calculation.Furt her more, acc or ding to M-C criteri on, the tensile fracture a ngl e and t hecompr essi on fr acture angle are complementar y angles, t his is obviouslyinconsistent with the large number of experi mental results. Ellipse criterionand Mises criteri on could not explain the strength-differential (SD) effecto f BM Gs. Alt h o u g h Dr u c k e r-Pra g e r (D-P) criteri on considered t he effect ofhydr ost ati c pre ssur e on the basis of Mises crit erion, it does not take i ntoaccount for t he yi eld purel y caused by hydr ostatic pressur e and thenonli nea r relationship bet wee n yi eld cu rves and hydr ost ati c pre ss ure. Twi nshear uni fied stre ngth (T SU S) criteri on (si mi lar t o M-C cri terion) has al sosea me d e dges and singularit ies, t here fore it is not suitable to the numericalcalcul ation. Mor eover, to the nonli near yi eld curves, TSUS crit eri on canonly linea r approxi mati on ca nnot be fully c onta i ned. 2. Zienkiewicz-Pande (Z-P) criterion is first to be introduced in BMGs field. The Z-P criterion both incorporates the effects of intermediate principal stress, pure hydrostatic pressure and strength difference (SD) effect on the yield behavior of BMGs, and takes into account of the nonlinear relationship between yield loci and hydrostatic pressure to a certain extent, and for this reason it could give a better interpretation to their yield characteristics under various loading conditions. The Yield surface of Z-P criterion in stress space is smooth and there are no seamed edges and singularities on its yield surface, overcoming the disadvantages of other yield criteria. What is more important, the Z-P criterion is a system of yield criteria, it can unify different yield criterion through transformation of different parameters. By using the values of stress triaxiality from several experiments and the atomic simulated data in plane stress state, Z-P criterion can explain effectively yield behaviors of BMGs.3. In terms of M-C criterion, D-P criterion and hyperbolic-type Z-P criterion, two types of formula for calculating the strength-differential factor are derived. Based on their tension and compression test data, the tension-compression (T-C) asymmetry of BMGs was discussed, suggesting that Z-P criterion is the most suitable to be used to explain experiment results among the above-mentioned criteria. Pressure dependence of BMGs is analyzed in detail, and it is further found that this dependence may have a major impact on the T-C asymmetry in BMGs. |
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