Hall-Petch relation and boundary strengthening

The effects of hydrogen on the physical and mechanical properties of iron and steel are reviewed. A new mechanism for the cold work peak for hydrogen in iron is considered. Together, internal friction and mechanical properties indicate that hydrogen softens iron by enhancing screw dislocation mobility at room temperature but hardens iron by core interactions at low temperatures. No single mechanism exists for the degradation of the properties of steel by hydrogen. Instead a complex process involving many of the proposed mechanisms as contributing factors is shown to account for most degradation phenomena.

Effects of hydrogen on the properties of iron and steel

On the validity of the hall-petch relationship in nanocrystalline materials

Two of the key parameters that are useful for understanding the deformation kinetics in metals with ultrafine grain (UFG) and/or nanocrystalline (nc) microstructure are the strain rate sensitivity (SRS) of the flow stress and the flow stress activation volume. In this paper, we present experimental results from our in-house tests on UFG Cu, Fe and Ta, along with those available in literature, regarding the behavior of SRS and activation volume of UFG/nc metals with face-centered cubic (fcc) and body-centered cubic (bcc) structures. The UFG metals were produced via severe plastic deformation (SPD), and confirmed using transmission electron microscopy. Our own results, as well as those in the literature, indicate that the SRS of the UFG/nc fcc metals is elevated, whereas that of UFG/nc bcc metals is much reduced compared to their coarse-grained counterparts. These trends appear to hold independent of the processing routes used to refine the grain size and of the testing method employed. The activation volumes in UFG fcc and bcc metals also exhibit different behavior. We present discussions to explain the observed trends. The implications of these findings for plastic instability are also discussed for the two lattice structures.

Effect of nanocrystalline and ultrafine grain sizes on the strain rate sensitivity and activation volume: fcc versus bcc metals

Microstructure and strength of nickel at large strains

Yielding in nickel as a function of grain or cell size

This chapter focuses on two kinds of environmental fracture, stress corrosion cracking (SCC) and hydrogen embrittlement, and discusses the role in such failures of a number of metallurgical variables. These include chemical composition; microstructural components such as precipitate type and structure, and grain size and shape; crystallographic texture; heat treatment and its effect on the foregoing variables; and processing, particularly the thermomechanical treatments (TMT), which are attracting increased attention for property optimization. These variables are expected to be of great importance in the development of new engineering materials to meet demanding service conditions.

The Role of Metallurgical Variables in Hydrogen-Assisted Environmental Fracture

Effect of grain size on work hardening in nickel

Eleven alloys similar to commercial A-286 but with small variations in chemistry were tested to assess whether the performance of this type of stainless steel, which shows a ductility (RA) loss of about 48 pct when hydrogen charged, could be improved. It was found that removal of Mn resulted in anRA loss of only 15 pct; low Mn plus a 5 pct increase in Ni gave further improvement to 11 pctRA loss. Reductions in C, Si and B had little effect on theseRA losses. None of the above changes had significant effect on strength. Other chemistry changes were unsuccessful: additions of Al, Ti, Ti + Mo, or decreases in Ni, increased the ductility loss. Extensive examination of thin foils permitted a structural rationale for behavior in hydrogen in terms of the effects of second-phase particles on ductile fracture, particularly as affected by the misfit between the austenite and the γ’ precipitates. This rationale was consistent with fractographic observations.

Hydrogen performance of precipitation-strengthened stainless steels based on A-286

Creep tests were conducted at 295 K on Ti-6 Al-4 V in the solution treated and aged (4 h at 815 K) condition, and in the as-welded condition Some aged specimens were tested after pre-straining Creep stresses ranged from 40 to 90 pct of the aged material yield strength Results showed that creep was of the primary or transient kind in all cases, and was much greater in welded than in aged material In general, pre-strains reduced creep, although a strain larger than 10-3 was needed to do this at the highest creep stress Activation areas A* were between 10 and 20 b2, and thus were similar to tensile results on titanium and its alloys The microstructural rationale applied to Ti-5 Al-25 Sn in earlier work, based on the character of dislocation sources, proved successful in understanding the effects of prestrain in this work

Anthony W. Thompson

Papers

Yielding in nickel as a function of grain or cell size

The Role of Metallurgical Variables in Hydrogen-Assisted Environmental Fracture

Effect of grain size on work hardening in nickel

Hydrogen performance of precipitation-strengthened stainless steels based on A-286

Low temperature creep of Ti-6 Al-4 V