D. P. Pope

The orientation and temperature dependence of the yield stress of Ni3 (Al, Nb) single crystals

The flow stress of Ni3(Al, Nb) single crystals has been measured as a function of orientation in the temperature range 77 to 910 K. While the increasing flow stress behavior is similar to that observed in other Ni3Al-based alloys, the absolute value of the stress was found to be much higher. Also, the effect of orientation changes was to produce much greater changes in the temperature at which the peak flow stress occurs than has been previously observed. The operative slip systems were analyzed by two surface slip trace analysis. Primary octahedral slip was found to be predominant at temperatures below the peak stress temperature, while primary cube slip is prevalent above the peak temperature. The anomalous increase in the flow stress of Ni3(Al, Nb) with increasing temperature is generally consistent with the thermally activated cross-slip of a/2<110> dislocations from {111} planes onto {100} planes. The cross-slip is shown to be aided not only be the resolved shear stress on the {100} cross-slip plane but also by the stress tending to constrict the a/<112> Shockley partial dislocations on the primary glide plane.

Temper embrittlement of Ni-Cr Steels by phosphorus

Temper embrittlement in 3.5 pct Ni, 1.7 pct Cr steels doped with P and isothermally aged at several temperatures was studied by measurements of ductile-to-brittle transition temperature and hardness, which were correlated with observations of the intergranular fracture surfaces by Auger electron spectroscopy and scanning electron fractography. It is shown that if all other factors remain constant, the effect of a small change in the matrix hardness can be very large; “overaging” (a maximum in embrittlement with respect to aging time) was found to result from softening rather than from a reversal of segregation of P. Nickel was found to be segregated at the grain boundaries, and both Ni and Cr appear to enhance the amount of segregated P. The major role of Cr was found to be its effect of increasing matrix hardness (by enhancing hardenability and resistance to softening during tempering), resulting in an increased susceptibility to temper embrittlement. The effect of variations in the roughness of grain boundary topography appears to be small. It is shown that the segregation of P to grain boundaries can be accounted for by diffusion from the matrix and is consistent with the hypothesis of equilibrium (Gibbsian) segregation. The results are in qualitative agreement with the thermo-dynamic theory of Guttmann.

Temper embrittlement of Ni-Cr steel by antimony: III. Effects of Ni and Cr

The individual effects of Ni and Cr on temper embrittlement in a Ni-Cr steel doped with Sb were studied by notched-bar testing and Auger electron spectroscopy. Nickel is required for significant segregation of Sb to grain boundaries (and concomitant embrittlement). The addition of Cr somehow makes Ni more effective in this respect; segregation of Cr was not detected. Precipitation of Cr-rich carbides, which depletes the α matrix of Cr, reduces the embrittlement susceptibility. The amount of segregated Ni is a function only of the amount of segregated Sb; it is independent of Cr or C content of the steel.

The effects of trace impurities on the ductility of a Cr-Mo-V steel at elevated temperatures

The development of current ideas about impurity effects on the ductility of steels at elevated (creep) temperatures is reviewed and the effects of low temperature intergranular failure (LTIF) studies on these ideas are emphasized,e.g., temper embrittlement studies. The possible mechanisms by which impurities can induce high temperature intergranular failure (HTIF) are then reviewed. It is shown that impurity effects on LTIF and HTIF can be quite different and therefore there need be no correlation between the two phenomena. Experiments on a bainitic 1 Cr-1.25 Mo-0.25V steel containing deliberately added Mn, P, Si, and Sn show that these additions can eitherdecrease or have no effect on the extent of HTIF compared to undoped material, depending upon the austenitizing temperature. These results may be due to a possible reduction of segregated grain boundary S due to competition between S and P for available grain boundary sites. (The steels used contain approximately 40 wt ppm residual S.) These results resolve some of the controversies in the literature on the effects of grain boundary impurities on HTIF.

Compositional effects on the creep ductility of a low alloy steel

The role which P plays in determining the creep ductility of 2.25Cr-1Mo steel is examined by notched bar creep rupture tests on high purity material selectively doped with combinations of Mn, Si and P. The impurity concentrations, hardness and grain size were carefully controlled. The ductility of as-tempered samples containing dopants was found to be higher than those without dopants; however the ductility of step cooled samples containing Mn and P was found to be lower than as-tempered samples. It is suggested that P, when segregated to the prior austenite grain boundaries, enhances the nucleation of grain boundary cavities while retarding their growth. Mechanisms for each process are proposed.

The cyclic stress-strain response of polycrystalline, pseudoelastic Cu-14.5 wt pct Al-3 wt pct Ni alloy

Limited results on the fatigue of pseudo-elastic material indicate that, as a class, these materials should have truly outstanding fatigue properties. To check this, the mechanisms of cyclic deformation and fracture have been studied in Cu−Al−Ni chosen because its transformation behavior is well understood. Since this alloy is notoriously brittle, pulsating compression fatigue tests were carried out in polycrystalline material. The details of the stress-induced martensite behavior were studied byin situ video observations. The alloy was found to undergo cyclic hardening and failure eventually occurred by multiple nucleation of cracks at grain boundaries, by a mechanism similar in principle to that which occurs in regular metals cycled at high plastic strains. The Coffin-Manson law was obeyed.

The effect of oxygen on the structure and mechanical behavior of Aged Ti-8 Wt pct Al

It is shown that for a Ti-8 wt pct Al alloy aged at a temperature high in the two-phase region (695°C) to precipitate the ordered α2 phase, an increase in oxygen content from 600 ppm to 1200 ppm decreases the fracture strain from 20 to 1 pct elongation at room temperature and slightly increases the yield strength. The fracture mode is changed from dimpled rupture to predominantly cleavage. Further increase in oxygen content to 3000 ppm does not produce significant additional changes in ductility or yield strength. It is demonstrated that oxygen additions alter the position of the α/α + α2 coherent solvus, resulting in formation of coherent α2 in specimens containing ⪞ 1000 ppm oxygen aged at 968 K (695°C). For a given aging time the volume fraction of α2 increases with increasing oxygen up to 1300 wt ppm and then levels off. The changes in mechanical behavior are attributed to the presence of α2. The experimental evidence suggests that oxygen partitions preferentially into α2.

The dislocation and martensite substructures of a fatigued, polycrystalline, pseudoelastic Cu-AI-Ni alloy

Polycrystalline Cu-AI-Ni specimens, subjected to pulsating compression fatigue, while capable of pseudo-elastic deformation, nevertheless exhibit cyclic hardening and fatigue fracture, as reported inMet. Trans. A, 1977, vol. 8A, p. 955. In order to interpret this behavior, transmission electron microscopy has been used to study the microstructure. Increasing amounts of martensite and deformation substructure result from decreasing the test temperature and raising the stress level. Martensite of different morphologies has been observed and identified. Large plates ofβí, common to all samples in varying amounts, were determined to be of the 18R structure, with lattice parameters ofa = 4.382Å,b = 5.356Å, andc = 38.0Å. Samples deformed at low temperature or high stresscontain not only β1, but two distinct forms of γ both with lattice parameters ofa = 4.41Å,b = 5.31Å, andc = 4.222Å, and of the 2H crystal structure. The largeβí and γ plates seem to be characterized by interfacial dislocations between the matrix and plates. In all samples, antiphase boundaries (APB’s) can be imaged, even in heavily dislocated areas, indicating that the deformation has not destroyed the matrix order. It is concluded that hardening results from interactions between matrix dislocations and the stress-induced martensite, as well as by collisions between groups of martensite plates having different habits.

Compositional effects on the high temperature ductility of 1 Cr-1.25 Mo-0.25 V Steel

This paper describes the results of slow strain rate (ε = 4.4 × 10-5 s-1) tensile tests performed at temperatures between 25 and 700 °C on a high purity CrMoV steel containing various dopants. The materials all had a bainitic microstructure, a hardness of RC28, and a grain size of ASTM 0. Some samples were step cooled prior to tensile testing. Four different compositions were tested: undoped (HP), Mn + P doped (MnP), P doped (P), and Sn doped (Sn) materials. All four materials failed in a low ductility cleavage mode at low temperatures and by a low ductility grain boundary cavitation mode at high temperatures. At intermediate temperatures, around 500 °C, the MnP material showed the highest ductility, the HP and Sn materials showed the lowest, and the P material was intermediate. The beneficial effects of both Mn and P on the creep ductility are rationalized in terms of their control of the sulfur concentration on prior austenite boundaries. In addition, it is suggested that P on the grain boundaries can reduce the cavitation rate by reducing the grain boundary self diffusion rate.

Effects of sulfur and phosphorus on the creep ductility of a CrMoV steel

Abstract In this paper the results of creep rupture tests performed at 550°C on a high purity 1Cr1.5Mo0.25V steel containing residual sulfur and deliberately added phosphorus and manganese are described. The effects of impurities, hardness, grain size and step cooling were examined. The material with deliberately added phosphorus and manganese showed the highest ductility, the high purity (undoped) material with residual sulfur showed the lowest ductility and that of the material with deliberately added phosphorus was intermediate. The effects of hardness on the creep ductility are due to the change in the relative cavitation rates in the grain boundaries and matrix. The effect of prior austenite grain size is actually due to the effect of different austenitization temperatures (which affects the grain boundary chemical composition). The results obtained here are quite consistent with our previous tensile test results and are interpreted in terms of the effects of sulfur and phosphorus segregation on grain boundary cavitation.