Deformation models for two-phase materials
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features in controlling strength and ductility of dualphase steels. It is very promising if this type of simple deformation model could be used to direct the alloying and heat treatment of the steels. Therefore, we aim at discussing some features of this theory. The theory of Mileiko 5 is based on the following assumptions: i) the true stress-true strain relationships for both phases can be expressed by o = Ke" ii) equal strain in both phases iii) the flow stress of the composite is given by the law of mixtures. Using these assumptions the true stress-true strain relationship of the composite is obtained and the true uniform strain and true UTS is then given by the Consid6re criterion (do~de = o). According to the theory these properties of the composite only depend on the uniform strain and UTS of the ferrite and martensite respectively and on the volume fraction of martensite. If the uniform strain and UTS of the ferrite and the martensite respectively are independent of the volume fraction of martensite and if the volume fraction of martensite varies from zero to one, the uniform strain and UTS of the composite will vary continuously from the value of the ferrite to those of the martensite. In Fig. 1 four curves of this type are shown. We have followed Davies 1 in assuming that the uniform elongation is 0.31 for pure ferrite and 0.08 for martensite independent of their strengths. The strength values are also taken from Davies (of. Fig. 19 in ref. 1). The conclusions to be drawn from the curves in Fig. 1 are that at the desired strength of the composite (true UTS 750 MPa): i) the uniform elongation of the composite increases with the UTS of the ferrite ii) the uniform elongation of the composite increases with the UTS of the martensite iii) a larger increase in the uniform elongation of the composite can be gained by increasing (percentagewise) the UTS of the ferrite than by increasing the UTS of the martensite The first two conclusions only express the simple fact that the best properties of the composite are obtained
Equal
strain
case
0.30
025
Deformation Models for Two-Phase Materials
Numbers
z O
are
UTS ( f e r r l t e )
/ UTS ( m a r t e n s i t e )
< ID
~0,20
IJJ
1400
PETER OSTROM 0,15
In four recent papers TM R. G. Davies uses Mileiko's theory for composites of two ductile phases 5 to understand the relative importance of various microstructural
z
280 28012660 /
~0,10
%-
PETER OSTROM, formerlywith Swedish Institute for Metals Research, Drottning Kristinas vag 48, S-11428 Stockholm, Sweden is now with AB Volvo,TechnOlogicalDevelopment, Laboratory for Metallic Materials, S-405 08 GOTEBORG, Sweden. Manuscript submitted May 27, 1980.
I
I
I
0
500
1000
/
1500
I
I
2 000
2 500
3000
TRUE UTS (IPa)
Fig. 1--Calculated curves of the uniform elongation as a function of the ultimate tensile strength for four composites. Equal strain case.
ISSN 0360-2133/81/0211-0355500.75/0 METALLURGICALTRANSACTIONSA 9 1981 AMERICAN SOCIETY FOR METALS AND THE METALLURGICALSOCIETY OF AIME
VOLUME 12A
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