Anthony W. Thompson

The composition dependence of stacking fault energy in austenitic stainless steels

The stacking fault energy (SFE) of AISI 310 stainless steel was measured using the weak-beam, dark field technique on extended nodes. The SFE was found to be 40 ± 5 mJ/m2, well below the 104 mJ/m2 predicted for this composition in earlier work by Schramm and Read. Difficulties with the Schramm and Read linear composition approach to SFE were identified, and a representation of SFE using the Fe-Cr-Ni ternary diagram was suggested to provide a more realistic, flexible and accurate picture of the composition dependence.

Influence of deformation substructure on flow and fracture of fully pearlitic steel

Abstract Tensile stress-strain data over the whole strain range were obtained for a range of pearlites from very coarse to relatively fine (interlamellar spacings 0.53 and 0.13 μm, respectively). Transmission electron microscopy (TEM) for pearlite subjected to various amounts of strain was performed. Coupling mechanical data with TEM examination provided a detailed picture of how pearlite yields, deforms, work hardens, and fails under uniaxial tension. It is shown that yielding and work hardening of pearlite are largely controlled by processes occurring in ferrite. The role of a cementite plate at low stresses is mainly to limit the slip distance in ferrite. It is found that the tensile fracture is determined by processes in the colonies with lamellae parallel to the tensile axis and that the stress necessary to break a cementite plate corresponds to the true U.T.S. The influence of interlamellar spacing on the yield strength, flow stress, and the true U.T.S. is quantitatively explained.

Roughness-Induced Crack Closure: An Explanation for Microstructurally Sensitive Fatigue Crack Growth

The concept of roughness-induced crack closure is utilized to explain the role of prior austenite grain size and pearlite interlamellar spacing on near-threshold fatigue crack propagation in fully pearlitic eutectoid steel tested at low and high stress ratio in lab air and purified helium. It is shown that at low load ratios, near-threshold growth rates are significantly reduced for coarse-grained microstructures, compared to fine-grained at constant yield strength, due to roughness-induced crack closure. Using roughness-profile microscopy, it was found that fracture surface roughness near threshold scaled with grain size and inversely with yield strength, macroscopic roughnesses at threshold being considerably larger than the conventionally calculated cyclic crack tip opening displacement. Auger analysis of near-threshold corrosion products showed it to be iron oxide; the oxide thickness was seen to be decreased by increased stress ratio. The significance of this model to near-threshold fatigue crack growth behavior, in terms of load ratio, microstructure, and environment is discussed.

Substructure strengthening mechanisms

The mechanisms which give rise to substructural strength originate in the dislocation character of the substructure boundary, in the size of sources for continued slip which the boundaries permit, and in the size of the cells or subgrains. The interrelation of these factors must be considered in obtaining a general view of the strengthening.

On macroscopic and microscopic analyses for crack initiation and crack growth toughness in ductile alloys

Relationships between crack initiation and crack growth toughness are reviewed by examining the crack tip fields and microscopic (local) and macroscopic (continuum) fracture criteria for the onset and continued quasi-static extension of cracks in ductile materials. By comparison of the micromechanisms of crack initiationvia transgranular cleavage and crack initiation and subsequent growthvia microvoid coalescence, expressions are shown for the fracture toughness of materials in terms of microstructural parameters, including those deduced from fractographic measurements. In particular the distinction between the deformation fields directly ahead of stationary and nonstationary cracks are explored and used to explain why microstructure may have a more significant role in influencing the toughness of slowly growing, as opposed to initiating, cracks. Utilizing the exact asymptotic crack tip deformation fields recently presented by Rice and his co-workers for the nonstationary plane strain Mode I crack and evoking various microscopic failure criteria for such stable crack growth, a relationship between the tearing modulusTR and the nondimensionalized crack initiation fracture toughnessJIc is described and shown to yield a good fit to experimental toughness data for a wide range of steels.

Evidence for Dislocation Transport of Hydrogen in Aluminum

The use of concurrent plastic straining during cathodic charging of equiaxed-grain, high purity 7075 aluminum has provided evidence that dislocations can transport large amounts of hydrogen deep into the interior of the alloy; as a direct consequence of this, highly brittle intergranular fracture ensues. This effect is most pronounced for heat treatments that produce a microstructure which allows for planar dislocation arrays and long slip lengths. The implications of these findings to the occurrence of hydrogen embrittlement in other alloy systems have been assessed.

Microstructural effects on the cleavage fracture stress of fully pearlitic eutectoid steel

The microstructural parameter(s) controlling the critical cleavage fracture stress, σF, of fully pearlitic eutectoid steel have been investigated. Independent variation of the pearlite interlamellar spacing,Sp, and the prior austenite grain size were accomplished through heat treatment. Critical cleavage fracture stresses were measured on bluntly-notched bend specimens tested over the temperature range -125 °C to 23 °C. The cleavage fracture stress increased with decreasingSp, and was independent of prior austenite grain size. Fine pearlitic microstructures exhibited temperature, strain-rate, and notched-bar geometry independent values for σF, consistent with propagation-controlled cleavage fracture. Coarse pearlitic specimens exhibited temperature-dependent values for σF over a similar temperature range. Inclusion-initiated fractures were generally located at or beyond the location of the peak normal stress in the bend bar, while cracking associated with pearlite colonies was observed to be closer to the notch than the predicted peak stress location. The calculated values for σF were independent of both the type and location of initiation site(e. g., inclusion, pearlite colony). Thus, although inclusions may provide potent fracture initiation sites, their presence or absence does not necessarily change σF in fully pearlitic microstructures.

The compositional dependence of antiphase-boundary energies and the mechanism of anomalous flow in Ni3 Al alloys

Abstract The theory of the anomalous flow behaviour of Ll2 compounds has developed over the last 30 years. This theory has a foundation in the early estimates of the crystallographic anisotropy of antiphase-boundary (APB) energy in these compounds. In spite of this critical aspect of the theory, it is only in the last 5 years that electron microscopy has been employed to quantify the APB energies and to determine the detailed nature of dislocation structures at each stage of deformation. The present study examined binary Ni3Al single crystals having compositions which span the single-phase region, and selected ternary compositions expected to have a dominant influence on either {001} or {111} plane APB energies. Crystals were deformed in compression at an orientation near the [001] direction over the temperature range from — 196 to 700°C. Weak-beam dark-field microscopy was employed to quantify the APB energies and the APB energy anisotropy over the composition range. Experimental uncertainties associated...

Micromechanisms of brittle fracture

Mechanical processes operating in materials on the scale of the microstructure have come to be called “micromechanisms.” The fundamental science and the micromechanisms of brittle fracture are reviewed here, with particular emphasis on cleavage and intergranular fracture. Extant micromechanisms for these fracture types are evaluated. The role of solutes, particularly in intergranular fracture, is also discussed in terms of the fundamentals of brittle fracture.

The role of microstructure in hydrogen-assisted fracture of 7075 aluminum

Underaged, peak strength (T6), and overaged (T73) microstructures were studied in 7075 plate material. Hydrogen charged and uncharged tensile specimens of longitudinal orientation were tested between −196°C and room temperature. The results confirm a hydrogen embrittlement effect, manifested mainly in the temperature dependence of the reduction of area loss; a classical behavior of hydrogen embrittlement. The maximum embrittlement shifted to lower temperatures with further aging. The effect of hydrogen was largest for the underaged condition and smallest for the overaged, thus following the pattern found for the sensitivity to stress-corrosion cracking in high strength aluminum alloys. The fracture path was predominantly transgranular, with minor amounts of intergranular fracture.