Effect of carbon addition on the strength and creep resistance of FeAl alloys

We investigated the effect of carbon content (0.05, 0.12, and 0.2 wt pct C) and heat-treatment temperature (1100°C and 1300°C for 2 hours and air cooled) on the tensile and the creep properties of Fe-24 wt pct Al alloy. The increase of carbon content increased the yield strength without affecting the tensile ductility of the alloys. Carbon content appears to be beneficial in suppressing the hydrogen embrittlement at the grain boundary, because the fracture mode changes from predominantly intergranular failure in a low carbon (0.05 wt pct C) alloy to a predominantly transgranular cleavage failure in a high carbon (0.2 wt pct C) alloy. With the increase of carbon content, the anomalous yield strength peak shifted to a higher temperature possibly due to the interaction between carbon and vacanies. Significant improvements were noted in the tensile and the creep properties of medium (0.12 wt pct C) and high carbon (0.2 wt pct C) alloys after heat treating at 1300°C. The improvements in the tensile and the creep properties were attributed to the synergetic effect of retained vacancies and fine carbide precipitates present in the alloys after 1300°C heat treatment. However, the improved strength and creep properties associated with 1300 °C heat treatment were lost when the heat-treated alloys were further subjected to a vacancy removal annealing. Our results suggest that the retained vacancies present in the FeAl alloys after high-temperature heat treatment and air cooling are effective in improving the creep resistance at 700°C, and yield strength up to 800°C. The creep resistance of the present high carbon FeAl alloy is comparable to or better than several grades commercial heat-resistant Fe-based and Ni-based alloys. The work was carried out when the authors were with Chrysalis Technologies Inc., Richmond, VA. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.     

The columnar to equiaxed transition in tin-lead alloys

Tin-lead alloys were solidified directionally and the position of the columnar to equiaxed transition determined on vertical sections of the ingots. The columnar length was found to increase with decreasing lead concentration and increasing heat transfer coefficient. A mathematical model of the heat flow in the system was used to determine local temperatures, temperature gradients, and velocities in the solidifying alloy. Comparing the position of the columnar to equiaxed transition and local thermal conditions, it was found that the transition occurred when the temperature gradient in the melt at the liquidus temperature was 0.11°C/mm for Sn 10 wt pct Pb, 0.10°C/mm for Sn 5 wt pct Pb, and 0.13°C/mm for Sn 15 wt pct Pb. The position of the transformation was found to be independent of melt superheat for the conditions considered.     

Solidification and solid-state transformation mechanisms in Si alloyed high-chromium white cast irons

Chromium white cast irons are widely used in environments where severe abrasion resistance is a dominant requirement. To improve the wear resistance of these commercially important irons, the United States Bureau of Mines and CSIRO Australia are studying their solidification and solid-state transformation kinetics. A ternary Fe-Cr-C iron with 17.8 wt pct (pct) Cr and 3.0 pct C was compared with commercially available irons of similar Cr and C contents with Si contents between 1.6 and 2.2 pct. The irons were solidified and cooled at rates of 0.03 and 0.17 K · s^-1 to 873 K. Differential thermal analysis (DTA) showed that Si depresses the eutectic reaction temperature and suggests that is has no effect upon the volume of eutectic carbides formed during solidification. Microprobe analysis revealed that austenite dendrites within the Si alloyed irons cooled at 0.03 and 0.17 K·s^-1 had C and Cr contents that were lower than those of dendrites within the ternary alloy cooled at the same cooling rate and a Si alloyed iron that was water quenched from the eutectic temperature. These lower values were shown by image analysis to be the result of both solid-state growth (coarsening) of the eutectic carbides and some secondary carbide formation. Hardness measurements in the as-cast condition and after soaking in liquid nitrogen suggest an increase in the martensite start temperature as the Si content was increased. It is concluded that Si’s effect on increasing the size and volume fraction of eutectic carbides and increasing the matrix hardness should lead to improved wear resistance over regular high-chromium white cast irons.     

Interphase boundary precipitation in liquid phase sintered W-Ni-Fe and W-Ni-Cu alloys

The microstructure of liquid-phase sintered, tungsten-based heavy alloys comprises a continuous network of spheroidal tungsten single crystals embedded in a ductile, fcc matrix phase, and the integrity of the tungsten-matrix interphase boundaries established during processing is of major importance in determining the resultant mechanical properties. A serious potential source of embrittlement in these systems involves the precipitation of a brittle third phase along these boundaries. In the present work the techniques of selected area and convergent beam electron diffraction, energy dispersive X-ray microanalysis, and scanning Auger electron spectroscopy have been used to identify the embrittling interphase boundary precipitate formed in a commercial W-4.5 wt pct Ni-4.5 wt pct Fe alloy. The interphase boundary precipitation of an intermetallic phase in a W-7.2 wt pct Ni-2.4 wt pct Cu alloy under controlled conditions of heat treatment has also been confirmed. The precipitate phase observed in the W-Ni-Fe alloy in the as-sintered furnace-cooled condition has been found to be an eta carbide with a diamond cubic crystal structure (space group Fd3m,a ₀ = 1.092 ± 0.005 nm) and a tentative composition of the form (Ni,Fe)₆W₆C, where the Ni:Fe atom ratio is approximately 2:3. Neither the carbide nor any evidence of an intermetallic phase was observed in the as-sintered, furnace-cooled W-Ni-Cu alloy, but a continuous interphase boundary film of intermetallic precipitate could be induced in specimens solution treated at 1350°C, water quenched, and aged isothermally in the temperature range 600 to 900°C. Selected area electron diffraction indicated that the phase was isomorphous with the intermetallic Ni₄W of the binary Ni-W system. This paper is based on a presentation delivered at the symposium “Activated and Liquid Phase Sintering of Refractory Metals and Their Compounds” held at the annual meeting of the AIME in Atlanta, Georgia on March 9, 1983, under the sponsorship of the TMS Refractory Metals Committee of AIME. Formerly with Department of Mechanical and Industrial Engineering and Materials Research Laboratory, University of Illinois at Urbana-Champaign     

The effect of grain boundary chemistry on Intergranular stress corrosion cracking of Ni-Cr-Fe alloys in 50 Pct NaOH at 140 °C

The role of chromium, carbon, chromium carbides, and phosphorus on the intergranular stress corrosion cracking (IGSCC) resistance of Ni-Cr-Fe alloys in 50 pct NaOH at 140 °C is studied using controlled-purity alloys. The effect of carbon is studied using heats in which the carbon level is varied between 0.002 and 0.063 wt pct while the Cr level is fixed at 16.8 wt pct. The effect of Cr is studied using alloys with Cr concentrations between 5 and 30 wt pct. The effect of grain boundary Cr and C together is studied by heat-treating the nominal alloy composition of Ni-16Cr-9Fe-0.035C, and the effect of P is studied using a high-purity, P-doped alloy and a carbon-containing, P-doped alloy. Constant extension rate tensile (CERT) results show that the crack depth increases with decreasing alloy Cr content and increasing alloy C content. Crack- ing severity also correlates inversely with thermal treatment time at 700 °C, during which the grain boundary Cr content rises and the grain boundary C content falls. Phosphorus is found to have a slightly beneficial effect on IG cracking susceptibility. Potentiodynamic polarization and potentiostatic current decay experiments confirm that Cr depletion or grain boundary C enhances the dissolution at the grain boundary. Results support a film rupture-anodic dissolution model in which Cr depletion or grain boundary C (independently or additively) enhances dissolution of nickel from the grain boundary region and leads to increased IG cracking.     

The effect of cold work on the precipitation of Ω and θ′ in a ternary Al-Cu-Mg alloy

The effect of plastic deformation on the microstructural evolution of an Al-5.0Cu-0.5Mg (wt pct) ternary alloy was investigated. Hardness measurements and quantitative precipitate analysis were performed on specimens that were water quenched from a solution heat treatment, stretched either 0 or 6 pct and immediately aged at ambient temperature or artificially aged at 200 °C or 250 °C for times up to 3000 hours. Quantitative transmission electron microscopy (TEM) was used to characterize Ω and θ′ number density, diameter, and thickness as a function of preage mechanical stretch and artificial aging condition. Age hardening curves for naturally and artificially aged specimens revealed an increase in hardness corresponding with a preage stretch. Quantitative TEM verified an increase in the number density and a refinement of precipitates for both Ω and θ′ between the 0 and 6 pct stretch condition for those samples artificially aged. When aged at 200 °C, θ′ exhibited superior coarsening resistance relative to the Ω phase. The quantified Ω coarsening kinetics were greater than similar Ag-containing alloys. To investigate the effects of trace Si additions on subsequent microstructural evolution, a series of Al-Cu-Mg-Si quaternary alloys were produced. The addition of 0.1Si (wt pct) was found to suppress Ω precipitation in most Al-4.0Cu-xMg alloys investigated. These initial results indicate that Ω precipitation may be related to the Mg/Si ratio.     

The use of a boron addition to prevent intergranular embrittlement in Fe-12Mn

Fe-12 Mn alloys undergo failure by catastrophic intergranular fracture when tested at low temperature in the as-austenitized condition, a consideration which prevents their use for structural applications at cryogenic temperatures. The present research was undertaken to identify modifications in alloy composition or heat treatment which would suppress this embrittlement. Chemical and microstructural analyses were made on the prior austenite grain boundaries within the alloy in its embrittled state. These studies failed to reveal a chemical or microstructural source for the brittleness, suggesting that intergranular brittleness is inherent to the alloy in the as-austenitized condition. The addition of 0.002 to 0.01 wt pct boron successfully prevented intergranular fracture, leading to a spectacular improvement in the low temperature impact toughness of the alloy. Autoradiographic studies suggest that boron segregates to the austenite grain boundaries during annealing at temperatures near 1000 °C. The cryogenic toughness of a Fe-12Mn-0.002B alloy could be further improved by suitable tempering treatments. However, the alloy embrittled if inappropriate tempering temperatures were used. This temper embrittlement was concom-itant with the dissolution of boron from the prior austenite grain boundaries, which reestablishes the intergranular fracture mode.     

Ductility and impact resistance of powder-metallurgical molybdenum-rhenium alloys

Mo-Re alloys containing between 5 and 47.5 wt pct Re were fabricated from Mo and Mo-Re powders by sintering and hot radial forging. The mechanical properties of as-forged, stress-relieved, and recrystallized specimens were examined. Up to a concentration of 41 wt pct Re, the Charpy ductile-to-brittle transition temperature decreased monotonically with increasing rhenium concentration. Consistent with this, bend angles for fracture at T=−100 °C increased monotonically with increasing rhenium concentration. Between 10 and 41 wt pct rhenium, the room-temperature tensile ductility of recrystallized Mo-Re remained nearly constant with values on the order of 35 to 45 pct. This result differs from the low ductility values observed previously by Lundberg (1997) for compositions on either side of Mo-13 wt pct Re.     

Fatigue crack propagation in aluminum-base copper alloys

Fatigue crack propagation ratesda/dN in binary Al alloys with 3.6 wt pct Cu and 6.3 wt pct Cu and commercial 2024 aged at 21°C were compared with 99.95⁺ wt pct aluminum. Omitting an anomalous region at lowΔK, the extrapolated rates for “pure” aluminum are more than 100 times greater than those in the three alloys at the same ΔK. The data for the alloys fit into a single scatter band of a factor of three. It was suggested thatda/dN varies inversely with the square of the strength of the alloy but that another parameter related to the fatigue crack propagation energy per unit area is also important. Theda/dN vs ΔK curves were determined for 3.6 wt pct Cu single crystals aged seven days at 21°C which containGP zones and two and seven days at 160°C which contain mixtures ofθ′ andθ′’. No systematic variation of (da/dN _Δ with crystallographic orientation was discerned, but the naturally aged specimen had a strong orientation dependence on crack initiation. At low ΔK 21°C aged specimens gave the lowestda/dN while at high ΔK the warm aged specimens gave the lower values ofda/dN. Measurement ofda/dN vs ΔK curves were conducted on specimens of 3.6 wt pct Cu with 1 mm equiaxed grains aged for various times at 130°C, 160°C, and 190°C. All warm aged specimens experienced brittle intergranular fracture at sufficiently high ΔK. The transition ΔK where intergranular fracture first appears is inversely proportional to the aging temperature. The change of fracture mode from intra to intergranular occurs gradually over a broad range of ΔK which shifts to lower ΔK with increase in aging temperature.     

Thermodynamics of carbon in nickel,iron-nickel and iron-chromium-nickel alloys

Iron-nickel alloys with 8 and 16 wt pct nickel and iron-chromium-nickel alloys with 8 pct nickel and chromium contents in the range of 2 to 22 pct were equilibrated with iron and nickel in flowing CH₄-H₂ gas mixtures and in sealed capsules under partial vacuum at temperatures between 700 and 1060°C. Carbon activities in these alloys were established from the carbon concentrations in the nickel by applying Henry’s law to the solubility of carbon in nickel that was determined in the temperature range of 500 to 1000°C. First-order free-energy interaction parameters were used to relate the carbon activities to composition and temperature in the single-phase austenitic Fe-Ni and Fe-Cr-Ni alloys. An expression was also developed to evaluate carbon activities in Fe-Cr-Ni alloys in the region of higher chromium contents (〉4 wt pct) that result in a two-phase austenite plus carbide mixture at these temperatures.     

Evaluation of vanadium and vanadium-base alloys in hot-trapped sodium

Corrosion tests were conducted with vanadium-base alloys in zirconium-purified sodium from 450° to 750°C for 500 to 4000 hr. The materials tested were vanadium, binary vanadium alloys containing from 10 to 40 wt pct Ti, and two ternary alloys: V-5Ti-15Cr and V-15Ti-7.5Cr. The sodium was analyzed to contain from 1.7 to 8.7 ppm O by the vacuum-distillation technique. The behavior of these materials was determined from weight change data, and from chemical, metallographic, and electron-microprobe analyses of the test specimens. At temperatures up to 650°C, the maximum weight-change varied from a weight loss of 0.45 for vanadium to a weight gain of 0.46 mg per sq dm-day for the V-40Ti alloy. Chemical analyses of the corrosion test specimens indicated that vanadium-base materials were sensitive to the activity of oxygen, carbon, and/or nitrogen in sodium. Alloys containing >10 wt pct Ti increased in oxygen, carbon, and in some tests, nitrogen content, indicating that alloys of this titanium content are not compatible with sodium of the present purity. Complex titanium carbide precipitates were observed near the surface. V-5Ti-15Cr alloy was more resistant to liquid sodium than the other vanadium materials investigated.     

Quantification of overaging hardening kinetics of aluminum alloys

The overaging hardening kinetics in several aluminum alloys has been quantified, based on a relation between hardness and time for precipitation-hardened alloys. The relation takes into account precipitate coarsening and Orowan bowing mechanisms. Hardness-time curves from literature data for Al-2.5Cu-1.5Mg-(Si,Ag) (wt pct) alloys at 200° C and for an Al-1.23Mg-1.64Cu-6.15Zn (wt pct) alloy at 120° C are analysed, which are found to obey the overaging kinetics relation.     

Laves phase precipitation in Fe-Ta alloys

Development of an iron-base alloy hardened by particles of an intermetallic compound rather than a carbide is a desirable goal because of the greater thermal stability of such a dispersion. As a first step in the development of iron-base alloys hardened with the Laves phase, structural studies of binary Fe-Ta alloys have been undertaken. The structures of two phase Fe-Ta alloys have been studied by means of optical and transmission electron microscopy, X-ray diffraction, electron beam microprobe analysis, and scanning electron microscopy. The hardness change as a function of time at 600°, 700°, and 800°C has been determined for binary alloys with 1 at. pct Ta and 2 at. pct Ta in iron. Also, the uniaxial tensile strengths of solution treated, quenched, and aged samples have been determined. These studies suggest that the compound, Fe₂Ta, is isomorphous with the structure type, MgZn₂, (C14) and has a range of compositional homogeneity. The latter results correspond with the predictions of the Engel-Brewer correlation. Also, it has been found that precipitation occurs at grain boundaries, dislocations, and randomly throughout the matrix. Particles which form at dislocations have a (100)α habit plane; whereas a (110)α habit plane has been reported by others^1,3 for the hexagonal Laves phase in α iron. Hypereutectoid composition alloys quenched from the ö phase field have a completely retained § structure. Isothermal decomposition at 600°, 700°, and 800°C of alloys with the retained § structure results in a sizable hardness increase in 2 at. pct Ta alloys but only a modest increase in 1 at. pct Ta alloys. Brittle fracture of aged tensile specimens tested at room temperature reveals that the ductile-brittle transition temperature in tension is above room temperature.     

Fatigue crack propagation in aluminum-base copper alloys

Fatigue crack propagation ratesda/dN in binary Al alloys with 3.6 wt pct Cu and 6.3 wt pct Cu and commercial 2024 aged at 21°C were compared with 99.95⁺ wt pct aluminum. Omitting an anomalous region at lowΔK, the extrapolated rates for “pure” aluminum are more than 100 times greater than those in the three alloys at the same ΔK. The data for the alloys fit into a single scatter band of a factor of three. It was suggested thatda/dN varies inversely with the square of the strength of the alloy but that another parameter related to the fatigue crack propagation energy per unit area is also important. Theda/dN vs ΔK curves were determined for 3.6 wt pct Cu single crystals aged seven days at 21°C which containGP zones and two and seven days at 160°C which contain mixtures ofθ′ andθ′’. No systematic variation of (da/dN _Δ with crystallographic orientation was discerned, but the naturally aged specimen had a strong orientation dependence on crack initiation. At low ΔK 21°C aged specimens gave the lowestda/dN while at high ΔK the warm aged specimens gave the lower values ofda/dN. Measurement ofda/dN vs ΔK curves were conducted on specimens of 3.6 wt pct Cu with 1 mm equiaxed grains aged for various times at 130°C, 160°C, and 190°C. All warm aged specimens experienced brittle intergranular fracture at sufficiently high ΔK. The transition ΔK where intergranular fracture first appears is inversely proportional to the aging temperature. The change of fracture mode from intra to intergranular occurs gradually over a broad range of ΔK which shifts to lower ΔK with increase in aging temperature. This research was supportd by U.S. Air Force Office of Scientific Research, Office of Aerospace REsearch, Grant No. AF-AFOSR-73-2431.     

The Role of Silicon in the Solidification of High-Cr Cast Irons

This work analyzes the effect of different additions of silicon (0 to 5.0 pct) on the structure of a high-Chromium white cast iron, with chromium content of 16.8 pct and carbon 2.56 pct. The alloys were analyzed in both as-cast and heat-treated conditions. Casting was undertaken in metallic molds that yielded solidification rates faster than in commercial processes. Nevertheless, there was some degree of segregation of silicon; this segregation resulted in a refinement in the microstructure of the alloy. Silicon also generated a greater influence on the structure by destabilizing the austenitic matrix, and promoted greater precipitation of eutectic carbides. Above 3 pct silicon, pearlite formation occurred in preference to martensite. After the destabilization heat treatment, the matrix structure of the irons up to 3 pct Si consisted of secondary carbides in a martensitic matrix with some retained austenite; higher Si additions produced a ferritic matrix. The different as-cast and heat-treated microstructures were correlated with selected mechanical properties such as hardness, matrix microhardness, and fracture toughness. Silicon additions increased matrix microhardness in the as-cast conditions, but the opposite phenomenon occurred in the heat-treated conditions. Microhardness decreased as silicon content was increased. Bulk hardness showed the same behavior. Fracture toughness was observed to increase up to 2 pct Si, and then decreased for higher silicon contents. These results are discussed in terms of the effect of eutectic carbides’ size and the resulting matrix due to the silicon additions.     

Diffusion and electrotransport of carbon in γ-phase iron-nickel alloys

The diffusivity and electrotransport behavior of carbon in an iron-32.5 wt pct nickel alloy was investigated for the temperature range of 1220 to 1415 K for two carbon concentrations. The electrical resistivity was measured as a function of temperature between room temperature and 1625 K for carbon contents of 0.002, 0.10, and 0.21 wt pct. Values for the activation energy for diffusion of 32.0 ± 5.8 and 36.0 ± 5.2 kcal per mole were determined for alloys containing 0.10 and 0.21 wt pct carbon respectively, with slightly smaller values being found for the activation energy for electrotransport. The sign of the effective valence indicates that carbon migrates toward the cathode.     

Transverse creep and stress-rupture of borsic-aluminum composites and borsic-aluminum composites containing stainless steel and titanium

The transverse creep and stress rupture behavior of a number of Borsic®-aluminum composites was investigated at temperatures from 200° to 400°C. The cpmposites studied consisted of nominally 50 vol pct Borsic fiber and included matrices of 6061, 2024, 2219, and 5052 aluminum alloys. The effect of heat treatment was studied in the heat-treatable alloys. Where transverse composite behavior differed from matrix alloy behavior, the difference was found to be due primarily to a change in fracture mode at higher matrix strength levels from matrix failure to one which involves longitudinal fiber splitting. Of the four basic matrix alloys tested, the best creep resistance was obtained with the 2024 matrix. Additional improvement of transverse creep and stress rupture resistance was realized by incorporating transverse reinforcements such as SAP alloy foil, titanium alloy foil, and 0.002 in. stainless steel wire in the composites. These reinforcements made possible good transverse properties at 400°C with density increases of ≤15 pct. The two best additions were 21 pctβ III titanium foil and 6 pct AFC-77 stainless steel wire. A transverse fracture mode incorporating longitudinal fiber splitting was documented and characterized, and its effect on composite behavior determined. The use of nonsplitting fibers such as 5.6 mil B and 5.7 mil Borsic in preventing this fracture mode was investigated.     

Thermodynamics of carbon in austenite and Fe-Mo austenite

A Cahn Electrobalance has been used to determine directly and very accurately the carbon content of iron, iron-0.48 wt pct molybdenum and iron-1.16 wt pct molybdenum specimens which were equilibrated with a series of methane-hydrogen gas mixtures of constant composition. The equilibria investigated involved the austenite phases of the alloys at 783, 813 and 848‡C. The experimental results permit direct calculation of the activities of carbon in the samples, relative to graphite as unity, and of the enthalpy and entropy of solution of carbon. The results are compared with the experimental measurements of a number of other investigators. The results are in excellent agreement with those of Smith and Schenck and Kaiser for the Fe-C system at 800‡C, and indicate -H _C ^/M values of 9700 ± 500 cal/mole for pure Fe, 10,030 ± 500 cal/mole for an Fe-0.48 wt pct Mo alloy, and 10,150 ± 500 cal/mole for an Fe-1.16 wt pct Mo alloy. The effect of molybdenum in austenite is to decrease the activity coefficient of carbon in austenite.     

Thermodynamics of carbon in austenite and Fe-Mo austenite

A Cahn Electrobalance has been used to determine directly and very accurately the carbon content of iron, iron-0.48 wt pct molybdenum and iron-1.16 wt pct molybdenum specimens which were equilibrated with a series of methane-hydrogen gas mixtures of constant composition. The equilibria investigated involved the austenite phases of the alloys at 783, 813 and 848‡C. The experimental results permit direct calculation of the activities of carbon in the samples, relative to graphite as unity, and of the enthalpy and entropy of solution of carbon. The results are compared with the experimental measurements of a number of other investigators. The results are in excellent agreement with those of Smith and Schenck and Kaiser for the Fe-C system at 800‡C, and indicate -H _C ^/M values of 9700 ± 500 cal/mole for pure Fe, 10,030 ± 500 cal/mole for an Fe-0.48 wt pct Mo alloy, and 10,150 ± 500 cal/mole for an Fe-1.16 wt pct Mo alloy. The effect of molybdenum in austenite is to decrease the activity coefficient of carbon in austenite.