| METALLURGICAL AND MATERIALS TRANSACTIONS A | |
|---|---|
|
|
Volume 28A, No. 10, October 1997 This Month Featuring: The 1996 Howe Memorial Lecture; 1996 Distinguished Lecture in Materials & Society; Alloy Phases; Transformations; Mechanical Behavior; Pysical Chemistry; Solidification; Material Processing; Composite Materials. View October 1997 Contents.
|
|
The know-how for these process improvements and revolutionary technologies can be purchased, if it exists. However, since the U.S. industry has a unique set of drivers, it may be necessary to develop many of the new technologies through its own research and development. The current status of research and development in the United States and selected international producers was examined. As expected, it was found that the industry's research capabilities have been greatly reduced. Furthermore, less than half of the companies that identified a given technology as critical have significant research and development programs addressing the technology. It is clear that, in many cases, these technologies must be developed collaboratively using all of the intellectual resources available, including universities. Much of the basic process understanding and data for optimization can be obtained from basic research, which is highly focused on the requirements of the new process, thus eliminating some expensive pilot plant trials.
Examples of how basic research aided in process improvements in the past are given. The examples include demonstrating how fundamentals of reaction kinetics, improved nitrogen control, and thermodynamics of systems helped reduce nozzle clogging and how fluid flow studies reduced defects in casting. However, in general, basic research did not play a major role in processes previously developed but helped our understanding and aided optimization. To have a major impact, basic research must be focused and be an integral part of any new process development. An example where this has been done successfully is the AISI Direct Ironmaking and Waste Oxide Recycle projects, in which fundamental studies on reduction, slag foaming, and postcombustion reactions have led to process understanding, control, and optimization. Industry leaders recognize the value and need for basic research but insist it be truly relevant and done with industry input. From these examples, the lessons learned on how to make basic research more effective are discussed.
' precipitates. This study focuses on the microstructural development of IN738LC during various heat treatments. The 1120°C/2 h/accelerated air-cooled (AAC) solution treatment, given in the literature, already produces a bimodal precipitate microstructure, which is, thus, not an adequate solutionizing procedure to yield a single-phase solid solution in the alloy at the outset. However, the 1235°C/4 h/water quenched (WQ) solution treatment does produce the single-phase condition. A microstructure with fine precipitates develops if solutionizing is carried out under 1200°C/4 h/AAC conditions. Agings at lower temperatures after 1200°C/4 h/AAC or 1250°C/4 h/AAC or WQ conditions yield analogous microstructures. Agings below ~950°C for 24 hours yield nearly spheroidal precipitates, and single aging for 24 hours at 1050°C or 1120°C produces cuboidal precipitates. Two different ' precipitate growth processes are observed: merging of smaller precipitates to produce larger ones (in duplex precipitate-size microstructures) and growth through solute absorption from the matrix. Average activation energies for the precipitate growth processes are 191 and 350 kJ/mol in the ranges of 850°C to 1050°C and 1050°C to 1120°C, respectively, calculated using the precipitate sizes from microstructures in the WQ condition, and 150 and 298 kJ/mol in the analogous temperature ranges, calculated from precipitate sizes in the microstructures in the slow furnace-cooled condition.
Competitive Formation of Inter- and Intragranularly Nucleated Ferrite
S.J. JONES and H.K.D.H. BHADESHIA
A recent trend in the development of tough steels has been to stimulate the heterogeneous nucleation of ferrite on nonmetallic particles, at the expense of that nucleated at the austenite grain surfaces. This leads to a refinement of the microstructure, which also becomes less organized, thus giving better mechanical properties. This article deals with a model for the competitive growth of ferrite nucleated both at austenite grain surfaces and intragranularly on inclusions. The classical Johnson-Mehl-Avrami theory for overall transformation kinetics has been adapted to deal with such simultaneous transformations. The theory is demonstrated to reproduce known trends in experimental observations and is shown to be of use in the design of steels.
The Role of Manganese and Copper in the Eutectoid Transformation of Spheroidal Graphite Cast Iron
JACQUES LACAZE, ALINE BOUDOT, VALÉRIE GERVAL, DJAR OQUAB, and HENRIQUE SANTOS
The decomposition of austenite to ferrite plus graphite or to pearlite in spheroidal graphite (SG) cast iron is known to depend on a number of factors among which are the nodule count, the cooling rate, and the alloying additions (Si, Mn, Cu, etc.). This study was undertaken in order to deepen the understanding of the effect of alloying with Mn and/or Cu on the eutectoid reaction. For this purpose, differential thermal analyses (DTAs) were carried out in which samples were subjected to a short homogenization treatment designed to smooth out the microsegregations originating from the solidification step. The effect of various additions of copper and manganese and of the cooling rate on the temperature of the onset of the stable and metastable eutectoid reactions was investigated. A description of the conditions for the growth of ferrite and of pearlite is given and shows that these reactions can develop only when the temperature of the alloy is below the lower boundary of the ferrite/austenite/graphite or ferrite/austenite/cementite related three-phase field. The experimental results can be explained if the appropriate reference temperature is used. The cooling rate affects the temperature of the onset of the ferrite plus graphite growth in the same way as for the eutectic reaction, with a measured undercooling that can be extrapolated to a zero value when the cooling rate is zero. The growth undercooling of pearlite had values that were in agreement with similar data obtained on silicon steels. The detrimental effect of Mn on the growth kinetics of ferrite during the decomposition of austenite in the stable system is explained in terms of the driving force for diffusion of carbon through the ferrite ring around the graphite nodules. Finally, it is found that copper can have a pearlite promoter role only when combined with a low addition of manganese.
Effect of the Ti/N Ratio on the Hardenability and Mechanical Properties of a Quenched-and-Tempered C-Mn-B Steel
YULIN SHEN and STEVEN S. HANSEN
Ten experimental 0.18 pct C-1.2 pct Mn- 0.002 pct B steels with various Ti/N ratios were evaluated in this study. The hardenability of these steels was first determined using Jominy tests. Slab sections were then rolled to produce 12.5-mm-thick plates, and subsequently quenched and tempered for mechanical property evaluation. The volume fraction of coarse (greater than 1 µm) TiN particles was measured in all steels using quantitative metallographic techniques. Scanning transmission electron microscopy was used to investigate fine precipitates, and scanning electron microscopy was used to examine the fracture surface of Charpy specimens. The results show that a complete boron (B) hardenability effect is obtained with Ti/N ratios >2.9, a value slightly below the stoichiometric Ti/N ratio of 3.4. Any excess Ti, above that which combines with N, provides an additional increase in hardenability on quenching (effect of Ti in solution) and an increase in strength on tempering (Ti (C,N) precipitation). Steels with a higher (Ti)(N) product develop a higher volume fraction of coarse TiN particles during solidification. These coarse TiN particles result in reduced toughness levels of the heat-treated plates evaluated in the present study.
Communications: Bainitic Transformation in Austempered Ductile Iron with Reference to Untransformed Austenite Volume Phenomenon
M. NILI. AHMADABADI
Influence of Cerium Additions on High-Temperature-Impact Ductility and Fracture Behavior of Iridium Alloys
A.N. GUBBI, E.P. GEORGE, E.K. OHRINER, and R.H. ZEE
High-temperature tensile impact testing was carried out on Ir + 0.3 wt pct W alloys doped with cerium and thorium individually, and with cerium and thorium together. Impact ductility was evaluated as a function of grain size and test temperature. Cerium by itself was not as effective as thorium in improving the grain boundary cohesion, even though it segregated more strongly than thorium to the grain boundaries. This lower grain boundary cohesion was responsible for lower impact ductility and higher brittle-to-ductile transition temperature of cerium-doped alloys compared to those of the thorium- or thorium plus cerium-doped alloys. Reduction in thorium content by a factor of 5 (from 50 to 10 appm) in the bulk did not result in any significant reduction in high- temperature impact ductility or an increase in the brittle-to-ductile transition temperature as long as sufficient cerium was added to provide grain refinement. Grain boundary strengths of thorium- and thorium plus cerium-doped alloys were almost identical.
Fatigue Crack Growth Behavior in Niobium-Hydrogen Alloys
MARK CHING-CHENG LIN and K. SALAMA
Near-threshold fatigue crack growth behavior has been investigated in niobium-hydrogen alloys. Compact tension specimens (CTS) with three hydrogen conditions are used: hydrogen-free, hydrogen in solid solution, and hydride alloy. The specimens are fatigued at a temperature of 296 K and load ratios of 0.05, 0.4, and 0.75. The results at load ratios of 0.05 and 0.4 show that the threshold stress intensity range (
Kth) decreases as hydrogen is added to niobium. It reaches a minimum at the critical hydrogen concentration (Ccr), where maximum embrittlement occurs. The critical hydrogen concentration is approximately equal to the solubility limit of hydrogen in niobium. As the hydrogen concentration exceeds Ccr, Kth increases slowly as more hydrogen is added to the specimen. At load ratio 0.75, Kth decreases continuously as the hydrogen concentration is increased. The results provide evidence that two mechanisms are responsible for fatigue crack growth behavior in niobium-hydrogen alloys. First, embrittlement is retarded by hydride transformation-induced and plasticity-induced crack closures. Second, embrittlement is enhanced by the presence of hydrogen and hydride.
A Study of the Mechanism of Hardness Change of Al-Zn-Mg Alloy during Retrogression Reaging Treatments by SmallAngle X-ray Scattering (SAXS)
CHAOFU MENG, HOUWEN LONG, and YONG ZHENG
In this work, the changes in hardness of Al-Zn-Mg alloy during retrogression and reaging (RRA) treatments were detected and the mechanism of the hardness change was studied by Small Angle X-ray Scattering (SAXS). It was discovered that the hardness changes during RRA treatments are as follows. (1) Hardness decreases at the beginning of retrogression, achieves a minimum value at 90 seconds, and then increases and achieves the second maximum value at 6 minutes, and finally decreases simply. (2) Hardness of the reaged sample is higher than that of the retrogressed sample. The following conclusions were drawn from the experimental results of SAXS. (1) The drop in hardness for short retrogression time is attributed to the decrease of volume fraction of the precipitates and the growth of the particles; the drop in hardness with increasing retrogression time after the second maximum of hardness achieved is attributed to coarsening of the particles. 2. The increase in hardness during reaging is due to the occurrence of new precipitates and the increase of volume fraction of the precipitates.
The Influence of Martensite on the Strength and Impact Behavior of Steel
B. MINTZ
The influence of high C martensite on the strength and impact behavior of C-Mn-Nb-Al steels has been determined for two distributions: films surrounding the ferrite grains and distinct colonies. In the former case, the impact behavior markedly deteriorated, this deterioration increasing with martensite level. The changes in impact behavior could be explained by regarding the films of martensite as being similar to the brittle grain boundary carbides that are present in ferrite-pearlite steels. These films readily crack, yet are always thick enough to produce a sufficiently wide crack to render crack propagation easy. The critical event in fracture then becomes the ability to propagate the cracks through the grain boundaries. The greater the grain boundary coverage by these films, the easier this becomes and the worse is the impact behavior. When the martensite is present as colonies, again the impact performance is seriously impaired, but to a lesser extent than when the martensite is present as films, this probably being related to the difficulty in cracking a thick colony except at its extremities. The influence of martensite on strength was found to be similar whether the martensite was in the form of colonies or films. Replacing pearlite with martensite led to a large increase in the yield strength. Increasing the amount of martensite to ~7 pct caused the yield strength to fall to a minimum, after which it again increased. This behavior can be interpreted in terms of the manner in which the volume expansion accompanying the martensite transformation influences the generation of dislocations.
R Values of Fiber-Textured Tantalum Plates
JAMES O'BRIEN, ROGER LOGAN, and WILLIAM HOSFORD
Computations have been made of the effect of crystallographic texture on the R values in sheets of bcc metals with an upper-bound model based on <111> pencil glide. Sheets having [111] and [100] textural components with rotational symmetry about the sheet normal were considered. The predicted R values increase with the fraction [111] component up to a maximum of about 3 for pure [111]. Experiments were made on disks of tantalum produced by upsetting of rods, which produced textures having rotational symmetry about the disk normal. X-ray diffraction showed mixed [111] and [100] textures, with the fraction of [111] varying from about 34 to 96 pct. R values were measured in tension and compression tests on these sheets, and their dependence on fraction [111] agrees very well with the theoretical predictions.
The Effect of the Austenitizing Heat-Treatment Variables on the Fracture Toughness of High-Speed Steel
N. SARAFIANOS
The influence of austenitizing treatment and tempering on the fracture behavior of high-speed steel (DIN 1.3333) has been investigated. The fracture behavior has been characterized by determining the KIC and JIC values via the performance of modified compact tension (CT) and single edge notched (SEN) tests. The micromechanisms of crack initiation and propagation have been studied by metallographic examination of the fractured specimens. The results indicate that austenitizing conditions of temperature range 1050°C to 1190°C and time 0.25 to 6 minutes and tempering at 550°C to 650°C up to 150 minutes alter the microstructure and, subsequently, the fracture toughness. It was found that cracking occurs by nucleation at the interface of matrix/vanadium-enriched large carbides, where sulfur is segregated and where linkage of the microcracks bridges ductile ligament of voids at small Mo + W enriched carbides. The improvements of the fracture toughness and hardness by short austenitizing time of 15 to 75 seconds at 1190°C are attributed to (1) the optimum distribution of a dense network of small carbides, (2) the lack of grain growth as the boundaries are pinned down by these small carbides, and (3) the retained austenite at a level up to 16 vol pct transformed to martensite.
The Role of Coincidence-Site-Lattice Boundaries in Creep of Ni-16Cr-9Fe at 360°C
VISIT THAVEEPRUNGSRIPORN and GARY S. WAS
The objective of this study is to understand and quantify the role of the coincidence-site-lattice boundary (CSLB) population on creep deformation of Ni-16Cr-9Fe at 360°C. It is hypothesized that an increase in the CSLB population decreases the annihilation rate of dislocations in the grain boundary, leading to an increase in the internal stress and a decrease in the effective stress. The result is a reduction in the creep strain rate. The role of CSLBs in deformation is, thus, to increase the internal stress by trapping run-in lattice dislocations at the grain boundaries as extrinsic grain boundary dislocations (EGBDs), creating backstresses on following dislocations rather than annihilating them, as in the case of high-angle boundaries (HABs). The hypothesis was substantiated by showing (1) that dislocation absorption kinetics differ substantially between a CSLB and an HAB, and (2) that the CSLB fraction strongly affects the internal stress in the solid. Dislocation absorption kinetics were measured by comparing EGBD density in transmission electron microscopy (TEM). Results showed that CSLBs contain an EGBD density which is 3 times higher than HABs at 1.25 pct strain. Internal stress was measured by the stress dip test and was found to be 
30 MPa higher in the CSLB-enhanced sample. Steady-state creep rates of Ni-16Cr-9Fe in 360°C argon were also found to be strongly affected by the grain boundary character distribution. Increasing the CSLB fraction by approximately a factor of 2 resulted in a decrease in steady-state creep rates by a factor of 8 to 26 in coarse-grain (330 µm) samples and a factor of 40 to 66 in small-grain (35 µm) samples. It is postulated that annihilation of EGBDs only occurs at triple lines where at least two HABs intersect. By using a geometric relationship to evaluate the probability of EGBDs annihilating at a triple line, the model predicts a non-linear dependence of the creep rate with CSLB fraction, yielding excellent correlation with measurement. The model provides a physical basis for measurements which show that increasing the CSLB fraction by only moderate amounts can greatly reduce the steady- state creep rate in Ni-16Cr-9Fe.
Stress Corrosion Cracking of Superplastically Formed 7475 Aluminum Alloy
T.C. TSAI, J.C. CHANG, and T.H. CHUANG
The effects of biaxial superplastic deformation and postforming heat treatment upon the stress corrosion cracking (SCC) of a fine-grained 7475Al alloy plate have been investigated. For all postforming tempered conditions, increasing the extent of superplastic deformation, which created more cavitations, would decrease the mechanical properties, the SCC resistance, and the corrosion resistance. The influence of cavitation on the decay of elongation of the superplastically formed workpieces is larger than that on the decay of its strength. Post-forming tempered by retrogression and reaging (RRA) treatment could effectively improve the SCC resistance of workpieces in postforming T6 temper while not sacrificing the strength. However, the benefit of improving the SCC resistance by means of the postforming RRA temper was decreased with increasing the extent of superplastic deformation, because the SCC susceptibility increased as the extent of superplastic deformation increased for each postforming tempered condition. The cavitation led to more anodic corrosion potential and pitting potential and to an increase in both corrosion current density and passive current density, which would increase the SCC susceptibility.
Communication: Microfracture Behavior of Al-SiC Composites under Dynamic Loading
CHANG GIL LEE, DONGIL KWON, AND SUNGHAK LEE
Communication: On the Gegel's Stability Criterion in Processing Maps
S.V.S NARAYANA MURTY and B. NAGESWARA RAO
The Effect of Cooling Rate from the Melt on the Recrystallization Behavior of Aluminum Alloy 6013
BUNCHA THANABOONSOMBUT and T.H. SANDERS, JR.
In most commercial operations, the plant metallurgist likely has little control over the solidification rate of the process. However, solidification rate is affected by the dimensions of the ingot, and product form (plate ingot vs extrusion billet, for example) determines the dimensions of the ingot to be cast. Consequently, understanding the effects of solidification rate might be useful in explaining differences in microstructure that are often observed in various product forms cast from equivalent compositions. To provide this microstructural information, the effect of cooling rate from the melt on the microstructural changes in hot-rolled and solution heat treated (SHT) aluminum alloy 6013 was investigated. The range of cooling rates in this investigation is comparable to what might be observed through the thickness of a plate ingot. Over the cooling rate range investigated (0.5 to 5 K/s), recrystallization behavior of the alloy appears to be primarily affected by the size and number density of the coarse 
(AlFeMnSi) constituent particles, which act as sites for particle stimulated nucleation (PSN) of recrystallized grains. At intermediate cooling rates (1.5 K/s), the resistance to recrystallization is at a minimum. As the cooling rate increases beyond 1.5 K/s, the number of particles available for PSN decreases; thus, there is a decrease in the fraction of recrystallized grains after heat treating. On the other hand, as the cooling rate is decreased from 1.5 K/s, the size of the constituents increases; however, their number decreases, once again leading to a decrease in the fraction of recrystallized grains observed after heat treatment.
| Search | TMS Document Center | Tables of Contents | Subscriptions | Met. and Mat. Trans. | TMS OnLine |
|---|