| METALLURGICAL AND MATERIALS TRANSACTIONS A | |
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Volume 28A, No. 3, March 1997 This Month Featuring: Alloy Phases, Transformations, Transport Phenomena, Mechanical Behavior, Welding & Joining, Electronic, Magnetic, & Optical, Solidification, Materials Processing, Composite Materials. View March 1997 Contents.
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-(Fe3Zn10), 1-(Fe5Zn21), 
-(FeZn7), and 
-(FeZn13) with the addition of 5 pct Al (wt) were ball milled in an argon gas atmosphere to form homogenous alloys. Nonisothermal kinetic analyses of the mechanically alloyed materials, based on differential scanning calorimetry (DSC) measurements, revealed two diffusion-controlled processes during the evolution of the + 5 pct Al and + 5 pct Al compositions with activation energies of 227 ± 2 and 159 ± 1 kJ/mole, respectively. Other endothermic and exothermic reactions detected for these compositions are consistent with the Fe-Zn-Al equilibrium phase systems with respect to the formation of the Fe3Al, Fe2Al5, and -FeZn7 phases. Based on the evidence of FeAl2 formation at 440°C for the + 5 pct Al composition from X-ray diffraction (XRD) and DSC measurements, the revision/re-evaluation of the Fe-Zn-Al equilibrium phase diagrams is proposed. The + 5 pct Al and 1 + 5 pct Al compositions evolved similarly through the same fields, except at 400°C, where the former consisted of 
-Fe + + , while the later was without the phase.
Thermodynamic Study of the Low-Temperature Phase B19' and the Martensitic Transformation in Near-Equiatomic Ti-Ni Shape Memory Alloys
WEIJIA TANG
Experimental information on the transformation temperatures and the thermodynamic properties of the near-equiatomic TiNi alloys is analyzed. Special attention is paid to the estimation of To temperature from experimental Ms and Af temperatures. The properties of the TiNi low-temperature phase (B19') are evaluated from selected experimental data by using a two-sublattice model. The Ti-Ni phase diagram including the B19' phase is then calculated. It reveals that the equiatomic TiNi parent phase (B2) remains stable from high temperatures until 370 K, and then the B19' phase becomes thermodynamically stable as a linear compound under 370 K. Thermodynamic quantities such as the To temperature and transformation enthalpy are also calculated and compared with experimental data. Further, the Ms temperature is predicted and compared with data from different sources.
Aging and Tempering Behavior of Iron-Nickel-Carbon and Iron-Carbon Martensite
M.J. VAN GENDEREN, M. ISAC, A. BÖTTGER, and E.J. MITTEMEIJER
The aging at room temperature (RT) and the tempering behavior in the temperature range 293 to 973K of ternary iron-nickel-carbon martensite (containing 14.4 at. pct Ni and 2.35 at. pct C) was investigated principally by using X-ray diffractometry to analyze changes in the crystalline structure and differential scanning calorimetry to determine heats of transformation and activation energies. These techniques also were used in the parallel study performed in this work of the tempering behavior of FeC martensite (containing about 4.4 at. pct C) in the temperature range 298 to 773 K. Analysis of the structural changes revealed that in both FeNiC and FeC the following processes occurred: (1) formation of carbon enrichments and development of a periodic arrangement of planar carbon-rich regions up to 423 K; (2) precipitation of 
/
transition carbide and transformation of a part of the austenite into ferrite under simultaneous enrichment with carbon of the remaining austenite (between 423 and 523 K); (3) decomposition of the retained austenite into ferrite and cementite between 523 and 723 K (only partly for FeNiC); (4) precipitation of cementite between 523 and 723 K; and (5) for FeNiC, reformation of austenite from ferrite and cementite above 773 K. A short comparative discussion concerning the first stage of martensite decomposition for FeC, FeNiC, FeN, and FeNiN martensites is given.
Communication: Discussion of "A Study on Morphology and Plate Mean Dimensions in Fe-Ni and Fe-Ni-Cr Alloys"
V. RAGHAVAN and R. DATTA
Q, are compared to the values of H2 + E calculated from the Lazarus LeClaire theory for several solute impurities in ten solvent metals. The calculated results agree very well with the experimental values for the large majority of solutes. The theory presented adequately describes solute impurity diffusion in both -Fe and 
-Fe, Al, Ni, and the noble metals. In particular, the low activation energies for impurity diffusion of the alkali metals (ground state valence of one) in Al (ground state valence of three) are accounted for by the theory. It is shown that the diffusion of the electronegative solute impurities (Cr, Mn, Fe, and Co) in Al is not anomalous when the relative valence is calculated by the proposed theory. The diffusion of electronegative solute impurities in the noble metals, which has been problematic in the past, is also well described by the proposed theory. The proposed theory introduces a simple method of estimating the effective electron densities of solute impurities and illustrates that the Lazarus LeClaire theory adequately describes solute impurity diffusion in the ten solvent metals studied. It is expected that more accurate calculations of effective electron density for solute impurities would result in even better agreement between experimental and calculated results.
A Modified "Hole" Theory for Solute Impurity Diffusion in Liquid Metals
J.R. CAHOON
A modified ``hole'' theory for solute impurity diffusion in liquid metals is presented. The theory calculates an "effective" valence for the solute impurities by assuming that the bulk valences of the solvent and solute metals are those given by the Engel Brewer theory (one for bcc structures, two for cph structures, and three for fcc structures). The effects of balancing the Fermi energy level of the solute with that of the solvent and differences in zero point energy are then considered in calculating the effective valence. The effective valence is then used to calculate a value of diffusional activation energy from a modified Lazarus LeClaire approach. It is shown that the proposed theory works very well for solute impurity diffusion of a number of solutes in Al, Cu, Ga, Ag, Pb, and Sn, but the paucity of diffusion data is a hindrance to development of theoretical models.
The Effect of SiC Particle Reinforcement on the Creep Behavior of 2080 Aluminum
P.E. KRAJEWSKI, J.E. ALLISON, and J.W. JONES
The influence of SiC particle reinforcement on the creep behavior of 2080 aluminum is investigated between 150°C and 350°C. The effect of particle size (F-280, F-600, and F-1000), volume fraction (10, 20, and 30 vol pct), and heat treatment (T6 and T8) on creep behavior is studied. In both the T6 and T8 conditions all composites are less creep resistant than similarly heat-treated monolithic materials when crept at 150°C. These results contradict continuum mechanics predictions for steady-state creep rate, which predict composite strengthening. A high dislocation density is observed near SiC particles. It is proposed that strain localization near the reinforcements leads to microstructural breakdown and the subsequent reduction in creep resistance. When both materials are severely overaged or when they are tested at very high temperatures (350°C), composite materials exhibit improved creep resistance relative to monolithic material. In these cases, the strengthening is consistent with continuum predictions for direct composite strengthening.
Effect of Alloying Additions on Secondary Hardening Behavior of Mo-Containing Steels
H. KWON, C.M. KIM, K.B. LEE, H.R. YANG, and J.H. LEE
The effect of alloying additions on secondary hardening behavior in Fe-Mo-C steels has been investigated by means of the successive alloying additions of Cr, Co, and Ni. The Cr additions promote M3C cementite formation. The Ni additions destabilize the cementite formation, while the Co additions retard dislocation recovery and present the necessary sites for M2C formation which provides the secondary hardening.
Quasi-Steady-State Creep Crack Growth in a 3.5NiCrMoV Steel
S.H. RYU, JIN YU, and S.H. HONG
Creep crack growth rate (å) is usually characterized in terms of acroscopic load parameters, such as C*, Ct and C(t), through the constant load test. However, load parameters are continuously changing during the test, and so is å. Here, by conducting constant Ct and constant tests, quasi-steady-state crack growth was obtained where å remained almost constant. Results indicate the å~[Ct]0.76 correlation, which differ from the å~[Ct]0.96 correlation of the constant load test. Discrepancies can be ascribed to the inclusion of the stage II data, which showed no correlation between å and Ct, in the constant load analysis. Finally, the crack growth rate was well predicted using the Monkmam Grant analysis in creep crack growth.
Microstructural Modification of Plain Carbon Steels Irradiated by High-Energy Electron Beam
DONGWOO SUH, SUNGHAK LEE, and YANGMO KOO
The main objective of the present study is to analyze the microstructural modification of the surface hardened by the irradiation of high-energy electron beam in 0.18 pct C and 0.38 pct C plain carbon steels. Steel samples were irradiated using an electron accelerator (1.4 MeV), and the detailed microstructures of the irradiated surface were examined. Upon irradiation, the ferrite-pearlite structure near the sample surface was changed to the dual-phase structure, i.e., ferrite and martensite, and fine particles or needlelike lamellae were observed in the ferrite/martensite interface. In order to investigate these complex microstructures as well as the martensitic transformation mechanism, the simulation test, including thermal cycles of abrupt heating and quenching, was carried out. The test results indicated that the irradiated surface was heated up to about 1100°C and then quenched to room temperature, which was enough to obtain the surface hardening through martensitic transformation. Thermal analysis of the irradiated surface was also carried out for systematic understanding of the microstructural modification in terms of the irradiation parameters such as beam travel speed.
Effect of Phosphorus on the Microstructure and Stress Rupture Properties in an Fe-Ni-Cr Base Superalloy
W.R. SUN, S.R. GUO, D.Z. LU, and Z.Q. HU
This article describes the effect of phosphorus on the microstructure and stress rupture property at 650°C in an Fe-Ni-Cr base superalloy. The results showed that phosphorus markedly improved the intergranular precipitation in the range of 0.0005 to 0.016 wt pct, which facilitated M23C6 and M3B2 precipitation but inhibited the formation of MC carbide. A too high phosphorus addition (0.051 wt
pct P) resulted in an excessive precipitation at grain boundaries, while a too low phosphorus content (0.0005 wt pct P) led to many precipitate-free grain boundaries. Phosphorus also enlarged the size of the ' particles and lowered its stability, that -Ni3Ti preferred to form in the alloy with 0.051 wt pct P. Due to the improvement of the microstructure, appropriate amount of P content significantly prolonged the rupture life of the alloys in the range of 0.0005 to 0.016 wt pct. The peak value was 660 hours at 0.016 wt pct, more than 4 times that of the alloy with 0.0005 wt pct phosphorus, but phosphorus reduced the fracture elongation. The mechanism by which phosphorus influenced the alloy is discussed.
Hydrogen Embrittlement of Ni-Cr-Fe Alloys
D.M. SYMONS
The purpose of this work was to investigate the role of chromium on hydrogen embrittlement of Ni-Cr-Fe alloys and thus to develop a better understanding of the low-temperature stress corrosion cracking (SCC) phenomenon. The effect of chromium on hydrogen embrittlement was examined using tensile tests followed by material evaluation via scanning electron microscopy (SEM) and light optical microscopy. Four alloys were prepared with chromium contents ranging from 6 to 35 wt pct. In the uncharged condition, ductility, as measured by the percent elongation or reduction in area, increased as the alloy chromium content increased. Hydrogen appeared to have only minor effects on the mechanical properties of the low-chromium alloys. The addition of hydrogen had a marked effect on the ductility of the higher-chromium alloys. In the 26 pct chromium alloy, the elongation to failure was reduced from 53 to 14 pct, with a change in fracture mode from mixed ductile dimple and ductile intergranular failure to a brittle appearing intergranular failure. A maximum in embrittlement was observed in the 26 pct Cr alloy. The maximum in embrittlement coincided with the minimum in stacking-fault energy. It is proposed that the increased hydrogen embrittlement in the high-chromium alloys is due to increased slip planarity caused by the lower stacking-fault energy. Slip planarity did not appear to affect the fracture of the uncharged specimens.
Crystallographic Study of Fatigue Cracking in Ni3Al(CrB) Single Crystal
G.P. ZHANG, Z.G. WANG, G.Y. LI, and S.D. WU
The effect of crystallographic orientation on the fatigue-crack initiation and propagation in Ni3Al(CrB) single crystal was studied using a compact-tension specimen. Stage I crystallographic cracking and cleavage fracture were observed. Crystallographic cracking can occur on two or more {111} slip planes simultaneously. It was shown that the threshold stress intensity for crack initiation from the notch root exhibits a dependence on crystallographic orientation. In addition, an effect of orientation on microcracking behavior was also shown. The number of {111} planes intersecting with each other determines the different microscopic features on the cleavage fracture surface.
A New Nickel-Aluminum Bronze Alloy with Low Magnetic Permeability
P. WENSCHOT
This article reports on the development of a nickel-aluminum bronze alloy with a low magnetic permeability in combination with good mechanical properties, a high corrosion resistance, and corrosion-fatigue strength. Investigations have revealed that the ferromagnetism of the iron-rich K phase can be reduced by use of an addition while maintaining a fine grain structure. This has resulted in a new nickel-aluminum bronze alloy with low magnetic properties. The properties of this new alloy have been measured and are compared with those of commercial nickel-aluminum bronzes.
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