Prevention of the intergranular fracture by addition of silicon and aluminum to a high-purity Fe-0.2 Pct P alloy with a trace of boron

The effect of addition of 0.25 pct silicon and aluminum on the intergranular fracture and the grain-boundary segregation of solutes in a high-purity Fe-0.2 pct P alloy with a trace of boron has been investigated by impact test, tensile test at low temperatures, optical and scanning electron microscopy, and Auger electron spectroscopy (AES). The addition of 0.25 pct silicon and aluminum remarkably reduces the susceptibility of an Fe-0.2 pct P alloy to the intergranular fracture and decreases the ductile-brittle transition temperature (DBTT). The addition of silicon and aluminum causes considerable segregation of boron to grain boundaries and, simulta-neously, remarkably decreases the segregation of phosphorus in the Fe-0.2 pct P alloy. The cause for these effects is discussed. Formerly Professor, Institute for Materials Research, Tohoku University     

The effect of trace additions of sn on precipitation in Al-Cu alloys: An atom probe field ion microscopy study

We have studied the early stages of θ′ precipitation in an Al-Cu-Sn alloy by atom probe field ion microscopy (APFIM) and transmission electron microscopy (TEM). Clusters of Sn atoms were observed in as-quenched samples, and their formation clearly precedes the formation of the θ′ phase. Transmission electron microscopy also revealed that elevated temperature aging leads to the formation of spherical particles. Atom probe analysis and microbeam electron diffraction suggested that these particles were β-Sn (I4₁/amd, α = 0.583 nm, and c = 0.318 nm). The TEM observations showed that θ′ nucleated heterogeneously on these particles. We have also examined the θ′/α interface following further precipitate growth to check for possible Sn segregation. Atom probe analysis of both the broad face and the narrow rim of the platelike θ′ precipitates has shown no evidence of Sn segregation at θ′/α interfaces. It was also found that cold work prior to aging inhibits the formation of Sn particles resulting in a lower number of these types of nucleation sites for the θ′ phase. Atom probe analysis has also revealed solute depletion at grain boundaries during the early stages of aging in the Al-Cu-Sn alloy, and this, in conjunction with vacancy depletion, explains the formation of narrow precipitate-free zones observed following further aging. Formerly Japan Society for the Promotion of Science Postdoctoral Research Fellow, Institute for Materials Research, Tohoku University Formerly Research Associate, Institute for Materials Research, Tohoku University     

Site competition between sulfur and carbon at grain boundaries and their effects on the grain boundary cohesion in iron

Grain boundary segregation in iron-sulfur-carbon alloys containing up to 100 wt ppm sulfur and up to 90 wt ppm carbon has been investigated with Auger electron spectroscopy (AES). The results show the site compctition on grain boundaries between the segregation of sulfur and carbon. The segregation energy of sulfur is estimated to be 75 kJ/mol. Impact tests of these alloys were carried out. Iron-sulfur alloys with less than 20 wt ppm carbon fractured by the intergranular mode with high ductile-brittle transition temperatures (DBTT’s). Addition of up to 90 wt ppm carbon to the binary alloys prevented the intergranular fracture caused by the grain boundary segregation of sulfur, and decreased the DBTT. Carbon, when segregated to grain boundaries, drives sulfur away from the boundaries and also increases the grain boundary cohesion. The DBTT values of the iron-sulfur-carbon alloys are analyzed in terms of the degree of grain boundary segregation of sulfur and carbon. It is shown that sulfur decreases the grain boundary cohesion of iron more severely than phosphorus if compared at the same degree of grain boundary segregation. Formerly Graduate Student     

Atom probe field ion microscopy study of the partitioning of substitutional elements during tempering of a low-alloy steel martensite

The partitioning behavior of Mn and Si at the cementite/ferrite interface during tempering of Fe-C-Si-Mn steel martensite has been studied by atom probe field-ion microscopy (APFIM). It has been shown that cementite can form without partitioning of Si and Mn during the early tempering stage at a low temperature. The atom probe compositional analysis shows no evidence of segregation or of concentration spikes of substitutional elements at the interface. This suggests that the early stage of cementite growth occurs by paraequilibrium mode and is controlled only by C diffusion in the matrix. In addition, significant C concentration fluctuations are measured in the as-quenched condition. The onset of partitioning of both Si and Mn occurs after prolonged time or by increasing the tempering temperature. Formerly Research Associate, Institute for Materials Research, Tohoku University     

Pearlite phase transformation in Si and V steel

Systematic research has been undertaken on the effects of single and combined additions of vanadium and silicon on the phase transformation and microstructure of pearlitic steels. Both alloy additions were found to result in the formation of nonlamellar products in the vicinity of austenite grain boundaries in hypereutectoid compositions (0.77 to 0.95 wt pct C). The products comprise discrete initial cementite particles and grain boundary ferrite, which is embedded with interphase precipitates of vanadium carbide. As the carbon content is increased further (up to 1.05 wt pct), the amount of grain boundary ferrite gradually decreases without any dramatic change in the morphology of the initial cementite particles. No continuous embrittling grain boundary cementite network was formed. The aspect ratios of the grain boundary cementite particles were decreased from 60:1 to 25:1 by the addition of the alloy elements. A compre-hensive model has been suggested to explain these effects. Other effects of these alloy elements on the microstructure of pearlitic steels have also been examined. For given austenitization conditions, an increase in carbon and vanadium content produced a decrease in austenite grain size. Silicon was found to increase the rate of interphase precipitation of vanadium carbides. Formerly Graduate Student, Department of Materials, Oxford University Formerly University Lecturer, Department of Materials, Oxford University     

Evaluation of the effects of segregation on austenite grain boundary energy in Fe-C-X alloys

A scanning Auger microprobe study has been made of the segregation of substitutional alloying elements to austenite grain boundaries in Fe-C-X alloys (where X = Mn, Ni, Si, Co, and Mo). The grain boundary enrichments in X are considerably smaller than those previously estimated from the thermal grooving method but appear consistent with other SAM results in the literature. Mo exhibits the highest enrichment factor, those for Si and Mn are intermediate, and no appreciable grain boundary enrichment of either Co or Ni is observed. In view of the special relevance of this information to nucleation kinetics of austenite decomposition products at austenite grain boundaries, the reductions in grain boundary energy attending the measured enrichments are evaluated using the model of interactive segregation of interstitial and substitutional solutes formulated by Guttmann and McLean. These calculations were performed under two different (limiting) conditions: (i) equilibrium segregation of both solutes is fully achieved at the isothermal reaction temperatures, and (ii) the boundary concentration of X is fully inherited from the austenitizing temperature and only paraequilibrium segregation of carbon is achieved. Various characteristics of interactive segregation are also discussed in terms of the interaction and binding energies of each solute. Formerly a Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA15213 Formerly with Oak Ridge National Laboratory, Oak Ridge, TN 37830     

The normalized coffin- manson plot in terms of a new damage function based on grain boundary cavitation under creep- fatigue condition

A new damage function based on a model for the creep-fatigue life prediction in terms of nucleation and growth of grain boundary cavities is proposed. This damage function is a combination of the terms related to the cavitational damage in the life prediction equation and is generally applicable to the materials in which failure is controlled by the grain boundary cavitational damage. The creep-fatigue data from the present and other investigations are used to check the validity of the proposed function, and it is shown that they satisfy the reliability of damage function. Additionally, using this damage function, one may realize that all the Coffin-Manson plots at the various levels of tensile hold time and temperature under strain-controlled creep-fatigue tests can be normalized to make the master curve. Formerly Graduate Student, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology Formerly Graduate Student, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology Formerly Graduate Student, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology     

The effect of an air environment on the creep and rupture behavior of a nickel-base high temperature alloy

The behavior of wrought and cast polycrystalline Udimet 700 tested in static tension at 1200 K (1700†F) has been determined. An air environment decreases rupture life and ductility, except in very coarse-grained cast specimens, because of premature failure by stress-assisted grain boundary oxidation and cracking. In very coarse-grained cast specimens greater life and ductility are found in air than in vacuum, presumably due to the paucity of transverse grain boundaries and to some type of surface hardening effect. Wrought specimens exhibit greater grain boundary sliding, hence more cracking, shorter life, and lower ductility, than cast specimens in any environment. This is attributed to differences in grain boundary topography. Formerly Research Fellow, Department of Metallurgy and Materials Science and LRSM, University of Pennsylvania, Philadelphia     

Effect of phosphorus on solidification process and segregation of directionally solidified IN738 superalloy

The effect of phosphorus on the solidification and the solute segregation of a directionally solidified IN738 Ni-based superalloy was investigated experimentally employing a method of partially directional solidification and subsequent quick quenching. It was found that the phosphorus addition widened the solidus-liquidus temperature interval of the alloy. Both the P content and the solidification rate affected the morphology of the solid-liquid interface and the precipitation of phosphides in the alloy. Phosphorus segregated largely in the intercellular/interdendritic regions and promoted the segregation of other alloying elements in the solidified structure. Formerly Graduate Student, Institute of Metal Research, Academia Sinica     

On the mechanisms of high-temperature intergranular embrittlements of Ni3Al-Zr Alloys

Recent studies on the room-temperature fracture behavior of Ni₃Al-Zr alloys after preexposure at elevated temperatures show various types of intergranular failure. In the presently studied Ni₇₈Al₂₁Zr₁B_0.2 alloy, a strong intergranular fracture tendency at room temperature has been found after preexposure at 750 °C, which is caused by the grain boundary precipitation in this alloy. After short-term exposure above 1200 °C and bending fracture at room temperature, the alloy also suffers intergranular embrittlement due to grain boundary melting. The intergranular fracture appearance is quite different from that observed in a previous study for a Ni_77.4Al₂₂Zr_0.6B_0.2 alloy after air exposure for 100 hours at 1200 °C. In that case, the intergranular fracture was accompanied by grain boundary diffusion (invasion) and segregation of oxygen. The mechanisms of these types of grain boundary failure are discussed. Formerly Doctoral Candidate, Institute of Materials Science and Engineering, National Taiwan University.     

Embrittlement of austempered nodular irons: Grain boundary phosphorus enrichment resulting from precipitate decomposition

The microstructures, mechanical properties, and fracture behavior were characterized for a series of Mg treated nodular cast iron specimens austenitized at 1170, 1255, and 1340 K and subsequently austempered at 640 K. The ductility and toughness of the alloy decreased as austenitization temperatures were increased, which is contrary to the behavior anticipated from the observed micro-structural evolution. Fractographic and surface chemical analyses demonstrated that the mechanical property degradation was associated with embrittlement of the austenite grain boundaries by phosphorus. The primary mechanism of grain boundary phosphorus enrichment does not appear to be equilibrium segregation, and an alternative mechanism based on the decomposition of P rich precipitates is proposed and discussed. Formerly with Michigan Technological University, Houghton, MI     

The formation of austenite in a low-alloy steel

Austenite formation from different microstructural conditions has been studied in an Fe-lV-0.2C steel. The starting microstructures of ferrite, bainite, and martensite, and the morphology of austenite formation have been examined in detail by light microscopy and transmission electron microscopy. Retained austenite in quenched martensitic areas has been used to establish the crystallographic relationships during austenite nucleation, and to indicate the behavior of the initial vanadium carbide dispersion present in the ferrite starting microstructure. Limited measurements were also made of the kinetics of austenitization. Formerly Research Student, Department of Metallurgy and Materials Science, University of Cambridge. Formerly of Royal Society, Warren Research Fellow, Department of Metallurgy and Materials Science, University of Cambridge.     

The influence of intercritical heat treatment on the temper embrittlement susceptibility of aP-Doped Ni-Cr steel

Temper embrittlement of a Ni-Cr steel doped with 0.06 wt pct P aged at 480°C after an intercritical heat treatment (IHT) has been compared with that of the conventionally heat treated steel with a range of prior austenite grain sizes. The IHT virtually eliminated the embrittlement susceptibility, but low temperature brittle fracture was still intergranular. It appears that most of the benefit of IHT in this steel comes from microstructural refinement; however, IHT also reduced the amount of segregation of Ni and P to grain boundaries. This is believed to be connected with partitioning of Cr during IHT. Formerly Post Doctoral Fellow, Department of Metallurgy and Materials Science, University of Pennsylvania Formerly on the Research Staff, LRSM, University of Pennsylvania     

Sulfur segregation to grain boundaries in Ni3Al and Ni3(AI,Ti) alloys

The segregation of S to grain boundaries in Ni₃Al and Ni₃(Al, Ti) has been studied using Auger electron spectroscopy. The S concentration at the grain boundaries decreases more slowly with increasing temperature than would be predicted by segregation models based on a single solute binding energy to the grain boundaries. This behavior, which can be interpreted as an increase in the effective solute binding energy for a grain boundary as a function of temperature, is consistent with predictions of a model based on the existence of a spectrum of solute binding energies for grain boundaries. Formerly Professor and Head of the Department of Metallurgical Engineering, Michigan Technological University     

Temper embrittlement of Ni-Cr steel by Sn

Tin-induced temper embrittlement of 3.5 pct Ni, 1.7 pct Cr steels containing 0.2 and 0.4 pct C was studied by means of notched-bar testing, scanning electron fractography, and Auger electron spectroscopy of isothermally aged specimens. The ductile-brittle transition temperature varied linearly with Sn concentration on grain boundaries at a rate which increased with hardness of the steel. The rate of approach to the steady state level of embrittlement was faster in the higher C steel. The grain boundary concentrations of Ni and Sn bear a unique relationship in a manner analogous to the behavior found previously in Sb-doped steels. The potency of Sn as an embrittling element is somewhat less than that of Sb, but considerably greater than that of P. Formerly Research Fellow, Department of Metallurgy and Materials Science, University of Pennsylvania, Philadelphia,PA Formerly Post Doctoral Fellow, Department of Metallurgy and Materials Science, University of Pennsylvania H. C. Feng Formerly with the Research Staff, LRSM, University of Pennsylvania     

Measurements of the electrical conductivity of Wood's alloy and other low melting point alloys

Formerly Graduate Student Research Assistant, Department of Materials Science and Mineral Engineering, University of California.     

Grain boundary segregation of carbon in iron

The distribution of C¹⁴ in a series of a-phase Fe-C alloys was studied autoradiographically. Excess carbon was present near grain boundaries in specimens equilibrated at 500‡ and 600‡C; the amount of carbon near the grain boundaries was found to decrease as the equilibration temperature increased. Electron microscopy of the specimens subsequent to the autoradiography showed that no grain boundary precipitates were present. The results are compared with the theories of nonequilibrium vacancy-aided segregation and Gibbsian adsorption, and also with McLean's model of grain boundary segregation. It is shown that the technique measures directly the quantity of thermodynamic interest, the total excess solute per unit area of grain boundary. Formerly Graduate Student, Henry Krumb School of Mines, Columbia University, New York, N. Y.. This paper is based on a thesis submitted by J. M. PAPAZIAN in partial fulfillment of the requirements of the degree of Doctor of Philosophy at Columbia University.     

Influence of Interfacial Structure upon Carbide Precipitation at Austenite : Ferrite Boundaries in an Fe-C-Mo Alloy

During conventional isothermal transformation of an Fe-0.11 pct C-1.95 pct Mo alloy, eutectoid decomposition occurs by the interphase boundary carbide precipitation and the fibrous carbide mechanisms at 770° to 825 °C. When proeutectoid ferrite is formed and then recrystallized within the α + γ region, and subsequently further transformed at 770° to 825 °C, however, both of these eutectoid decomposition mechanisms are rendered inoperative. Carbide precipitation occurs instead entirely as isolated particles. This result supports the deduction that carbide precipitation at austenite : ferrite boundaries can occur only when these boundaries are locally immobilized by a partially coherent interfacial structure. A general approach to explaining the development of planar and curved interphase boundary precipitation, fibrous structure, and pearlite is developed in terms of two crystallographic factors. Formerly Research Associate in the Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931 and the Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15213 Formerly Graduate Student, Michigan Technological University, and Visiting Graduate Student, Camegie-Mellon University Formerly Professor at Michigan Technological University     

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. Formerly a Postdoctoral Research Fellow at the University of Pennsylvania formerly a Visiting Scientist at the University of Pennsylvania Formerly a Graduate Student at the University of Pennsylvania     

Grain-boundary relaxations in an Fe-Ni-Cr alloy

Grain boundary internal friction spectra have been studied in a stabilized and an unstabilized Fe-20 pct Cr-25 pct Ni alloy. The effect of grain size and of prior creep on the relaxation phenomena have been studied. It has been found that the spectrum consists of three constituent peaks occurring in the temperature regions of 630°, 690°, and 770°C at 1 Hz. The relaxation strength and the width (seen through the β factor) of each of the peaks have been observed to decrease and the peak temperature to increase with increasing grain size. It has been suggested that the relaxation processes at the grain boundaries are associated with the solvent iron, solute chromium, and solute nickel atoms, respectively. Deformation by creep increases the peak height, width, and temperature of the peaks in the stabilized alloy, but has no effect on the grain-boundary relaxation spectrum of the unstabilized alloy. From the study of stabilized and unstabilized alloys it can be implied that the dislocation structure at the grain boundaries influences the relaxation strength of the grain-boundary peaks. Formerly Graduate Student, Department of Metallurgy and Materials Technology, University of Surrey, United Kingdom