Siegfried S. Hecker

Effects of Strain State and Strain Rate on Deformation-Induced Transformation in 304 Stainless Steel: Part I. Magnetic Measurements and Mechanical Behavior

The γ→α transformation in 304 stainless steel can be induced by plastic deformation at room temperature. The kinetics of strain-induced transformations have been modeled recently by Olson and Cohen. We used magnetic techniques to monitor the progress of the γ→α transformation in 304 stainless steel sheet loaded in uniaxial and biaxial tension at both low (10-3 per second) and high (103 per second) strain rates. We found that using the von Mises effective strain criterion gives a reasonable correlation of transformation kinetics under general strain states. The principal effect of increased strain rate was observed at strains greater than 0.25. The temperature increase resulting from adiabatic heating was sufficient to suppress the γ→α transformation substantially at high rates. The consequences of the γ→α transformation on mechanical behavior were noted in uniaxial and biaxial tension. Uniaxial tension tests were conducted at temperatures ranging from 50 to -80°C. We found that both the strain hardening and transformation rates increased with decreasing temperature. However, the martensite transformation saturates at ≈85 pct volume fraction α. This can occur at strains less than 0.3 for conditions where the transformation is rapid. Once saturation occurs, the work hardening rate decreases rapidly and premature local plastic instability results. In biaxial tension, the same tendency toward plastic instability associated with high transformation rates provides a rationale for the low biaxial ductility of 304 stainless steel.

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 ε-martensite) intersections. In cases where shear bands consisted of bundles of intermixed faults, twins, and ε-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.

Stretching limits in sheet metals: In-plane versus out-of-plane deformation

Limiting principal strains were measured by two different techniques of biaxial stretching of sheets, one of which permits free deformation in a flat plane, while the other causes constrained deformation in contact with a rigid or rubber punch. The latter method, which relates well with industrial experience, produces larger limiting strains under identical degrees of biaxiality. A possible explanation is based on the process of instability and strain localization.

Nucleation and evolution of strain-induced martensitic (b.c.c.) embryos and substructure in stainless steel: A transmission electron microscope study

Abstract The deformation of type 304 stainless steel produces a preponderance of strain-induced α′ (b.c.c.) martensite, which nucleates as stable embryos at micro-shear band or twin-fault intersections as proposed by Olson and Cohen. The two intersecting micro-shear bands must have a specific defect (fault-displacement) structure, and for stable martensite embryos to form requires a minimal micro-shear band thickness ranging from 50–70 A. The critical nature of nucleation is influenced by the local temperature and strain. The structure, geometry, and morphology of strain-induced martensite embryos is essentially invariant regardless of the strain rate, strain state or temperature. Larger volume fractions of martensite evolve at large strains (⩾20%) as a result of embryo coalescence to produce a blocky-type morphology. Martensite embryos and coalesced volume elements of α′ are frequently characterized by an irregular, non-homogeneous distribution of smaller b.c.c. regimes which result from the irregular satisfaction of the necessary and specific fault-displacement requirements within a larger intersection volume.

Brittle Fracture in Iridium

Brittle fracture in fcc metals is uncommon. It is not common knowledge that single crystals of iridium, a high melting point fcc metal, fail by brittle cleavage at room temperature. Furthermore, polycrystalline iridium fails predominantly by brittle inter granular fracture at temperatures below 1000°C. With the aid of several models of brittle fracture we have demonstrated that cleavage in iridium is intrinsic, resulting from apparently very strong and directed atomic binding forces. Intergranular fracture in iridium has been generally assumed to arise from the segregation of harmful impurities to the grain boundaries. We were able to demonstrate using Auger electron spectroscopy that impurity segregation to grain boundaries in iridium was not necessary for grain boundary fracture to occur, thereby demonstrating that intergranular brittle fracture in polycrystalline iridium is also intrinsic and not impurity related.

Failure in thin sheets stretched over rigid punches

Criteria for diffuse and local necking in sheet metals stretched over a hemispherical punch have been developed. During stretching, a peak develops in the distribution of strain across the dome. This peak is a ring of thinned material around the pole which moves outward radially with continued deformation. The present approach essentially focuses on the loading rate of the region bounded by this ring of maximum thinning referred to here as thecrown. While diffuse instability is identified with a vanishing rate of crown loading, local necking is associated with the onset of a rapid rate of thinning under falling crown load. The principal surface strains were measured from incremental stretching of several different materials and time dependent constitutive relations were used to predict the instability strains. Such predictions agree with experimentally observed forming limits defined by the onset of visible necking and provide an understanding of the failure of biaxially stretched sheets deformed by a rigid punch. Since no satisfactory plasticity analyses have been developed to predict the strain distribution during punch stretching, the empirical input to the present instability criterion is required.

Yield surfaces in prestrained aluminum and copper

The analysis of strain-hardening materials subjected to multiaxial states of stress requires more detailed experimental information about the effects of previous plastic deformation on the yield surfaces of real materials than is presently available. To provide insight into some of these effects thin walled tubular specimens of 1100-0 aluminum and annealed OFHC copper were subjected to biaxial stresses through the application of simultaneous axial tension and internal pressure, and the effects of the magnitude, direction, and sequence of prestraining operations on subsequent yield surfaces were determined. It was found that the yield surface behavior depends greatly upon the definition of yielding employed. Use of small proof strain definitions resulted in very anisotropic yield surface characteristics which reflected the effect of previous deformation. On the other hand, use of large proof strains resulted in isotropic yield surface characteristics which were devoid of previous deformation influence. The small proof strain yield curves were found, in general, to expand and translate in the direction of prestrain and, for biaxial prestrains, to be distorted in the vicinity of the loading point. Multiple prestrain sequences in normal directions induce a large negative cross effect similar to Bauschinger effect observed under reversed loading. Such anisotropic behavior was found to contradict the two most commonly used continuum mechanics predictions, the isotropic and kinematic hardening rules.

Quantitative evidence for dislocation emission from grain boundaries

The study of strain and strain rate on the residual microsture of deformed 304 stainless steel, it was found that small strains grain boundaries and twin boundaries serve principally as sources of dislocations which are generated in response to the requirement of strain compatibility. Although the features of emitted arrays may not be distinguishable from the features of pile-ups, the comparison of dislocation densities near these boundaries to the grain interior and the lack of evidence of interior sources favors the concept of boundary emission. Dislocation pile-ups may still play an importatnt role in work hardening of poly crystals, especially at large strains. (FS)

Large strain plastic deformation of commercially pure nickel

AbstractCommercially pure nickel was deformed to very large strains by cold rolling. Tensile tests on prestrained sheet were used to establish strain hardening behaviour and residual ductility. The hardening of nickel was found to be peculiar: initial rapid hardening was followed by an apparent plateau, which, in turn, was followed by another stage of rapid hardening at strains larger than 4. The residual elongation dropped precipitously with initial prestrain, levelled off, and then increased again during the final stage of hardening. Nickel rolled to a strain of 6·1 exhibited a remarkable tensile strength of 1400 MN m−2 with 4% total elongation. Extensive in-plane and limited edge-on transmission electron microscopy was used to examine the evolution of substructure. Dislocation cells formed at strains <0·10 and continued to refine in size with increasing strain. Definite evidence of dynamic recovery was found. The formation of elongated, well defined subgrains appears to coincide with a saturation of ha...

Influence of deformation history on the yield locus and stress-strain behavior of aluminum and copper

Thin-walled tubular specimens of 1100-0 aluminum and OFHC copper were loaded biaxially through the application of simultaneous axial load and internal pressure. The effects of loading path and deformation history on the stress-strain and yield locus characteristics were studied at strains less than 2.0 pct. The observed plastic strains for both materials depended on the loading path to a given stress point, whereas the loading path during prestraining did not affect subsequent deformation. Deformation subsequent to prestraining depended on the prestraining magnitude and direction at strains less than 0.2 pct and only on the magnitude at larger strains. The resulting plastic response was, therefore, anisotropic at small strains and isotropic at large strains. The small strain behavior cannot be predicted by present continuum plasticity theories, whereas the large strain behavior agrees with the isotropic hardening rule. It was also found that a prestraining operation of sufficient plastic strain can erase some of the prior deformation history.