Deformation and Fracture Behavior of Rapidly Solid

Authors: JE Wittig G Frommeyer

Publish Date: 2008/01/03

Volume: 39, Issue: 2, Pages: 252-265

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Abstract

In this study the mechanical properties deformation behavior and fracture modes of ironsilicon meltspun ribbons are related to changes in silicon composition from 45 to 65 wt pct and the influence of ordering phase transformations The assolidified meltspun ribbons which exhibit plasticity even for the Fe65 wt pct Si composition provide the opportunity to characterize dislocation glide Tensile deformation with a plastic strain of ∼2 pct produced planar slip of pure edge and pure screw mainly on 112 slip planes although slip on 011 and 123 planes was also observed The extended dislocations became qualitatively more planar as silicon concentration increased owing to reduced crossslip For the Fe65 wt pct Si material pairs of dislocations with the same Burgers vector were observed As ribbon thickness increased the material’s ductility decreased Thinner ribbons provide a reduced mean free path of gliding dislocations and fewer impediments to glide before they reach the ribbon surface which removes strain hardening effects In the assolidified state the B2 and DO3 order has been suppressed Heat treating the 65 wt pct silicon ribbons induces the ordering phase transformations and reduces the ductility The most embrittled condition occurs for the coexistence of B2 and D03 ordered domains after annealing at 400 °C to 500 °CAlloys of iron and silicon are technologically important owing to their combination of superior soft magnetic properties and relative low cost1 Most common grades of electrical steel contain about 3 wt pct Si and are available in both nonoriented and oriented states In the oriented condition a series of thermomechanical processes are required to produce the preferred crystallographic cube on edge texture with the easiest direction of magnetization leftlangle text001 rightrangle appropriately aligned for the application The worldwide steel industry produces over 1 million tons of grain oriented “Goss textured” ironsilicon sheet each year Improving the production methods is an ongoing process2 The ideal soft magnetic material must have high magnetic permeability and induction while minimizing core losses from eddy currents and hysteresis during cyclic magnetization Raising the silicon content in iron causes a decrease in the saturation induction but this is of minor consequence compared to the beneficial effects of higher permeability lower magnetostriction and reduced core losses from a combination of lower intrinsic coercivity and increased resistivity A composition of approximately 65 wt pct silicon is reported to achieve the optimum soft magnetic properties with a maximum in permeability a minimum in coercivity and a zero leftlangle text001 rightrangle magnetostriction coefficient for noise free power transmission1 Unfortunately accompanying these improved soft magnetic properties is a drastic decrease in ductility at about 4 to 5 wt pct Si which prohibits conventional processing specifically cold rolling of high siliconcontaining alloys3 Concomitant with the change in the room temperature fracture behavior with increasing silicon content are B2 and D03 superlattice phase transformations occurring at silicon composition higher than 5 wt pct 95 at pct4 The influence of atomic order on the electrical and magnetic properties is still an active area of research56Because slip of a second dislocation on the same plane would reorder the B2 superlattice the APB interfacial energy would be removed Thus a pair of dislocations a superlattice dislocation having a net Burgers vector of a leftlangle text111 rightrangle is predicted to move through the lattice at a lower stress compared to single dislocations12 Although superlattice dislocations would be energetically favorable this complicated arrangement of dislocations would be expected to severely limit dislocation crossslip and inhibit accommodation of plastic deformationMarcinkowski and Brown have described the crystallographic configuration of superlattice dislocations in the D03 superlattice13 Ordinary dislocations in the expanded D03 unit cell a o ′ = 2 a o have Burgers vectors a′/4 leftlangle text111 rightrangle For the D03 superlattice of type A3B where B atoms have both first and second unlike nearest neighbors the passage of four ordinary dislocations with b = a′/4 leftlangle text111 rightrangle is required to completely reorder the lattice Slip of an ordinary dislocation on a 110 plane produces an APB by changing both first and second nearest neighbors Although a second ordinary dislocation reorders first nearest neighbors with a reduction in APB energy second nearest neighbors remain disordered A third ordinary dislocation again disorders nearest neighbors raising the APB energy and finally a fourth dislocation transforms the lattice back to the initial state Because the APB energy between dislocations 2 and 3 is relatively low compared to the APB energy between dislocations 1–2 and 3–4 the two dislocation pairs would not be strongly connectedSuperlattice dislocations were first experimentally observed with TEM by Marcinkowski et al in the Cu3Au L12 superlattice based on the facecenteredcubic fcc structure1415 Because perfect dislocations in an fcc structure with low stacking fault energy tend to split into partials the superlattice dislocation configuration for the Cu3Au structure consists of two pairs of partial dislocations held together by an antiphase domain boundary However in B2 and D03 structures superlattice dislocations are not always observed Experimental evidence from stoichiometric Fe3Al alloys which exhibit both B2 and D03 order suggests that dislocations move at room temperature as ordinary a/2 leftlangle text111 rightrangle types13 Superlattice dislocations have only been detected in deformed D03ordered Fe3Al at specific elevated temperatures16 or in nonstoichiometric Fe3Al Fe31 at pct Al17 The lack of superlattice dislocations in Fe3Al has been accounted for by the relatively low APB energy present in the ordered Fe3Al lattice13 However a theoretical analysis predicts that the APD energy in Fe3Si is at least twice as large as for the Fe3Al superlattice which would increase the probability for the existence of superlattice dislocations in the D03 superlattice of Fe3Si13

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