Deformation characteristics of Ti-6Al-4V alloy under isothermal forging conditions

An investigation was undertaken to determine the influence of forge temperature, ram rate, and starting microstructure on the deformation characteristics of isothermally forged Ti-6A1-4V alloy. Both yielding and finish forge pressures were measured in the practical range of forge temperatures and ram rates. With the absence of die-chilling, the results obtained can be quantitatively related to the hot deformation properties. The yielding and finish forge pressures and the stress-strain relationship are strongly dependent on forge temperature, ram rate, and initial microstructure. Although the forge pressures do not vary significantly with strain, an apparent yield-drop was observed, particularly in the β preforms. On the basis of the above experimental findings and the activation analysis, the rate-controlling deformation process under isothermal forgings is discussed with respect to the dynamic softening. Additional observation was made on dynamic recrystallization in the temperature range of 1500°F (1089 K) to 1650°F (1172 K). Structural features and tensile properties of isothermally forged material are also presented.     

Microstructure development during conventional and isothermal hot forging of a near-gamma titanium aluminide

The breakdown of the lamellar preform microstructure in the ingot metallurgy near-gamma titanium aluminide, Ti-45.5Al-2Cr-2Nb (atomic percent), was investigated. Microstructures developed during canned, conventional hot forging were compared to those from isothermal hot forging. The higher rate of deformation in conventional forging led to considerably finer and almost completely broken-down structures in the as-forged condition. Several nontraditional approaches, including the isothermal forging of a metastable microstructure (so-called “alpha forging”) and the inclusion of a short static recrystallization anneal during forging, were found to produce a more fully broken-down structure in as-isothermally forged conditions. Despite the noticeable microstructure differences after forging, conventionally and isothermally forged material responded similarly during heat treatment. In both cases, almost totally recrystallized structures of either equiaxed gamma or transformed alpha grains surrounded by fine gamma grains were produced depending on the heat-treatment temperature. Metallography on forged and heat-treated pancake macroslices was useful in delineating small differences in composition not easily detected by analytical methods.     

High-temperature deformation processing of Ti-24Al-20Nb

Power dissipation maps have been generated in the temperature range of 900 ‡C to 1150 ‡C and strain rate range of 10^-3 to 10 s^-1 for a cast aluminide alloy Ti-24Al-20Nb using dynamic material model. The results define two distinct regimes of temperature and strain rate in which efficiency of power dissipation is maximum. The first region, centered around 975 ‡C/0.1 s^-1, is shown to correspond to dynamic recrystallization of the α₂ phase and the second, centered around 1150 ‡C/0.001 s^-1, corresponds to dynamic recovery and superplastic deformation of the β phase. Thermal activation analysis using the power law creep equation yielded apparent activation energies of 854 and 627 kJ/mol for the first and second regimes, respectively. Reanalyzing the data by alternate methods yielded activation energies in the range of 170 to 220 kJ/mol and 220 to 270 kJ/mol for the first and second regimes, respectively. Cross slip was shown to constitute the activation barrier in both cases. Two distinct regimes of processing instability—one at high strain rates and the other at the low strain rates in the lower temperature regions—have been identified, within which shear bands are formed. Formerly with the Defence Metallurgical Research Laboratory     

Effects of TMCP Parameters on Microstructure and Mechanical Properties of Hot Rolled Economical Dual Phase Steel in CSP

 Effects of chemical compositions, finish rolling temperature, isothermal temperature on runout table and coiling temperature on microstructure and mechanical properties of economical dual phase steel produced on CSP line were investigated. Experimental results showed that martensite volume fraction could be enhanced and banding microstructure could be reduced by controlling Mn, Si contents and applying proper finish rolling temperature. Optimized processing-parameters were obtained for DP580 production on CSP line of Wuhan Iron and Steel (group) Co (WISCO) in China. Optimal microstructure and mechanical properties could be achieved when the strip was finished rolling at the range of 790 to 830 ℃, isothermally holding at 680 to 740 ℃ and coiling below 250 ℃.     

The influence of matrix microstructure and particle reinforcement on the creep behavior of 2219 aluminum

The influence of matrix microstructure and reinforcement with 15 vol pct of TiC particles on the creep behavior of 2219 aluminum has been examined in the temperature range of 150 ‡C to 250 ‡C. At 150 ‡C, reinforcement led to an improvement in creep resistance, while at 250 ‡C, both materials exhibited essentially identical creep behavior. Precipitate spacing in the matrix exerted the predominant influence on minimum creep rate in both the unreinforced and the reinforced materials over the temperature range studied. This behavior and the high-stress dependence of minimum creep rate are explained using existing constant structure models where, in the present study, precipitate spacing is identified as the pertinent substructure dimension. A modest microstructure-independent strengthening from particle reinforcement was observed at 150 ‡C and was accurately modeled by existing continuum mechanical models. The absence of reinforcement creep strengthening at 250 ‡C can be attributed to diffusional relaxation processes at the higher temperature.     

Microstructure and mechanical behavior of Cr-Cr2Hfin situ intermetallic composites

A detailed investigation of the effects of microstructural changes on the mechanical behavior of twoin situ intermetallic composites with Cr and Cr₂Hf phases in the Cr-Hf system was performed. The nominal compositions (at. pct) of the alloys were Cr-5.6Hf (hypoeutectic) and Cr-13Hf (eutectic). The study included evaluations of strength, ductility, and fracture toughness as a function of temperature and creep behavior. Two microstructures in each alloy were obtained by heat treatments at 1250 ‡C (fine microstructure) and 1500 ‡C (coarse microstructure). A decrease in elastic strength (stress at the onset of inelastic response in the load-deflection curve) with the coarsening of the microstructures was noted for both alloys below 1000 ‡C. The Cr-13Hf alloy retained strength to a higher test temperature, relative to Cr-5.6Hf alloy, under both microstructural conditions. The alloys showed no evidence of ductility at room temperature. However, in the coarse microstructure of the Cr-5.6Hf alloy, the primary Cr exhibited ductility at and above 200 ‡C; ductility in primary Cr could be seen only at and above 1000 ‡C for the fine microstructure. In other words, the temperature at which ductility was first observed decreased from about 1000 ‡C to about 200 ‡C due to high-temperature heat treatment in this alloy. Both microstructures of Cr-5.6Hf alloy showed a significant increase in fracture toughness with increasing test temperature. However, the increases in fracture toughness with temperature for the Cr-13Hf alloy microstructures were relatively small. Both alloys showed about four orders of magnitude reduction in steady-state creep rates relative to pure Cr at 1200 ‡C. The results are analyzed in the light of deformation characteristics and fracture micromechanisms. The effects of microstructural factors, such as the size and continuity of phases, solubility levels of Hf as well as interstitial elements in Cr, on the observed mechanical behavior are discussed. Formerly Research Scientist, Materials and Processes, UES, Inc.     

Elevated temperature flow strength,creep resistance and diffusion welding characteristics of Ti-6Al-2Nb-1Ta-0.8Mo

A study of the flow strength, creep resistance and diffusion welding characteristics of the titanium alloy Ti-6Al-2Nb-1Ta-0.8Mo has been conducted. Two mill-processed forms of this alloy were examined. The forged material had been essentially processed above the beta transus (∼1275 K) while the rolled form had been subjected to work below the beta transus. Between 1150 and 1250 K, the forged material was stronger and more creep resistant than the rolled alloy. Both forms exhibit superplastic characteristics in this temperature range. Strain measurements during diffusion welding experiments at 1200 K reveal that weld interfaces have no measurable effect on the overall creep deformation. Significant deformation appears to be necessary to produce a quality diffusion weld between superplastic materials. A “soft” interlayer inserted between faying surfaces would seemingly allow manufacture of quality diffusion welds with little overall deformation.     

Tensile behavior of the L12 compound Al67Ti25Cr8

Temperature-related variations in tensile yield strength and ductility were studied on cast, homogenized and isothermally forged Al₆₇Ti₂₅Cr₈. Yield strength dropped discontinously between 623 K and 773 K and then decreased gradually with increasing temperature. Below 623 K, fracture occurred prior to macroscopic yielding. Ductility decreased from 0.2% at 623 K to zero at 773 K, but increased again at higher temperatures. At 1073 K, an elongation of ∼ 19% was obtainable, and ultimate tensile strength and localized necking were observed. Fracture surfaces and deformed microstructures were examined. The 1073 K tensile specimen that exhibited ∼ 19% elongation showed grain boundary serrations and some evidence of recrystallization (likely dynamic) although fracture occurred predominantly via an intergranular mode.     

The microstructure of rolled and annealed tungsten rod

This paper reports a study of the microstructural changes that occur when potassium-doped tungsten ingots are rolled at elevated temperatures. The effect of annealing on the microstructure of the rolled material is also considered. All samples were rolled on a Kocks mill. At low levels of deformation, the grain boundaries are primarily high-angle boundaries, and many grains are dislocation free. Both of these features probably result from dynamic recrystallization during rolling. As deformation increases, the grains become more elongated, and more low-angle boundaries are found within the material. Also, the potassium gets drawn into narrower and longer tubes. When these rolled rods are annealed at temperatures between 1275 ‡C and 1950 ‡C, several changes occur in the microstructure. The material undergoes abnormal grain growth. The temperature at which this occurs depends on the length of the anneal, the amount of de-formation the rod has received, and the spatial location in the rod. This spatial distribution most likely results from strain gradients that exist in the rolled rod. The abnormal grain growth is accompanied by a decrease in hardness. The potassium-containing tubes in the rod also break up into bubbles during annealing. The temperature at which this breakup occurs again depends on the length of the anneal and the amount of deformation.     

Microstructural effects on moisture-induced embrittlement of isothermally forged TiAl-based intermetallic alloys

By using isothermally forged TiAl-based intermetallic alloys, various microstructures (of γ-grain, duplex, dual-phase, and fully lamellar microstructures) were prepared. These TiAl-based intermetallic alloys were tensile tested in vacuum and air as functions of strain rate and temperature to investigate microstructural effects on the moisture-induced embrittlement. All the intermetallic alloys with different microstructures showed different levels of reduced tensile stress (or elongation) in air at room temperature. The reduction in tensile stress (or elongation) due to testing in air diminishes as the testing temperature (or strain-rate) increases. From the fracture stress-temperature curves, it was found that the γ-grain microstructure was the most resistant to the moisture-induced embrittlement, and the dual-phase microstructure was the most susceptible to the moisture-induced embrittlement. Also, the moisture-induced embrittlement of the TiAl-based intermetallic alloys with a fully lamellar microstructure depends on the lamellar spacing and is reduced with decreasing lamellar spacing. The possible reasons for the observed microstructural effect on the moisture-induced embrittlement were discussed, in association with hydrogen behavior and properties in the constituent phases and at some interfaces.     

Investigation of the thermodynamics of V-O solid solutions by distribution coefficient measurements in the V-O-Na system

The activity coefficient of oxygen in vanadium was determined at 600‡, 650‡, 700‡, and 750"C by equilibrium distribution coefficient measurements in the V-O-Na system. The activity of oxygen in vanadium exhibits a significant negative deviation from Raoult's law (standard states: γ= 1at N = N‡). The activity coefficient is adequately expressed by the reciprocal temperature dependence, which is characteristic of a regular solution. The low oxygen pressures required for this investigation were attained by controlling the oxygen concentration in liquid sodium at levels below 20 ppm. The V(O sat)-O_Na-V₅O phase boundary and the equilibrium distribution of oxygen between vanadium and sodium were also determined in the temperature range 500‡ to 850‡C.     

Structure and strengthening of Al-Mg alloys during hot working

Hot deformation studies using torsion testing were conducted on high purity Al and Al-4 at. pct Mg alloy systems in the strain rate range of 0.1 to 1.0 s⁻¹ and temperatures up to 810 K (1000‡F). At all test temperatures, the flow stress of the Al-Mg alloy was higher than that of pure AL The strengthening in hot working (above 522 K (480°F)) is suggested to be due to a higher equilibrium subgrain forest dislocation density. Special quenching procedures were required to show this correlation. Conventional quenching fails to show this because structural details are lost when quenching from high temperatures. Formerly with Olin Metals Research Laboratories.     

Thermomechanical processing of microalloyed powder forged steels and a cast vanadium steel

The effects of controlled rolling on transformation behavior of two powder forged (P/F) microalloyed vanadium steels and a cast microalloyed vanadium steel were investigated. Rolling was carried out in the austenitic range below the recrystallization temperature. Equiaxed grain structures were produced in specimens subjected to different reductions and different cooling rates. The ferrite grain size decreased with increasing deformation and cooling rate. Ferrite nucleated on second phase particles, deformation bands, and on elongated prior austenite grain boundaries; consequently a high fractional ferrite refinement was achieved. Deformation raised the ferrite transformation start temperature while the time to transformation from the roll finish temperature decreased. Cooling rates in the cast steel were higher than in P/F steels for all four cooling media used, and the transformation start temperatures of cast steels were lower than that of P/F steel. Intragranular ferrite nucleation, which played a vital role in grain refinement, increased with cooling rate. Fully bainitic microstructures were formed at higher cooling rates in the cast steel. In the P/F steels inclusions and incompletely closed pores served as sites for ferrite nucleation, often forming a ‘secondary’ ferrite. The rolling schedule reduced the size of large pores and particle surface inclusions and removed interconnected porosity in the P/F steels. Formerly Postgraduate Researcher in the Department of Metallurgy and Materials Science, UMIST/University of Manchester, United Kingdom     

The effect of forging history on the strength and microstructure of TDNiCr (Ni-20Cr-2ThO2)

Forging variables were evaluated to determine their influence on the elevated temperature strength and microstructure of TDNiCr. Grain size was the principal microstrucrural feature related to elevated temperature strength and was controlled primarily by the thermomechanical variables of forging temperature and final annealing condition. Tests at 2000°F (1366 K) revealed a factor of eight increase in tensile strength as grain size increased from 1 to 150 μm, while stress-rupture strength improved by three to five times as grain size increased from 15 to 1500 μm. Forged material of grain size ≥ ∼150 μm displayed a level of elevated temperature strength comparable to that of optimized TDNiCr sheet. The presence of a preponderance of small twins and a strong preferred orientation may have also been factors contributing to the excellent high temperature strength of large grain forged material.     

Powder metallurgy T15 tool steel: Part II. Microstructure and properties after heat treatment

The effect of powder particle size and heat treatment on the micro structure and properties of hot isostatically pressed (“hipped”) T15 tool steel has been evaluated. Gas-atomized powder was screened into size fractions covering the range of ≤44 to 1200 /i-m and hipped at 1130 ‡C or 1195 ‡C. The consolidated powders were austenitized at 1175 ‡C or 1225 ‡C and tempered at 538 ‡C, 552 ‡C, or 565 ‡C to control prior austenite grain size, carbide type, carbide volume fraction, and carbide size distribution. Properties measured were bend strength, C-notch impact toughness, and hot hardness. Prior austenite grain size increases with hot isostatic pressing (“hipping”) temperature and austenitizing temperature but is independent of the particle size; similarly, the influence of austenitizing temperature on dissolution of MC and M₆C is independent of the particle size. In each particle size fraction, the volume fraction and size distribution of MC are independent of the tempering temperature. For M₆C, the volume fraction increases and the size distribution is skewed to coarser sizes with increasing tempering temperature. No significant differences in strength and toughness were detected as a function of particle size. Hot hardness is not affected by the particle size. The hot hardness of a powder blend (≤1200 Μm) hipped at 1130 ‡C was superior to that of commercial powder metallurgy (PM) T15 tool steel hipped at 1195 ‡C; this is attributed to a finer carbide size in the noncommercial material. It is established that the subcommercial hipping temperature (1130 ‡C) results in significant microstructural refinement; there is an associated small amount of residual porosity, and this controls the mechanical properties.     

Mechanical Behaviors of Ultrafine-Grained 301 Austenitic Stainless Steel Produced by Equal-Channel Angular Pressing

The technique of equal-channel angular pressing (ECAP) was used to refine the microstructure of an AISI 301 austenitic stainless steel (SS). An ultrafine-grained (UFG) microstructure consisting mainly of austenite and a few martensite was achieved in 301 steel after ECAP processing for four passes at 523 K (250 °C). By submitting the as-ECAP rods to annealing treatment in the temperature range from 853 K to 893 K (580 °C to 620 °C) for 60 minutes, fully austenitic microstructures with grain sizes of 210 to 310 nm were obtained. The uniaxial tensile tests indicated that UFG 301 austenitic SS had an excellent combination of high yield strength (>1.0 GPa) and high elongation-to-fracture (>30 pct). The tensile stress–strain curves exhibited distinct yielding peak followed by obvious Lüders deformation. Measurements showed that Lüders elongation increased with an increase in strength as well as a decrease in grain size. The microstructural changes in ultrafine austenite grains during tensile deformation were tracked by X-ray diffraction and transmission electron microscope. It was found that the strain-induced phase transformation from austenite to martensite took place soon after plastic deformation. The transformation rate with strain and the maximum strain-induced martensite were promoted significantly by ultrafine austenite grains. The enhanced martensitic transformation provided extra strain-hardening ability to sustain the propagation of Lüders bands and large uniform plastic deformation. During tensile deformation, the Lüders bands and martensitic transformation interacted with each other and made great contribution to the excellent mechanical properties in UFG austenitic SS.     

Fatigue-crack propagation behavior of type 304 stainless steel at elevated temperatures

The fatigue-crack propagation behavior of Type 304 stainless steel was investigated within the framework of linear elastic fracture mechanics at temperatures of 75‡, 600‡ 1000‡ and 1200‡F. The cyclic frequency for the elevated temperature tests was 4 cpm. It was found that, in general, fracture mechanics concepts may be used to describe the crack propagation behavior at these temperatures, and that increasing the temperature had a significant effect in increasing the fatigue-crack growth rate.     

Effect of single aging on microstructure and impact property of INCONEL X-750

The microstructural changes and grain boundary chemistry of high strength, age hardenable Ni-Cr-Fe alloy, INCONEL * X-750, have been studied using electron and Auger microscopy following a sequence of thermal treatments in the carbide precipitation temperature zone of 704 ‡C to 871 ‡C. The thermal treatment consisted of a solution anneal and quench from 1075 ‡C followed by aging up to 200 hours in this temperature region. An attempt has been made to correlate the microstructural data with Charpy impact test results, hardness values, and modified Huey Corrosion Test results (ASTM G28-72). Aging was conducted in a vacuum and in air from which the specimens were cooled at different rates. Aging at 871 ‡C for 50 to 100 hours under both air and vacuum furnace cooling conditions resulted in increased mechanical strength and corrosion resistance compared with aging at 704 ‡C or 760 ‡C, in which temperature range both apparent fracture toughness and corrosion rate deteriorate. The reprecipitation of secondary carbides along with a possible 17 phase precipitation upon aging at 871 ‡C for 200 hours under vacuum furnace cooling resulted in poor corrosion resistance and inferior impact properties.     

Softening and microstructural change following the dynamic recrystallization of austenite

To characterize the dynamic recrystallization behavior of austenite, continuous-torsion tests were carried out on a Mo steel over the temperature range 950 ‡C to {dy1000} ‡C, and at strain rates of 0.02, 0.2, and 2 s^-1. Interrupted-torsion tests also were performed to study the characteristics of postdynamic recrystallization. Quenches were performed after increasing holding times to follow the development of the postdynamic microstructure. Finally, torsion simulations were carried out to assess the importance of metadynamic recrystallization in hot-strip mills. The postdynamic microstructure shows that the growth of dynamically recrystallized grains is the first change that takes place. Then metadynamically recrystallized grains appear and contribute to the softening of the material. The rate of metadynamic recrystallization and the meta-dynamically recrystallized grain size depend on strain rate and temperature and are relatively independent of strain, in contrast to the observations for static recrystallization. True dynamic recrystallization-controlled rolling (DRCR) is shown to require such short interpass times that it does not occur in isolation in hot-strip mills. As these schedules involve 20 to 80 pct softening by metadynamic recrystallization, a new concept known as metadynamic recrystallization-controlled rolling (MDRCR) is introduced to describe this type of situation. 1 C. ROUCOULES, formerly with the Department of Mining and Metallurgical Engineering, McGill University, Montreal, PQ, Canada