Earl R. Parker

STRESS INDUCED MOVEMENT OF CRYSTAL BOUNDARIES

Abstract The stress-induced movement of small angle boundaries in zinc crystals was investigated in the temperature range of 25° to 400°C. The dynamic behavior of the boundaries seemed to require that they consist of an array of edge dislocations of like sign distributed more or less uniformly over the plane of the boundary. The activation energy for the movement of the boundaries in the temperature range of 300 to 400°C was determined to be about 21,500 calories per mole. Attention was drawn to the close agreement between this value and those of the activation energies for self-diffusion, creep, and recovery in zinc crystals. A mechanism was suggested for the formation of a substructure in crystals plastically deformed at low temperatures which depended only on the stress-induced movement of small angle boundaries. This mechanism was based on observations that the rate of boundary motion decreased with increasing boundary angle. Small angle boundaries overtook and united with those of larger angle, forming slower moving, or even stationary substructure boundaries.

Rapid Method for Determining Ternary‐Alloy Phase Diagrams

A rapid method for determining isothermal sections of ternary‐alloy diagrams has been developed. Three elements are evaporated simultaneously onto a heated substrate in such a way that the composition of the deposit varies from point to point as it does in a ternary‐phase diagram. An isothermal section of the Fe–Cr–Ni system prepared by the new technique is compared with one reported in the literature. The applicability of this technique to the preparation of ternary phase diagrams is discussed.

STABILITY AND MECHANICAL PROPERTIES OF SOME METASTABLE AUSTENITIC STEELS

The relation between austenite stability and the tensile properties, as affected by testing temperature and processing, was studied for a series of alloys of increasing compositional complexity, viz., the Fe-Ni, Fe-Ni-C, and Fe-Ni-Cr-Mn-C systems. The “stress” and “strain induced” modes of transformation to martensite differed significantly in their influence on the shape of the stress-strain curve. Under certain testing conditions, unusually low yield strengths and high work hardening rates were observed in some of these alloys. Maxima in yield strengths were observed for all austenitic alloys containing carbon that were processed at deformation temperatures between 200° and 300°C. Evidence gleaned from electron microscopy and magnetic and mechanical testing suggested that the maxima were due to the formation of carbon atmospheres on dislocations during processing. The influence of austenite stability on the mechanical properties of steels, varied by systematic changes in test temperature (22° to -196°C), composition (8 pct, 12 pct, 16 pct, and 21 pct Ni) and deformation temperature (25° to 450°C), was evaluated quantitatively.

Effects of silicon additions and retained austenite on stress corrosion cracking in ultrahigh strength steels

A study has been made of the effects of silicon additions and of retained austenite on the stress-corrosion cracking (SCC) behavior of commercial ultrahigh strength steels (AISI 4340 and 300-M) tested in aqueous solutions. By comparing quenched and tempered structures of 4340 and 300-M i) at equivalent strength and ii) at their respective optimum and commercially-used heat-treated conditions, the beneficial role of silicon addition on SCC re-sistance is seen in decreased Region II growth rates, with no change in K’ISCC. The beneficial role of retained austenite is demonstrated by comparing isothermally transformed 300-M, containing 12 pct austenite, with conventionally quenched and tempered structures of 300-M and 4340, containing less than 2 pct austenite, at identical yield strength levels. Here, the isothermally transformed structure shows an order of magnitude lower Region II SCC growth rates than quenched and tempered 300-M and nearly two orders of magnitude lower Region II growth rates than 4340, K ISCC values remaining largely unchanged. The results are discussed in terms of hydrogen embrittlement mechanisms for SCC in martensitic high strength steels in the light of the individual roles of hydrogen diffusivity and carbide type.

Microstructural features affecting fracture toughness of high strength steels

Abstract The fracture toughness of quenched and tempered steels, such as AISI 4340, AISI 4130 and 300M, can be increased by 50–100% by minor changes in heat treating procedures. Certain microstructural features, particularly blocky ferrite, upper bahnte and twinned martensite plates, are deleterious to fracture toughness. Similarly, the presence of undissolved carbides and sulfide inclusions, which act as crack nuclei, can lower fracture toughness by 25–50%. Other microstructural constituents, such as lower bainte, autotempered martensite, and retained austenite can enhance fracture toughness. By controlling the amounts and distributions of the microstructural constituents, the fracture toughness values of AISI 4340, AISI 4130 and 300M can be raised to the fracture toughness level of 18Ni maraging steel at equivalent values of yield strength.

Enhancement of fracture toughness in high strength steel by microstructural control

Abstract The development of new alloys with improved mechanical properties has been seriously hampered in the past by the inability of a metallurgist to relate quantitatively the variables of microstructure and fracture toughness. The emergence of a unified theory of fracture toughness in the past decade has done much to alleviate this difficulty. As a consequence of a recent interdisciplinary research effort involving both the disciplines of physical metallurgy and experimental fracture mechanics, we have been able to develop alloys with engineering properties superior to those of commercially available materials. This research has required the creation of new and unusual microstructures, utilizing a variety of thermal and thermomechanical processes. The quantitative relationships of mechanical properties (strength, ductility, work hardening, and fracture toughness) with composition and microstructure are discussed in detail for the newly developed TRIP steels. In the report of another development, it is shown how the fracture toughness of low alloy quenched and tempered steels with yield strengths over 200,000 psi can be improved by as much as 70 per cent by microstructural control. Lastly, the initial results of research on alloys intended for cryogenic service are described. The composition, heat treatment, microstructure and properties of an alloy having more than three times the toughness of the presently used alloys are discussed.

Tensile properties of 0.05 to 0.20 Pct C TRIP steels

The uniaxial tensile properties of a series of TRIP steels of varying carbon contents and processing histories were determined over a wide range of test temperatures. The yield strengths at room temperature varied both with the deformation temperature (over the range 250° to 550°C) and with the carbon content (0.05 to 0.20 pct). Possible reasons for these variations are advanced. For all steels, the −100°C yield strengths were substantially lower than the 100°C yield strengths. The minima and maxima in the yield strengths vs temperatures curves were especially pronounced for the steels processed at the lowest deformation temperatures. Both the rate of work hardening and the elongation were influenced by the strain-induced austenite-to-martensite transformation. The rate of strain hardening and the rate of production of strain-induced martensite (per unit strain) increased with decreasing temperature.

SOME SUPERCONDUCTING PROPERTIES OF SEVERAL CARBIDES AND NITRIDES OF THE TRANSITION METALS

Abstract Analogous to the transition elements, their solid solutions, and intermetallic compounds, a correlation is shown to exist between the total number of valence electrons and the superconducting critical temperature for transition metal carbides and nitrides crystallizing in the NaCl structure. The critical temperature increases as the total number of valence electrons increases from eight to about ten. A number of experiments were undertaken to substantiate the correlation. The critical temperature of cubic molybdenum carbide (NaCl structure) was found to be 13.0°K. The critical temperature of the two-phase mixtures of cubic and hexagonal molybdenum carbide was found to be dependent upon the relative amounts of the two phases present. This behavior was tentatively explained on the basis of the structural similarities of the two phases. A critical temperature for the hypothetical compound, cubic TaN, of 12–14°K is predicted from the variation of the critical temperature with composition for the cubic solid solutions of TaN and TaC.

Isothermal studies of bainitic and martensitic transformations in some low alloy steels

The transformation kinetics of the upper and lower bainitic reactions were determined for four commercial quenched and tempered steels, AISI 4130, AISI 4140, D6AC and AMS 6416 (300-M). Their chemical compositions are given. The kinetics of the bainite reaction below the martensite start temperature (M/sub s/) was determined.

A rapid magnetometric technique to plot isothermal transformation diagrams

A sensitive magnetic permeability method for rapid determination of isothermal transformation diagrams in steels and iron base alloys is described. The method consists of quenching the sample from an austenitizing temperature to a subcritical temperature in an isothermal bath, and holding it within the magnetic field of an inductor coil. The increase in permeability accompanying austenite decomposition increases the inductance of the coil, and this changes the resonant frequency of the circuit. An automatic continuous recording of the corresponding period provides a convenient and accurate method for following the austenite decomposition of AISI 4340 in the bainite and martensite temperature ranges. This method provides quantitative information on austenite decomposition kinetics within two seconds after the start of quenching.