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Volume 27A, No. 5, May 1996 This Month Featuring: Transformations; Mechanical Behavior; Environment; Surface Treatment; Solidification; Materials Processing and Composite Materials. View May 1996 Table of Contents.
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Precipitation Behavior in a Medium Carbon, Ti-V-N Microalloyed Steel
M. PRIKRYL, A. KROUPA, G.C. WEATHERLY, and S.V. SUBRAMANIAN
The precipitation behavior of a medium carbon steel microalloyed with Ti, V, and N has been studied by analytical transmission electron microscopy in the as-cast and isothermally heat-treated states, as well as at different stages in the thermomechanical processing of the steel. Mixed (Ti,V) nitrides were found in all the structures, but there was no evidence for mixed carbonitride formation. The Hillert-Staffansson model was used to predict the composition of the nitrides as a function of temperature. Upon prolonged aging many of the precipitates became ``fragmented'' and were no longer single crystals. At the same time, the volume fraction of precipitates dropped, while their average Ti content increased. Possible explanations for this unexpected behavior are discussed in this article.
Microstructural Stability on Aging of an 
+ 
Titanium Alloy: Ti-6Al-1.6Zr-3.3Mo-0.30Si
A.K. SINGH, T. ROY, and C. RAMACHANDRA
The development of the microstructure on aging of an (
+ 
) type titanium alloy containing 6Al-1.6Zr-3.3Mo-0.3Si (VT9) (in weight percent) has been studied. The -transus temperature of this alloy is approximately 1243 K. Solution treatment in the -phase field of the alloy followed by quenching in water at room temperature resulted in the formation of a single-phase martensite structure. The martensitic structure was confirmed to be orthorhombic ('') using X-ray diffraction. The water-quenched (WQ) specimens were subjected to aging treatments at temperatures of 823, 873, and 973 K for various lengths of time. Aging at 823 K for times between 24 and 100 hours did not bring about any noticeable change in the microstructure. Aging at 823 K for 200 and 300 hours resulted in the heterogeneous precipitation of s2 silicide particles and thin films of sandwiched between the interplatelet boundaries of martensite. Electron diffraction analysis confirms that the crystal structure of silicide particles is hexagonal with lattice parameters a = 0.70(1) nm and c = 0.36(8) nm. Aging at 873 K for 12 and 24 hours resulted only in the precipitation of s2 silicide particles, while aging at the same temperatures for longer times (48, 100, and 200 hours) and also at 973 K for 6 to 100 hours resulted in the precipitation of silicides and also thin films of and acicular martensite. The relative sizes of silicide precipitates and width of thin films of phase increase with increasing aging time. The sites for silicide precipitation are mainly at '-' boundaries, - interfaces, and sometimes within regions of transformed . The kinetics of s2 silicide precipitation in this alloy is faster than in commercial near- titanium alloys. This is attributed to the presence of Mo, a strong stabilizer.
Electron Microscopic Study of Cr2N Formation in Thermally Aged 316LN Austenitic Stainless Steels
D. SUNDARARAMAN, P. SHANKAR and V.S. RAGHUNATHAN
The formation of dichromium nitride phase in low nitrogen austenitic stainless steels has been studied by transmission electron microscopy (TEM). The electron diffraction patterns unambiguously confirm the formation of Cr2N phase on aging these steels in the range of 1023 to 1123 K, for time durations up to 100 hours. It is found that while no perceptible microstructural features could be recognized, formation of dislocation pairs is an important characteristic that could be associated with the nitrogen ordering in the matrix. The precipitation sequence processes have been discussed on the basis of stress-induced interactions that are predominant in interstitial alloys. Further, certain aspects of mechanical behavior are explained on the basis of our study.
Computer Simulation of Reversible Martensitic Transformations
PING XU and J.W. MORRIS, JR.
This article reports the results of computer simulation studies of reversible, athermal martensitic transformations in idealized, two-dimensional crystals. The transformation is accomplished by sequentially transforming elementary cells. The model accounts for the elastic strain developed during the transformation, assuming homogeneous elastic constants, negligible interfacial tension, and no external stress. The effects of frictional resistance to the transformation and plastic relaxation of the elastic strain are included in a simple way. The model is used to study the sources of hysteresis in the temperature-transformation (TT) curve and in the microstructural transformation path when the transformation is reversed. The central result is that some hysteresis is inevitable in a transformation of the type studied here. Even in the absence of friction and plastic relaxation, the transformation follows a path in which sequential elements of martensite relax the elastic strain of those that have previously formed. This causes the martensite to form in bursts and has the consequence that the reverse transformation does not reverse the path of the forward transformation. Friction and plastic relaxation increase hysteresis.
A Comprehensive Dynamical Study of Nucleation and Growth in a One-Dimensional Shear Martensitic Transition
B.P. VAN ZYL and R.J. GOODING
We have constructed a complete hydrodynamic theory of nucleation and growth in a one-dimensional (1-D) version of an elastic shear martensitic transformation with open boundary conditions, where we have accounted for interfacial energies with strain-gradient contributions. We have studied the critical martensitic nuclei for this problem: Interestingly, the bulk critical nuclei are twinned structures, although we have determined that the dominant route for the formation of martensite is through surface nucleation. We have analytically solved for the surface nuclei and evaluated exact nucleation rates showing the strong preference for surface nucleation. We have also examined the growth of martensite: There are two possible martensitic growth fronts, viz., dynamical twinning and so-called two-kink solutions. These transformation fronts are separated by a dynamical phase transition. We analytically derive this phase diagram and determine expressions for the speeds of the martensitic growth fronts.
MECHANICAL BEHAVIOR
Creep Deformation of Dispersion-Strengthened Copper
S.E. BROYLES, K.R. ANDERSON, J.R. GROZA and J.C. GIBELING
The creep behavior of an internally oxidized, Al2O3 dispersion-strengthened copper alloy, GlidCop Al-15, has been investigated in the temperature range of 745 to 994 K. The results exhibit a high apparent stress exponent (10 to 21) and a high apparent activation energy for creep (253.3 kJ/mole). To describe the creep behavior of this alloy, the Rösler-Arzt model for attractive particle/dislocation interaction is applied. The results are in good agreement with the model when account is taken of the effects of the fine elongated grains and heavily dislocated structures revealed through transmission electron microscopy. The analysis demonstrates that the dislocation/particle interaction is of moderate strength in this alloy, consistent with the observation that the particle/matrix interface is partially coherent. In addition, the analysis reveals that the choice of mechanism and corresponding activation energy for vacancy diffusion has only a small effect on the calculated model parameters. It is argued that the weak dependence of subgrain size on stress demonstrates that creep deformation is particle controlled, rather than subgrain size controlled. In addition, the poorly developed subgrain structure and high dislocation densities are attributed to the presence of the fine oxide particles. Finally, the dependence of rupture time on stress is shown to be consistent with a description of creep fracture based on diffusive cavity growth with continuous nucleation.
High-Temperature Deformation Properties of NiAl Single Crystals
K.R. FORBES, U. GLATZEL, R. DAROLIA, and W.D. NIX
The high-temperature deformation properties of stoichiometric NiAl single crystals have been studied in the temperature range from 850°C and 1200°C. We have established a basic data set for and have explored the high-temperature deformation characteristics of this intermetallic compound. The results provide a basis for determining the controlling mechanisms of high-temperature deformation. Constant stress tension creep and constant stress or constant strain rate compression experiments were conducted on crystals oriented with loading axes along the ``hard,'' [001] orientation, where no driving force exists for glide of b = <001> dislocations, and along various ``soft'' orientations, [223], [111], and [110], where deformation can occur by the glide of these dislocations. In addition to these monotonic tests, high-temperature deformation transients were studied using stress relaxation, strain rate change, and stress change experiments. These transient deformation experiments were conducted in an effort to further elucidate the mechanisms that control high-temperature deformation of this material. The steady-state deformation properties of these differently oriented single crystals can be characterized by creep activation energies that all coincide, within experimental error, with the activation energy for diffusion of Ni in NiAl, 308 ± 10 kJ/mol. The stress dependence of steady-state deformation can be characterized with stress exponents that range from about 9 at 850°C to about 4 at 1200°C. At all temperatures and stresses, the soft oriented crystals creep about two orders of magnitude faster than the hard oriented crystals at the same stresses and temperatures. Soft oriented crystals loaded along [223] and [111] axes tested in both tension creep and constant stress or constant strain rate compression are found to deform at the steady-state rate from the very beginning of the deformation experiment. Crystals with these orientations exhibit virtually no evidence of strain hardening. Transients associated with stress changes suggest that deformation is limited primarily by the mobility of dislocations and not by dislocation interactions. These characteristics of deformation are consistent with the operation of easy b = <001> glide processes in these crystals. Crystals loaded along [110] exhibit small deformation transients which indicate both sluggish dislocation motion and some substructure formation. We speculate that cross-slip of dislocations from {110} to {010} planes is responsible for this effect. Deformation in hard oriented crystals provides evidence for both mobility and substructure controlled deformation. Creep in hard oriented crystals is characterized by a dramatic sigmoidal transient suggesting very low dislocation mobility. However, the strain hardening observed in monotonic tests and the transient responses suggest that deformation is also limited by a dislocation substructure that forms during deformation. These findings support the conclusion, explored fully in a forthcoming article, that creep deformation in the hard orientation is controlled by the motion and interaction of b = <101> dislocations.
Analysis and Prevention of Vertical Cracking Phenomena During Deep Drawing of Hot-Rolled SG295 Steel Strips
SUNGHAK LEE, CHANG GIL LEE, DONGIL KWON, SUNG-HO PARK, and NACK J. KIM
Microscopic examination and microstructural analysis of vertical cracking phenomena in deep-drawn cups of hot-rolled SG295 steel strips were carried out in this study. Microvoids initiated preferentially at grain-boundary carbides were observed to form intergranular cracks. These grain-boundary carbides were identified as (Fe,Mn)3C carbides. The morphology of carbides was varied with processing variables, e.g., killing method and coiling temperature. In the Al-killed steels, the carbide shape tends to change from film type to bulk type, which may be beneficial to elongation and consequently formability. In addition, as the coiling temperature increased, the amount of banded pearlite structures and bulk-type carbides increased. These findings suggest that the deep drawability can be improved by modifying the carbide morphology in the microstructures.
Flow Stress and Microstructural Evolution During Hot Working of Alloy 22Cr-13Ni-5Mn-0.3N Austenitic Stainless Steel
M.C. MATAYA, C.A. PERKINS, S.W. THOMPSON, and D.K. MATLOCK
The stress-strain behavior and the development of microstructure between 850°C and 1150°C in an austenitic stainless steel, 22Cr-13Ni-5Mn-0.3N, were investigated by uniaxial compression of cylindrical specimens at strain rates between 0.01 and 1 s-1 up to a strain of one. The measured (anisothermal) and corrected (isothermal) flow curves were distinctly different. The flow stress at moderate hot working temperatures, compared to a number of other austenitic alloys, was second only to that of alloy 718. Both static and dynamic recrystallization were observed. Recrystallization was sluggish in comparison to alloy 304L, apparently due to the presence of a fine Cr- and Nb-rich second-phase dispersion, identified as Z phase, which tended to pin the high-angle grain boundaries even at a high temperature of 1113°C. Recrystallization may also be retarded by preferential restoration through the competitive process of recovery, which is consistent with the relatively high stacking-fault energy for this alloy. It is concluded that this alloy must be hot worked at temperatures higher than usual for austenitic stainless steels in order to minimize flow stress and refine grain size.
Prediction of Fatigue Crack Formation in 304 Stainless Steel
SHAUN M. McGUIRE and MORRIS E. FINE
Strain-controlled fatigue tests have been conducted on center-holed 304 stainless steel specimens. The fraction of total fatigue life spent until formation of an ``engineering'' crack ranged from about 15 to 85 pct, indicating the potential importance of being able to predict the fatigue crack formation life. A ``just formed engineering crack,'' as defined here, is a through crack long in the thickness direction, which has just emerged from the center hole. An energy based parameter, 
t, has been shown to correlate with the appearance of fatigue cracks in the center-holed 304 stainless steel specimens. This parameter is suggested to be more useful in predicting fatigue crack formation life than or t alone. A good correlation was found over the limited range of data for two types of 304 stainless steel, a powder metallurgy (PM) stainless steel with higher than normal strength properties and an ingot metallurgy (IM) stainless steel with normal strength properties. A better correlation was found for strain-controlled fatigue tests which did not go into compressive strain than for completely reversed fatigue.
The Normalized Coffin-Manson Plot in Terms of a New Damage Function Based on Grain Boundary Cavitation Under Creep-Fatigue Condition
SOO WOO NAM, YOUNG CHEOL YOON, BAIG GYU CHOI, JE MIN LEE, and JIN WAN HONG
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.
A Study of the Influence of Mischmetal Additions to Al-7Si-0.3Mg (LM25/356) Alloy
M. RAVI, U.T.S. PILLAI, B.C. PAI, A.D. DAMODARAN, and E.S. DWARAKADASA
This article deals with the effect of 0.25-1.5 wt pct mischmetal (MM) addition on the mechanical properties, microstructure, electrical conductivity, and fracture behavior of cast Al-7Si-0.3Mg (LM 25/356) alloy. Modification of eutectic silicon by MM is compared with strontium modification in terms of microstructure, mechanical properties, and fading behavior. Loss of magnesium encountered on holding the molten alloy and its resultant effect on mechanical properties of alloys modified with MM and Sr are compared with those in the unmodified alloy.
Effect of Strontium Modification on Near-Threshold Fatigue Crack Growth in an Al-Si-Cu Die Cast Alloy
M. SCHAEFER and R.A. FOURNELLE
The effects of strontium modification on microstructure and fatigue properties in a die cast commercial aluminum-silicon alloy are demonstrated. Strontium additions of 0.010 and 0.018 wt pct drastically change the morphology of the eutectic silicon. The influence of these microstructural changes on fatigue properties is evaluated through fatigue crack growth testing. Examination of the fracture surfaces and the crack path establish distinct fatigue fracture modes for the modified and unmodified eutectic structures. Changes in fracture mode and crack path are correlated to the microstructure changes. A higher energy fracture mode and increased crack path tortuosity explain the observed improvement in fatigue properties for the modified alloys. Strontium modified alloys exhibit a 10 to 20 pct higher fatigue crack growth threshold compared to an unmodified alloy for testing at a load ratio of 0.5. No difference was observed for testing at a load ratio of 0.1.
Effect of Strain Rate and Temperature on the Flow Stress of -Phase Titanium-Hydrogen Alloys
O.N. SENKOV and J.J. JONAS
Compression tests were carried out on seven titanium-hydrogen alloys containing hydrogen concentrations up to 31 at. pct. All the experiments were performed within the -phase field at strain rates of 0.001 to 1.0 s-1. The dependences of the steady-state flow stress on strain rate, temperature, and hydrogen concentration were determined. The strain rate sensitivity increases with temperature but decreases with hydrogen concentration. The experimental activation energy of deformation decreases when the flow stress or strain rate is increased. At a fixed strain rate, it decreases when the hydrogen concentration is increased. However, when measured at a fixed steady-state stress, the activation energies are nearly the same for all the alloys. The steady-state flow stress increases with hydrogen concentration as can be expressed by both linear and quadratic dependences. The flow behavior of the alloys can also be described in terms of thermally activated glide and the relation
= K -4 exp 
exp 
where the constants v and H0 are independent of hydrogen concentration, while the parameter K decreases exponentially when the hydrogen concentration is increased.
Communication: Mechanical Properties of Ru-Ni-Al Alloys
I.M. WOLFF and G. SAUTHOFF
Communication: On Microsuperplasticity in AA7475 Domes
M.G. ZELIN
ENVIRONMENT
Studies on the Influence of Metallurgical Variables on the Stress Corrosion Behavior of AISI 304 Stainless Steel in Sodium Chloride Solution Using the Fracture Mechanics Approach
H.S. KHATAK, J.B. GNANAMOORTHY, and P. RODRIGUEZ
Stress corrosion data on a nuclear grade AISI type 304 stainless steel in a boiling solution of 5M NaCl+ 0.15M Na2SO4+ 3 mL/L HCl (bp 381 K) for various metallurgical conditions of the steel are presented in this article. The metallurgical conditions used are solution annealing, sensitization, 10 pct cold work, 20 pct cold work, solution annealing + sensitization, 10 pct cold work + sensitization, and 20 pct cold work + sensitization. The fracture mechanics approach has been used to obtain quantitative data on the stress corrosion crack growth rates. The stress intensity factor, Kl, and J integral, Jl, have been used as evaluation parameters. The crack growth rates have been measured using compact tension type samples under both increasing and decreasing stress intensity factors. A crack growth rate of 5 x 10-11 m/s was chosen for the determination of threshold parameters. Results of the optical microscopic and fractographic examinations are presented. Acoustic signals were recorded during crack growth. Data generated from acoustic emissions, activation energy measurements, and fractographic features indicate hydrogen embrittlement as the possible mechanism of cracking.
Stress Corrosion Cracking of Pressure Vessel Steels in High-Temperature Caustic Aluminate Solutions
SU'E LIU, ZIYONG ZHU, HUI GUAN, and WEI KE
Stress corrosion cracking (SCC) behavior of three kinds of low alloy pressure vessel steels in high-temperature (200°C to 300°C) caustic aluminate (AlO2-) solutions has been studied by slow strain rate tests (SSRT). The results indicate that these pressure vessel steels are susceptible to SCC in caustic aluminate solution and that the SCC susceptibility increases with increasing temperature between 200°C to 300°C. Sulfide content and stringered sulfide inclusions severely and anisotropically affect the caustic SCC of these low alloy steels. The inclusions in the rare-earth-treated steel are predominantly globular rare-earth sulfides or oxysulfides, resulting in improved transverse properties. The effect of inclusions on SCC behavior correlates with the projected area of inclusions per unit volume at the crack tip, Av, on the plane perpendicular to the tensile direction. The susceptibility to SCC increases with increasing Av.
SURFACE TREATMENT
Corrosion Fatigue in Nitrocarburized Quenched and Tempered Steels
M. KARIM KHANI and D. DENGEL
In order to investigate the fatigue strength and fracture mechanism of salt bath nitrocarburized steels, specimens of the steels SAE 4135 and SAE 4140, in a quenched and tempered state, and additionally in a salt bath nitrocarburized and oxidizing cooled state as well as in a polished (after the oxidizing cooling) and renewed oxidized state, were subjected to comparative rotating bending fatigue tests in inert oil and 5 pct NaCl solution. In addition, some of the quenched and tempered specimens of SAE 4135 material were provided with an approximately 50-µm-thick electroless Ni-P layer, in order to compare corrosion fatigue behavior between the Ni-P layer and the nitride layers. Long-life corrosion fatigue tests of SAE 4135 material were carried out under small stresses in the long-life range up to 108 cycles with a test frequency of 100 Hz. Fatigue tests of SAE 4140 material were carried out in the range of finite life (low-cycle range) with a test frequency of 13 Hz. The results show that the 5 pct NaCl environment drastically reduced fatigue life, but nitrocarburizing plus oxidation treatment was found to improve the corrosion fatigue life over that of untreated and Ni-P coated specimens. The beneficial effect of nitrocarburizing followed by oxidation treatment on corrosion fatigue life results from the protection rendered by the compound layer by means of a well-sealed oxide layer, whereby the pores present in the compound layer fill up with oxides. The role of inclusions in initiating fatigue cracks was investigated. It was found that under corrosion fatigue conditions, the fatigue cracks started at cavities along the interfaces of MnS inclusions and matrix in the case of quenched and tempered specimens. The nitrocarburized specimens, however, showed a superposition of pitting corrosion and corrosion fatigue in which pores and nonmetallic inclusions in the compound layer play a predominant role concerning the formation of pits in the substrate.
Transmission Electron Microscopy Study on the Cross-Sectional Microstructure of an Ion-Nitriding Layer
XIAOLEI XU, LIANG WANG, ZHIWEI YU, and ZUKUN HEI
The cross-sectional microstructure of an ion-nitrided layer on an A3 steel (C = 0.15, Si = 0.2, Mn = 0.5, and balance Fe, in wt pct) was studied by transmission electron microscopy (TEM), and its electron diffraction patterns were analyzed. It has been shown that the compound layer consists of columnar -Fe2-3N and 
'-Fe4N. The former precipitates thin '-Fe4N phases related to each other by a 180 deg turning twin, and the orientation relationship between -Fe2-3N and '-Fe4N is
' // (0001) 
, <
> ' // <
>
The columnar '-Fe4N has a stacking fault substructure and accompanying lattice distortion. There are islandlike ferrite crystals between the columnar crystals. Near the compound layer is mostly '-Fe4N with a coexisting small amount of -Fe. The diffuse layer is composed of Guinier-Preston (GP) zones, ''-Fe16N2, '-Fe4N, and -Fe. The '-Fe4N with long period structure considered as the ordering of the stacking fault was found. The characters and transformation mechanism of the case were discussed.
SOLIDIFICATION
Macrosegregation During Dendritic Arrayed Growth of Hypoeutectic Pb-Sn Alloys: Influence of Primary Arm Spacing and Mushy Zone Length
S.N. TEWARI and R. SHAH
Thermosolutal convection in the dendritic mushy zone occurs during directional solidification of hypoeutectic lead tin alloys in a positive thermal gradient, with the melt on the top and the solid below. This results in macrosegregation along the length of the solidified samples. The extent of macrosegregation increases with increasing primary dendrite spacings for constant mushy zone length. For constant primary spacings, the macrosegregation increases with decreasing mushy zone length. Presence of convection reduces the primary dendrite spacings. However, convection in the interdendritic melt has significantly more influence on the spacings as compared with that in the overlying melt, which is caused by the solutal buildup at the dendrite tips.
MATERIALS PROCESSING
Rapid Solidification Processing of a Mg-Li-Si-Ag Alloy
A. MATSUDA, C.C. WAN, J.-M. YANG, and W.H. KAO
A Mg-13Li-4Si-1Ag (wt pct) alloy with improved ductility and thermal stability was developed via the rapid solidification (RS) processing technique. Silicon was added to the alloy as the third alloying element in order to form a thermally stable intermetallic dispersoid phase required for improved mechanical properties at ambient and elevated temperatures. The microstructure of the as-spun and heat-treated alloy was characterized using differential scanning calorimetry (DSC), X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Microhardness measurements were conducted on as-spun and heat-treated alloy in order to obtain qualitative property data and to investigate the extent of the degradation of properties at elevated temperatures. It was found that the melt-spun Mg-Li alloy possessed a microstructure consisting of a fine dispersion of Mg2Si phase in a fine-grained body-centered cubic (bcc) Mg-Li solid solution, resulting in the desired improvements in thermal stability and mechanical properties.
Tensile Properties of Mechanically Alloyed/Milled ODS-Ni-Based Alloys
J. ZBIRAL
Oxide dispersion strengthened (ODS) NiCr20 alloys were produced by ball milling blends of either prealloyed NiCr20 and yttria powders or elemental Ni and Cr powders and yttria powder. After milling, the powders were degassed in a vacuum furnace, sealed in steel cans, and consolidated by hot extrusion. The mechanical alloying process, which occurs during ball milling of elemental Ni and Cr powders, as well as the changes of microstructure, which occur during milling and extrusion, were characterized by X-ray diffraction (XRD) techniques. Measurements of microhardness, tensile strength, and elongation of extruded bars were done to gain information about the dispersoid partitioning and about the relation between milling parameters, microstructure, and tensile properties. These investigations showed that the attainable quality of ODS NiCr20 alloys is higher if they are produced by milling elemental Ni and Cr powders and yttria powders. Besides the dispersoid partitioning, the homogeneity of the mechanically alloyed powder strongly affects the quality. High-quality materials are only produced if the ball milling process yields a homogeneous dispersoid partitioning and a completely mechanically alloyed NiCr20 solid solution.
COMPOSITE MATERIALS
Interface Characterization of Ceramic Fiber-Reinforced Ti Alloy Composites Manufactured by Infrared Processing
S.G. WARRIER, S.H. CHEN, S.K. WU, and R.Y. LIN
Interfacial reactions between several ceramic fibers (SCS-0, SCS-6, and carbon fibers) and a liquid titanium-nickel-copper alloy were investigated using electron microscopic analysis. Composite specimens were produced using a rapid infrared manufacturing (RIM) process. In SCS-0/Ti alloy composites, SiC dissolved in the alloy. The main reaction product was discontinuous agglomerates of titanium carbide which formed from the reaction between dissolved carbon and titanium. Polygonal precipitates of Ti5Si3, which are believed to have formed during cooling, were also noticed. Two distinct interface morphologies were observed in these composites: uniform fronts caused by isothermal dissolution and scalloped fronts formed as a result of an accelerated dissolution mechanism caused by localized heating. The presence of the accelerated dissolution mechanism suggests that SiC fibers cannot be infiltrated with liquid alloy to form a continuous layer of TixC1-x. Further growth of this layer occurred by the diffusion of carbon atoms across the reaction. In SCS-6/Ti alloy composites, free carbon present in the coating formed a discontinuous layer of TixC1-x, whereas SiC particles dissolved in the alloy. Due to channeled dissolution in the coating, the accelerated dissolution mechanism was not observed in these composites. As a result, the presence of the carbon-rich coating prevented degradation of the fibers. Although the coating present on SCS-6 fibers moderately retarded reactions in the SiC/Ti alloy composite system during infrared liquid infiltration, it is recommended that the fibers be coated with pure carbon to effectively limit the attack of the fiber by molten titanium.
Communication: Thermal Stability of SiC-SCS-6 Fiber-Reinforced IMI834 Alloys
H.J. DUDEK, R. LEUCHT, and J. HEMPTENMACHER
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