Shear Band Intersections Research Articles

Effects of Strain State and Strain Rate on Deformation-Induced Transformation in 304 Stainless Steel: Part II. Microstructural Study

The γ→α transformation in 304 stainless steel was induced by plastic deformation under various conditions of strain, strain state, and strain rate, and the transformation microstructures were examined by transmission electron microscopy (TEM). The nucleation of α martensite embryos was always confined to microscopic shear band (faults, twins, and e-martensite) intersections. In cases where shear bands consisted of bundles of intermixed faults, twins, and e-martensite, α nucleated preferentially only within specific portions of the intersection volume. At sufficiently large strains α appeared to grow into polyhedral shapes. We postulate that growth occurs by repeated nucleation of new α embryos and coalescence of such embryos into polyhedral shapes. These shapes can grow either within an active slip plane or out of it, depending on how many shear band intersections are produced during deformation. Actual measurements of the number of intersections indicated that more intersections are formed in biaxial tension per unit effective strain than in uniaxial tension. This accounts for the more irregular, blocky α morphology observed in biaxial tension. At high strain rates we also found an increase in the number of intersections. However, adiabatic heating at large strains and high rates restricts repeated nucleation and coalescence and limits the amount of α transformation product.

A TEM Study of the Evolution of Strain-Induced Martensite

We have demonstrated previously the effects of strain, strain state, strain rate, and temperature on the morphology of strain-induced martensite in type 304 stainless steel. In all cases α' martensite nucleated at shear band intersections as suggested by Olson and Cohen, but the final morphologies were different. As the amount of martensite increased (with increasing strain, biaxial tension, low strain rates, and low temperatures), α' appeared more blocky and less lath-like. We have demonstrated that α' embryos grow by coalescence of new embryos, rather than actual growth.In the present study, we concentrated on the early stages of transformation to determine if α' embryos are identical for test conditions including uniaxial tension, biaxial tension, and shock loading, at room and cryogenic temperatures (as detailed in the caption of Fig. 1). Strain levels were controlled to produce only small amounts of α'. Sheet specimens, 0.18 mm thick, were used for all experiments, thinned to electron transparency, and examined in a JEOL 200B microscope operated at 200 kV.

Intersections of coarse shear bands in polystyrene

Three types of intersection between coarse shear bands are described. The first is the most numerous and happens in a single compression. The two bands shear each other. The reason why a new band can be sheared by an existing old band seems to be that it is easier for the new band to propagate along the striation direction of the old band rather than along its own direction. The merging of striations, the stretching of molecules, and the displacement of bands are described in some detail. The second type of intersection can be created by two mutually perpendicular compressions. As in the first type, the material inside the intersection is sheared twice, first by the old band and then by the new band. However, unlike the first, the two shears are in opposite directions in the second type of intersection. Both intersections cause sufficient disturbances so that microcracks and new shear bands (both coarse and fine bands) are generated to relieve some stresses. A third type of intersection which takes place also during two mutually perpendicular compressions has a small angle of intersection. The two shears of the material in the intersection are also in nearly opposite directions. However, the disturbance is so small that no microcracks or new shear bands are generated. As a result the bands offer little resistance to each other at the intersection of the third type. For all three types of intersection, the displacement of one band corresponds approximately to the shear strain of the other intersecting band. These observations suggest that molecular ordering or directional defects exist in the coarse shear bands in polystyrene.

The angles of intersection of coarse shear bands in polystyrene

The angles of intersection of coarse shear bands in polystyrene

Transformation behavior of TRIP steels

True-stress (σ), true-strain (e) and volume fraction martensite(f) were measured during both uniform and localized flow as a function of temperature on TRIP steels in both the solution-treated and warm-rolled conditions. The transformation curves(f vs e) of materials in both conditions have a sigmoidal shape at temperatures above Msσ (maximum temperature at which transformation is induced by elastic stress) but approach initially linear behavior at temperatures below Msσ where the flow is controlled by transformation plasticity. The martensite which forms spontaneously on cooling or by stress-assisted transformation below Msσ exhibits a plate morphology. Additional martensite units produced by strain-induced nucleation at shear-band intersections become important above Msσ. Comparison of σ-e andf-e curves indicate that a “rule of mixtures” relation based on the “static” strengthening effect of the transformation product describes the plastic flow behavior reasonably well above Msσ, but there is also a dynamic “transformation softening” contribution which becomes dominant below Msσ due to the operation of transformation plasticity as a deformation mechanism. Temperature sensitivity of the transformation kinetics and associated flow behavior is greatest above Msσ. Less temperature-sensitive TRIP steels could be obtained by designing alloys to operate with optimum mechanical properties below Msσ.

純チタン板のCup and Cone型破壊過程における剪断変形集中帯について

The process of “cup and cone” fracture was examined in commercial pure titanium plate pulled in uniaxial tension. Direct and continuous observation of the fracture process was possible because the fracture process of titanium plate used in this experiment was completed almost in two-dimensional state like plane strain state.The fracture process is summarized as follows: the formation of shear bands which intersect in the center of the necked portion of a specimen occurs prior to the onset of fracture. These macroscopic shear bands are composed of a number of highly localized fine bands of intense shear deformation in layers and along the direction of the macroscopic shear bands.After the formation of these intersecting macroscopic shear bands, the number and size of voids increase extraordinarily in the rhombus region where the shear bands intersected. Voids grow on the spots where the highly localized fine bands of intense shear deformation intersect and coalesce, resulting in the formation of a few small cracks. This is the onset of fracture.Connecting these small cracks, a central crack is formed. It propagates in zig-zag steps producing void sheets within the highly localized fine bands of intense shear deformation. The fracture is completed by separating void sheets having reached the edges of a specimen.The formation of intersecting macroscopic shear bands is a critical stage leading to the onset of fracture and intersecting shear bands also play a predominant role in the fracture process.Comparing the fracture process of the titanium plate with those of the copper round bar specimens in the previous papers, it is concluded that the above results may be applied to the ductile fracture of round bar specimens.

Kinetics of strain-induced martensitic nucleation

Intersections of shear bands in metastable austenites have been shown to be effective sites for strain-induced martensitic nucleation. The shear bands may be in the form of e’ (hcp) martensite, mechanical twins, or dense bundles of stacking faults. Assuming that shear-band intersection is the dominant mechanism of strain-induced nucleation, an expression for the volume fraction of martensite vs plastic strain is derived by considering the course of shear-band formation, the probability of shear-band intersections, and the probability of an intersection generating a martensitic embryo. The resulting transformation curve has a sigmoidal shape and, in general, approaches saturation below 100 pct. The saturation value and rate of approach to saturation are determined by two temperature-dependent parameters related to the fee-bee chemical driving force and austenite stacking-fault energy. Fitting the expression to available data on 304 stainless steels gives good agreement for the shape of individual transformation curves as well as the temperature dependence of the derived parameters. It is concluded that the temperature dependence of the transformation kinetics (an important problem in the development of TRIP steels) may be minimized by decreasing the fee, bec, and hep entropy differences through proper compositional control.