| METALLURGICAL AND MATERIALS TRANSACTIONS A | |
|---|---|
 |
|
Volume 29A, No. 6, June 1998 This Month Featuring: The 1997 Edward DeMille Campbell Memorial Lecture; Alloy Phases; Transformations; Transport Phenomena; Mechanical Behavior; Environment; Surface Treatment; Solidification; Materials Processing; Composite Materials. View June 1998 Contents.
|
|
The activity of platinum and the free energy, enthalpy, and entropy of mixing are derived. Activities exhibit moderate negative deviation from Raoult's law. The mixing properties can be represented by a pseudosubregular solution model in which excess entropy has the same type of functional dependence on composition as the enthalpy of mixing,
H = XRh (1 - XRh)[-10,970 + 45XRh]J/mol
SE = XRh (1 - XRh)[-3.80 + 1.55 X 10-2 XRh]J/mol·KThe negative enthalpy of mixing obtained in this study is in qualitative agreement with predictions of semiempirical models of Miedema and co-workers and Colinet et al. The results of this study do not support the solid-state miscibility gap suggested in the literature, but are consistent with liquidus data within experimental uncertainty limits.
Microstructural Characterization of 5 to 9 pct Cr-2 pct W-V-Ta Martensitic Steels
R. JAYARAM and R.L. KLUEH
The microstructure of 9Cr-2W-0.25V-0.1C (9Cr-2WV), 9Cr-2W-0.25V-0.07Ta-0.1C (9Cr-2WVTa), 7Cr-2W-0.25V-0.07Ta-0.1C (7Cr-2WVTa), and 5Cr-2W-0.25V-0.07Ta-0.1C (5Cr-2WVTa) steels (all compositions are in wt pct) have been characterized by analytical electron microscopy (AEM) and atom probe field ion microscopy (APFIM). These alloys have potential applications in fusion reactors because they exhibit reduced neutron activation in comparison to the conventional Cr-Mo steels. The matrix in all four alloys was 100 pct martensite. The precipitate type in the steels depended primarily on the chromium level in the alloy. In the two 9Cr steels, the stable phases were blocky M23C6 and small spherical precipitates previously identified as MC. The two lower-chromium steels contained blocky M7C3 and small needle-shaped carbonitrides in addition to M23C6. The AEM and APFIM analyses revealed that, in the steels containing tantalum, the majority of the tantalum was in solid solution. With the exception of a few of the small spherical precipitates in low-number densities in the 9Cr-2WVTa, none of the other precipitates contained measurable tantalum. The experimentally observed phases were in agreement with those predicted by phase equilibria calculations using the ThermoCalc software. However, a similar match between the experimental and predicted values of the phase compositions did not occur in some instances. Atom probe analyses directly confirmed the crucial role of trace amounts of nitrogen in the formation of vanadium-rich carbonitrides as predicted by thermodynamic equilibrium calculations.
F-Type Icosahedral Phase and a Related Cubic Phase in the Al-Rh-Cu System
X.Z. LI, K.-T. PARK, K. SUGIYAMA and K. HIRAGA
An F-type icosahedral phase and a related cubic phase (composition of Al66.1Rh21.5Cu12.3, lattice constant a = 1.5380(2) nm, and space group of Fm3) were observed in the Al63Rh18.5Cu18.5 alloy by transmission electron microscopy (TEM). The structure of the Al-Rh-Cu cubic phase was determined by single-crystal X-ray analysis. A high-resolution electron microscopic image of the Al-Rh-Cu cubic phase is presented together with a simulated image. The structure of the cubic phase can be described by two types of atom clusters, which have outer shells with icosahedral symmetry. It is suggested that the structure of the Al-Rh-Cu cubic phase is helpful for understanding the structure of the i-Al-Rh-Cu F-type icosahedral quasicrystal.
Structural Models of 
2-Inflated Monoclinic and Orthorhombic Al-Co Phases
Z.M. MO, H.X. SUI, X.L. MA, and K.H. KUO
A structural model of the C-centered monoclinic Al3Co, previously called 
2-Al13Co4 (a = 3.984 nm, b = 0.8148 nm, and c = 3.223 nm; 
= 107.97 deg), with a and c parameters about 2 times ( = (1 + 
5/2 = 1.61803...) larger than the corresponding ones in the C-centered monoclinic Al13Co4 (a = 1.5183 nm, b = 0.8122 nm, and c = 1.2340 nm; = 107.90 deg), has been derived from its high resolution electron microscopy (HREM) image and the known structure of the monoclinic Al13Co4. The simulated [010] HREM image and electron diffraction pattern agree reasonably well with the experimental results. Based on this structural model, the powder X-ray diffraction pattern was indexed. Furthermore, a structural model of the newly found 2-inflated orthorhombic phase (a = 3.79 nm, b = 0.81 nm, and c = 3.22 nm) was also proposed.
A New Equation for the Cr Equivalent in 9 to 12 Pct Cr Steels
S.H. RYU and JIN YU
In advanced 9 to 12 pct Cr steels, the Cr equivalent is used as a measure to check the formation of 
-ferrite. In the present analysis, 29 alloys of varying composition were vacuum induction melted, and the amounts of -ferrite were measured in as-tempered conditions. Based on this and previous results on 9 to 12 pct Cr steels, a new equation for the Cr equivalent is proposed and correlated with the amount of -ferrite formation. Results indicate that the new Cr equivalent equation shows better correlation than previous equations and predicts the amount of -ferrite formed reasonably well.
Communication: On the Observation of a New Ternary MgSiCa Phase in Mg-Si Alloys
Y. CARBONNEAU, A. COUTURE, A. VAN NESTE, AND R. TREMBLAY
-Martensitic Transformation and its Reversion in the FeMnSiCrNi Shape-Memory Alloy
-martensite were systematically investigated using transmission electron microscopy. It is found that, in the overlapped region of the stacking faults, the nucleus of -martensite may form by shear and grow along the {111}
plane, which is not the stacking fault plane. The nucleus may grow into a small, thin plate and may act as the basal structure unit of -martensite. When the stress increases, the thin plates continue to grow into an -martensite ribbon along this preferred orientation and, during the subsequent recovery annealing, the reverse transformation of these basal structure units occurs. As a result, the -martensite plates shrink in both length and thickness.
Microstructural Evaluation of Ti-6-22-22 Alloy
A.W. WILSON and J.M. HOWE
In this study, the microstructure of Ti-6-22-22 alloy as a function of aging time and temperature was examined and related to its fracture behavior. Fracture primarily occurred along prior grain boundaries, but the morphology of the fracture surfaces varied from very smooth to rough and dimpled with increasing fracture toughness. Changes in the density of acicular 
, silicides, and ordering in were found to influence the fracture toughness. Ordering in the phase was found to reduce fracture toughness significantly, while an increased density of acicular reduced the toughness only slightly. The amount of partitioning of alloying elements was not found to correlate directly with toughness.
Damage Process in Commercially Pure 
-Titanium Alloy without (Ti40) and with (Ti40-H) Hydrides
J. HUEZ, X. FEAUGAS, A.L. HELBERT, I. GUILLOT, and M. CLAVEL
The influence of hydrostatic stress and hydrogen content on damage process was investigated in -titanium alloy. In particular, void site, void nucleation criterion, void nucleation, and void growth kinetics were experimentally determined. Void nucleation and growth rates and fracture process have been discussed using a Gurson-Tvergaard model. The main result is that void growth rate is not affected by hydride inclusions contrary to the void nucleation kinetic. In Ti40, the nucleation process has less influence than the growth process, which involves fracture. On the contrary, with regard to Ti40-H, the growth process is negligible and the nucleation kinetics, which is predominant, is the main cause of fracture.
Microstructure and Mechanical Behavior of Reaction Hot-Pressed Titanium Silicide and Titanium Silicide-Based Alloys and Composites
R. MITRA
Titanium silicide (Ti5Si3) and its composites show promise for applications at temperatures higher than 1000°C. Dense Ti5Si3 was processed by reaction hot pressing of a TiH2/Si powder mixture, which involved decomposition of TiH2 into Ti and H2 at around 800°C, a chemical reaction between Ti and liquid Si at 1500°C forming Ti5Si3 in situ, and densification under pressure. The use of fine TiH2 particles led to the formation of a relatively fine-grained microstructure with fewer microcracks and higher hardness and fracture toughness values than those expected for a coarse-grained Ti5Si3. The addition of 8 wt pct Al as an alloying element led to the formation of Al0.67Si0.08Ti0.25 and Al2O3 in situ and a solid solution of Al in Ti5Si3. Both alloying with Al and the addition of TiC as a reinforcement phase improved the room-temperature fracture toughness. Fracture toughness measurements were performed by three-point bend testing of single-edge notch bend (SENB) specimens, as well as by indentation techniques using different models, and the data have been compared. The role of different operating toughening mechanisms such as crack deflection, bridging, branching, and energy dissipation through microcracracking have been examined. The investigation has also shown that Ti5Si3 maintains a high yield strength value up to 1200°C.
Behavior and Rupture of Hydrided ZIRCALOY-4 Tubes and Sheets
F. PRAT, M. GRANGE, J. BESSON and E. ANDRIEU
The mechanical behavior and rupture mechanisms of ZIRCALOY-4 guide tubes and sheet containing 150 to 1200 wt ppm hydrogen have been investigated at room temperature. Sheets were notched to study the influence of geometrical defects on rupture. It is shown that hydrides strengthened the material, as maximum stresses sustained by the material are increased with increasing hydrogen contents. On the other hand, ductility is reduced. The material also exhibits a strong anisotropy due to its pronounced texture. Metallographic examinations have shown that damage by hydride cracking is a continuous process that starts after the onset of necking. Notches reduce ductility. A modified Gurson-Tvergaard model was used to represent the material behavior and rupture. Numerical simulation using the finite element method demonstrates the strong influence of plastic anisotropy on the behavior of structures and rupture modes.
Superplastic Flow and Cavitation in Zn-22 Pct Al Doped with Cu
AHMADALI YOUSEFIANI and FARGHALLI A. MOHAMED
The sigmoidal relationship between stress and steady-state strain rate that has been reported for micrograin superplastic alloys is characterized by the presence of three regions: region I at low stresses, region II (the superplastic region) at intermediate stresses, and region III at high stresses. Recent results on the superplastic Zn-22 pct Al eutectoid have shown that the characteristics of region I are influenced by the impurity level of the alloy, and that neither region I nor significant cavitation is observed when such a level is reduced to about 6 ppm. These observations are in agreement with the suggestion that the origin of region I is related to strong impurity segregation at boundaries. The present investigation was conducted to study the effect of Cu, as a selected impurity, on superplastic deformation and cavitation in Zn-22 pct Al. The results show that Zn-22 pct Al-0.13 pct Cu exhibits two primary characteristics: region I is absent and cavitation is not extensive. These characteristics, which are essentially similar to those reported previously for high-purity Zn-22 pct Al but are different from those documented for a grade of the alloy containing a comparable atomic concentration of Fe, suggest that Cu has little or no tendency to segregate at boundaries. Indirect evidence in support of this suggestion is inferred from studying the effect of impurities on former boundaries that form in the microstructure of Zn-22 pct Al as a result of solution treatment above the eutectoid temperature. Although further studies are needed to provide direct evidence for the absence of Cu segregation at boundaries, the present results clearly indicate that superplastic flow and cavitation at low stresses are controlled not only by the impurity level, but also by its type.
to increase. Depending upon heat input, the was present as grain boundary allotriomorphs, secondary Widmanstäatten sideplates, and intragranular Widmanstätten precipitates. The reheated zones were of lower hardness and, at all heat inputs, consisted of a mixture of grain boundary allotriomorphs and Widmanstätten and martensite. Laser cladding improved the corrosion- and cavitation-erosion resistance of the surfaces but reduced their ductility. The properties of the clad surfaces depended on heat input.
Particle Engulfment and Pushing by Solidifying Interfaces: Part II. Microgravity Experiments and Theoretical Analysis
D.M. STEFANESCU, F.R. JURETZKO, B.K. DHINDAW, A. CATALINA, S. SEN, and P.A. CURRERI
Results of the directional solidification (DS) experiments on particle engulfment and pushing by solidifying interfaces (PEP), conducted on the space shuttle Columbia during the Life and Microgravity Science (LMS) Mission, are reported. Two pure aluminum (99.999 pct) 9 mm cylindrical rods, loaded with about 2 vol pct 500-µm-diameter zirconia particles, were melted and resolidified in the microgravity (µg) environment of the shuttle. One sample was processed at a stepwise increased solidification velocity and the other at a stepwise decreased velocity. It was found that a pushing/engulfment transition (PET) occurred in the velocity range of 0.5 to 1 µm/s. This is smaller than the ground PET velocity of 1.9 to 2.4 µm/s. This demonstrates that natural convection increases the critical velocity. A previously proposed analytical model for PEP was further developed. A major effort to identify and produce data for the surface energy of various interfaces required for calculation was undertaken. The predicted critical velocity for PET was 0.775 µm/s.
Grain Refining of Al-4.5Cu Alloy by Adding an Al-30TiC Master Alloy
KAZUAKI SATO and MERTON C. FLEMINGS
A particulate Al-30 wt pct TiC composite was employed as a grain refiner for the Al-4.5 wt pct Cu alloy. The composite contains submicron TiC particles. The addition of the TiC grain refiner to the metal alloy in the amount of 0.1 Ti wt pct effected a remarkable reduction in the average grain size in Al-4.5 wt pct Cu alloy castings. With the content of over 0.2 Ti wt pct, the grain refiner maintained its refining effectiveness even after a 3600-second holding time at 973 K. The TiC particles in the resulting castings were free of interfacial phases. It is concluded that the TiC are the nucleating agents and that they are resistant to the "fading effect" encountered with most grain refiners.
Synthesis of Nanocomposite Thin Films Based on the Mo-Si-C Ternary System and Compositional Tailoring through Controlled Ion Bombardment
S. GOVINDARAJAN, J.J. MOORE, and J. DISAM
A major advantage of sputtering processes compared to evaporation processes is the possibility of synthesizing films that replicate the composition of the source (target) material, particularly in the case of alloy targets. This is related to the unique feature of sputtering, viz, formation of an "altered layer" which facilitates reproduction of the target composition in the thin film. An exciting and novel area of research deals with the synthesis of nanocomposite thin films by sputtering composite targets. In this article, the feasibility of depositing a composite thin film based on the Mo-Si-C ternary system through RF magnetron sputtering of a MoSi2 + XSiC target, and the possibility of modifying the film composition by controlled ion bombardment (i.e., "ion plating" or "bias sputtering"), will be discussed. In this context, the role of the sputter yields for Mo, Si, and C will be examined with respect to the ability to vary the composition of as-deposited films. In addition, the modifications which were required to sputter a 58.4-mm-diameter composite target (produced in-house, by different synthesis reactions) using a 127 X 381-mm Vac Tec cathode will be discussed. Details of Auger electron spectroscopy (AES) scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses of the as-deposited films will be presented.
Communication: The Knoop-Hardness Yield Locus of an Orthorhombic Titanium Aluminide Alloy
P.D. NICOLAOU AND S.L. SEMIATIN
Dry Sliding Wear of a Ti50Ni25Cu25 Particulate-Reinforced Aluminum Matrix Composite
G. WANG, P. SHI, M. QI, J.J. XU, F.X. CHEN, and D.Z. YANG
The objective of this article is to characterize the sliding wear behavior of a 30 vol pct Ti50Ni25Cu25 particulate-reinforced aluminum matrix composite under dry conditions. The transformation temperatures of Ti50Ni25Cu25 particles were measured before and after the compounding procedure by the differential scanning calorimeter (DSC) method. The wear tests were carried out on a pin-on-disc machine. A 10 vol pct SiC particulate-reinforced composite and pure aluminum were chosen as the comparison specimens. The results indicate that Al-30 vol pct Ti50Ni25Cu25 composites exhibit higher wear resistance than their unreinforced matrices and are comparable with Al-10 vol pct SiC composites in this experiment. A self-adaptive mechanism that contributes to the wear resistance of an Al-30 vol pct Ti50Ni25Cu25 composite is proposed. Scanning electron microscopy (SEM) and energy diffraction spectrum (EDS) examinations were carried out to investigate the wear mechanism and interface reactions. The results indicate that the interfacial reaction is a predominant factor in determining the wear behavior of the Ti50Ni25Cu25/Al composite.
Effects of R-Ratio on the Fatigue Crack Growth of Nb-Si(ss) and Nb-10Si In Situ Composites
W.A. ZINSSER Jr. and J.J. LEWANDOWSKI
The effects of changes in R ratio on the fatigue crack growth behavior of a Nb-10 at. pct Si composite as well as bulk Nb-1.24 at. pct Si were determined. Fatigue crack growth experiments were performed over a range of K levels at R ratios of 0.1 and 0.4. Qualitative and quantitative scanning electron microscopy studies were performed to characterize the fatigue fracture features of the composites and alloys, in order to determine the factors controlling these fracture features. The results of this work indicate that increases in R ratio reduce the observed threshold stress intensities in both materials. Somewhat higher fatigue thresholds were observed in the Nb-Si(ss) compared to pure Nb in the literature. In contrast to the bulk Nb-Si(ss) alloy, which exhibited no evidence of cleavage fracture in fatigue at any R ratio or K level, the Nb-Si(ss) constituent in the Nb-10 at. pct Si composite exhibited a distinct fracture mode transition from ductile tearing near threshold and low K to cleavage fracture with an increase in K and Kmax. Possible reasons for such observations are provided.
| Search | TMS Document Center | Tables of Contents | Subscriptions | Met. and Mat. Trans. | TMS OnLine |
|---|