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.     

Microstructural basis for the effect of chromium on the strength and toughness of AF1410-based high performance steels

The variation in strength and Charpy impact toughness as a function of tempering temperature in the range of 200 ‡C to 650 ‡C was investigated in AF 1410 and AF 1410 + 1 pct Cr steels produced in a laboratory-scale, and a commercially produced AerMet 100 steel. The tensile test results showed that AF 1410 + 1 pct Cr had lower strength compared to AF 1410, while AerMet 100 had the highest strength of the three steels examined. Transmission electron microscopy (TEM) studies demonstrated that the strength variations among the steels can be attributed to differences in the matrix/carbide coherency strain and the volume fraction of the strengthening M₂C carbides. The toughness values of the three steels were comparable when tempered up to 424 ‡C. Tempering at and above 454 ‡C resulted in a relative enhancement of toughness in AF 1410 + 1 pct Cr steel compared to AF 1410. This toughening was attributed to the destabilization of cementite at lath and prior austenite boundaries and the formation of reverted austenite.     

Influence of aluminum and chromium on the activity of oxygen in nickel based melts at 1600?C

The activity of oxygen in technically important nickel melts containing 15 wt pct cobalt and 5 wt pct molybdenum has been determined at 1600‡C for various concentrations of chromium ranging from 0 to 30 wt pct and aluminum varying from 0 to 15 wt pct. The activity of oxygen was measured by an electrochemical technique using yttria-doped thoria electrolyte cells. The results obtained are analyzed in terms of activity coefficients of oxygen as a function of aluminum and chromium contents in the melts. Clear positive deviations of the experimentally determined from the calculated activity coefficients of oxygen were found when aluminum was added to Ni-Co-Mo melts with or without chromium. From the results obtained in the range between 1 and 10 wt pct aluminum, the following equilibrium constants for the reaction 2 [Al] + 3 [O] ⇋ Al₂O₃ in the nickel based melts at 1600‡C were calculated: loga ₀ =-2/3 log [pct Al] - 3.94 for 0 pct Cr loga ₀ = -2/3 log [pct Al] - 4.21 for 10 pct Cr loga ₀ = -2/3 log [pct Al] - 4.81 for 20 pct Cr loga ₀ = -2/3 log [pct Al] - 5.06 for 30 pct Cr.     

Coarsening rate of Β precipitates in Al-11Mg alloy

The coarsening rates of the Β precipitates in an Al-11 wt pct Mg alloy at 250 ‡C, 316 ‡C, and 330 ‡C aging temperatures were investigated. As predicted by the modified Lifshitz-Slyozov-Wagner (MLSW) theory of diffusion controlled particle coarsening, a linear dependence ofr ³ ont in the long time aging region was observed. Interfacial energies of 8.1, 1.8, and 1.7 J/m² for 250 ‡C, 316 ‡C, and 330 ‡C, respectively, were computed from these data using the MLSW theory. The high values of interfacial energy indicate that the Β precipitates are incoherent with respect to the Al matrix. The especially high value of interfacial energy and low activation energy for the precipitate coarsening at 250 ‡C suggest that significant short circuit diffusion occurs at this aging temperature.     

Retardation of intermetallic phase formation in experimental superferritic stainless steels

While superferritic stainless steels containing 29 pct chromium possess excellent resistance to corrosion, they may, under certain conditions, be embrittled by the precipitation of intermetallic phases. The extent to which the precipitation reactions can be retarded by alloying additions of aluminum and copper has been evaluated. It was found that additions of aluminum to an Fe-29 pct Cr-4 pct Mo-1.5 pct Ni base alloy suppress the precipitation of the undesirable sigma and chi intermetallic phases, but additions of up to 3 pct aluminum promote 475 ‡C embrittlement. Additions of copper slightly reduce the precipitation of sigma and chi phases under most conditions but enhance 475 ‡C embrittlement. The resistance to corrosion in 10 pct H₂SO₄ and 10 pct FeCl₃ was assessed. All the aluminum-containing alloys performed significantly better in H₂SO₄ than the base alloy; however, large additions of aluminum had a deleterious effect on the pitting resistance in FeCl₃. Additions of copper improved the resistance to FeCl₃ and lowered the rate of corrosion in the H₂SO₄ solution used.     

Effects of composition and microstructure on fatigue of γ/γ′-δ type eutectic alloys

Room temperature tension-tension fatigue tests were performed on two lamellar γ/γ′-δ alloys, one with 0 pct Cr and one with 6 pct Cr. The 6 pct Cr alloy was solidified at 3 cmJh while the 0 pct Cr alloy was solidified at 3 cm/h and 5.7 cm/h. Fatigue testing was done on both alloys in the as-directionally solidified condition and on the 0 pct Cr alloy after heat treatment. Increasing the growth speed of the 0 pct Cr alloy increased the fatigue life of the material at stresses above the 10⁷ cycle fatigue limit. Partial solution treating and aging of the 0 pct Cr alloy,R = 3 cm/h, increased the fatigue life relative to the as-directionally solidified material at high stresses, to the same extent as increasing the growth speed. Full solution treatment and aging of the 0 pct Cr alloy,R = 5.7 cm/ h, caused a reduction in the fatigue life relative to the as-directionally solidified material. Fatigue cracking tended to be faceted in the 6 pct Cr alloy as opposed to the more ductile failure of the 0 pct Cr alloy. Microstructural perfection, grain size and shape, interlamellar spacing, longitudinal cracking, and longitudinal and transverse ductility all are believed to have influenced the fatigue resistance of the alloys.     

The influence of Mo and Co on the embrittlement of an Fe-Ni-Mn alloy

In the “as rolled” condition an Fe-6 Ni-5 Mn maraging type alloy was found to be brittle exhibiting intergranular fractures. The addition of 2.5 pct Mo and 5.0 pct Mo increased the impact toughness of the “as rolled” material and changed the mode of brittle fracture to transgranular cleavage. The addition of 9 pct Co embrittled the alloy. On aging Mo and Co raised the peak hardness of the base Fe-6 Ni-5 Mn alloy, however, aging led to rapid embrittlement. The base alloy and an alloy containing 2.5 pct Mo showed brittle intergranular fractures on aging. The addition of 5 pct Mo gave rise to brittle transgranular cleavage fractures on aging at 450°C, but at temperatures less than 450°C there was always up to 20 pct intergranular fracture present in brittle fractures. At temperatures greater than 475°C brittle intergranular failure occurred in the 5 pct Mo alloy due to a grain boundary film of M₆C and Fe₂Mo. This paper is based upon a thesis submitted by D. R. Squires in partial fulfilment for a higher degree of CNAA at Sheffield Polytechnic.     

Structure and properties of a β solution treated,quenched, and aged si-bearing near-α titanium alloy

The microstructure, tensile properties, and fractographic features of a near-α titanium alloy, IMI 829(Ti-6.1 wt pct Al-3.2 wt pct Zr-3.3 wt pct Sn-1 wt pct Nb-05 wt pct Mo-0.32 wt pct Si) have been studied after aging over a temperature range of 550°C to 950°C for 24 hours following solution treatment in the β phase field at 1050°C and water quenching. Transmission electron microscopy studies revealed that aging at 625°C and above produced discrete silicides at α′ interplatelet boundaries. However, aging at 900°C and above has also resulted in the precipitation of β phase along the lath boundaries of martensite. The silicides have been found to have a hexagonal structure withc=0.36 nm anda=0.70 nm (designated as S₂ by earlier workers). There is a significant improvement in yield and ultimate tensile strength after aging at 625°C, but there is less improvement at higher aging temperatures. The tensile ductility is found to be drastically reduced. While the fracture surface of the unaged specimen shows elongated dimples, the aged samples show a mixed mode of fracture, consisting of facets, featureless parallel bands, and extremely fine dimples.     

Void formation during oxidation of the ODS alloy MA 6000

The present work deals with investigations on the subsurface void formation in the oxide dispersion strengthened (ODS) alloy MA 6000. The effect that results from oxidation has been analyzed for stress-free and stressed samples exposed to creep deformation up to a maximum exposure time of about 11,000 hours at 1050 ‡C. Additional tests in the stress-free condition have been performed at 1150 ‡C for up to 1300 hours. The depth of voiding increases with time following a parabolic rate law, and in the long-term range, it reaches about 2 mm at 1050 ‡C. For the stressed state during long-term exposure, the penetration was slightly deeper than for the stress-free state, and large voids were elongated perpendicular to the stress axis. For long exposure times at 1050 ‡C, the area of voids was about 1 to 2 pct of the total void-affected zone, and the maximum void diameter could reach about 25.0 Μm. A void-free zone, the width of which increased with time, formed beneath the scale. Among the mechanisms considered to explain the void formation, vacancy injection resulting from outward diffusion of mainly Cr seems the most consistent. J.L. GONZáLEZ-CARRASCO, formerly Postdoctoral Fellow, Commission of the European Communities, Institute for Advanced Materials, Joint Research Centro     

Substructure of ausformed martensite in Fe-Ni and Fe-Ni-C alloys

The martensite substructure after ausforming has been studied for two different martensite morphologies: partially twinned, lenticular martensite (Fe-33 pct Ni, M_s =-105‡C) and completely twinned “thin plate” martensite (Fe-31 pct Ni-0.23 pct C, M_s = -170‡C), and in both cases ausforming produces a dislocation cell structure in the austenite which is inherited, without modification, by the martensite. In the Fe-Ni alloy, the dislocation cell structure is found in both the twinned (near the midrib) and untwinned (near the interface) regions, the latter also containing a regular dislocation network generated by the transformation itself and which is unaltered by the austenite dislocation cell structure. Similarly, in the Fe-Ni-C alloy, the transformation twins are unimpeded by the prior cell structure. These observations show that carbide precipitation during ausforming is not necessarily required to pin the austenite cell structure and that the martensite-austenite interface, backed by either twins or dislocations, does not exhibit a ”sweeping” effect. Although the martensite transformation twins are not inhibited by the ausforming cell structure, they do undergo a refinement with increased ausforming, and it is indicated that the transformation twin width in martensite depends on the austenite hardness. However, the relative twin widths remain unchanged, as expected from the crystallographic theory. T. MAKI, Formerly with the University of Illinois     

Embrittlement of a continuously cooled Fe-25 Cr alloy

A study has been carried out on the effects of isothermal heat treatment at 475 and 550‡C and of continuous cooling at different rates from 850°C on the brittleness (as assessed by the ductile-brittle impact transition temperature) of a vacuum melted Fe-25 Cr alloy. The ductile-brittle transition temperature was found to be the lowest for the water quenched condition and highest for the furnace cooled condition and for material aged at 475‡C for long times (~500 h). An increase of brittleness with decreased cooling rate in the continuously cooled samples is attributed to the formation of more continuous and larger amounts of chromium nitrides and carbonitrides at the grain boundaries. Very little or no body centered cubic chromium-rich phase (alpha prime), associated with 475°C embrittlement, was observed. On aging at 550°C, the increased brittleness with time is also accounted for by the formation of grain boundary nitrides and carbonitrides. Although a similar effect takes place in the alloy heat treated at 475°C, the precipitation of alpha prime after long aging times enhances the brittleness. The tendency towards a more brittle condition with aging treatment and slower cooling rate is explained in terms of the Cottrell theory for brittle fracture.     

Strength of Initially Virgin Martensites at — 196 °C after Aging and Tempering

The compressive strength at —196°C of martensites in Fe-0.26 pct C-24 pct Ni, Fe-0.4 pct C-21 pct Ni, and Fe-0.4 pct C-18 pct Ni-3 pct Mo alloys, all with subzero M_s temperatures, has been determined in the virgin condition and after one hour at temperatures from —80 to +400 °C. The effects of ausforming (20 pct reduction in area of the austenite by swaging at room temperature prior to the martensitic transformation) were also investigated. For the unausformed martensites, aging at temperatures up to 0 °C results in relatively small increases in strength. Above 0 °C, the age hardening increment increases rapidly, reaching a maximum at 100 °C. Above 100 °C, the strength decreases continuously with increasing tempering temperature except for the molybdenum-containing alloy, which exhibits secondary hardening on tempering at 400 °C. For the ausformed martensites, the response to aging at subzero temperatures is greater than for unausformed material. Strength again passes through a maximum on aging at 100 °C. However, on tempering just above 100 °C, the ausformed materials show a slower rate of softening than the unausformed martensites. The strengthening produced by the ausforming treatment is largest for the Fe-0.4 pct C-18 pct Ni-3 pct Mo alloy, but there is no evidence of carbide precipitation in the deformed austenite to account for this effect of molybdenum. This paper is based on a presentation made at the “Peter G. Winchell Symposium on Tempering of Steel” held at the Louisville Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy and Heat Treatment Committees.     

Embrittlement of a continuously cooled Fe-25 Cr alloy

A study has been carried out on the effects of isothermal heat treatment at 475 and 550‡C and of continuous cooling at different rates from 850°C on the brittleness (as assessed by the ductile-brittle impact transition temperature) of a vacuum melted Fe-25 Cr alloy. The ductile-brittle transition temperature was found to be the lowest for the water quenched condition and highest for the furnace cooled condition and for material aged at 475‡C for long times (~500 h). An increase of brittleness with decreased cooling rate in the continuously cooled samples is attributed to the formation of more continuous and larger amounts of chromium nitrides and carbonitrides at the grain boundaries. Very little or no body centered cubic chromium-rich phase (alpha prime), associated with 475°C embrittlement, was observed. On aging at 550°C, the increased brittleness with time is also accounted for by the formation of grain boundary nitrides and carbonitrides. Although a similar effect takes place in the alloy heat treated at 475°C, the precipitation of alpha prime after long aging times enhances the brittleness. The tendency towards a more brittle condition with aging treatment and slower cooling rate is explained in terms of the Cottrell theory for brittle fracture.     

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.     

The effect of nickel,chromium, and primary

Elevated temperature creep tests were performed on Ti 6242Si deformed to small (<0.002) plastic strains using a highly aligned creep testing apparatus. Specimens were solution annealed at various temperatures below the beta transus(T _β - 6 °C toT _β − 52 °C) which controlled the volume fraction of primary alpha. Decreases in the amount of primary alpha are associated with decreased primary and steady-state creep rates. The effects of trace levels of the elements Ni and Cr on the creep properties of Ti 6242Si were also studied. Relatively small additions of Ni (0.075 to 0.093 wt pct), which appeared to segregate to the bcc beta phase, substantially in- creased the creep rates of this alloy, while additions of Cr up to 0.278 wt pct had little, if any, effect on the creep rates.     

Preaging effects in Al-Mg-Si alloys

The aging characteristics of aluminum alloy extrusions containing 0.60 to 0.90 wt pct Mg₂Si were determined. At low Mg₂Si levels, preaging treatments at room temperature and at elevated temperatures refined the G.P. zone dispersion and increased the alloy’s hardness after final aging. Preaging had little effect on hardness at the high end of the Mg₂Si range. These results are explained on the basis of current aging theories which invoke the concept of a critical temperature, above which homogeneous nucleation does not take place. This temperature varies from ≃150°C at 0.6 pct Mg2Si to ≃220°C at 1.5 pct Mg₂Si. The apparent activation energy for final aging was estimated to be 21 kcal/mole, a value which is intermediate between the activation energies for vacancy motion and solute (silicon and magnesium) diffusion in aluminum.     

The effect of sheet processing on the microstructure, tensile, and creep behavior of INCONEL alloy 718

The grain size, grain boundary character distribution (GBCD), creep, and tensile behavior of INCONEL alloy 718 (IN 718) were characterized to identify processing-microstructure-property relationships. The alloy was sequentially cold rolled (CR) to 0, 10, 20, 30, 40, 60, and 80 pct followed by annealing at temperatures between 954 °C and 1050 °C and the traditional aging schedule used for this alloy. In addition, this alloy can be superplastically formed (IN 718SPF) to a significantly finer grain size and the corresponding microstructure and mechanical behavior were evaluated. The creep behavior was evaluated in the applied stress (σ _a ) range of 300 to 758 MPa and the temperature range of 638 °C to 670 °C. Constant-load tensile creep experiments were used to measure the values of the steady-state creep rate and the consecutive load reduction method was used to determine the values of backstress (σ₀). The values for the effective stress exponent and activation energy suggested that the transition between the rate-controlling creep mechanisms was dependent on effective stresses (σ _e =σ _a σ₀) and the transition occurred at σ _e ≅ 135 MPa. The 10 to 40 pct CR samples exhibited the greatest 650 °C strength, while IN 718SPF exhibited the greatest room-temperature (RT) tensile strength (>1550 MPa) and ductility (ε _f >16 pct). After the 954 °C annealing treatment, the 20 pct CR and 30 pct CR microstructures exhibited the most attractive combination of elevated-temperature tensile and creep strength, while the most severely cold-rolled materials exhibited the poorest elevated-temperature properties. After the 1050 °C annealing treatment, the IN 718SPF material exhibited the greatest backstress and best creep resistance. Electron backscattered diffraction was performed to identify the GBCD as a function of CR and annealing. The data indicated that annealing above 1010 °C increased the grain size and resulted in a greater fraction of twin boundaries, which in turn increased the fraction of coincident site lattice boundaries. This result is discussed in light of the potential to grain boundary engineer this alloy. INCONEL is a registered trademark of Special Metals Corp., Huntington, WV. This article is based on a presentation made in the symposium entitled “Processing and Properties of Structural Materials,” which occurred during the Fall TMS meeting in Chicago, Illinois, November 9–12, 2003, under the auspices of the Structural Materials Committee.     

Role of Mo and W during sensitization of superaustenitic stainless steel—crystallography and composition of precipitates

This study concerns the crystallographic identification and compositions of precipitates formed in superaustenitic stainless steel. Three experimental alloys, all containing 24 wt pct Cr, 22 wt pct Ni, and 0.5 wt pct N but with varying amounts of Mo and W, were investigated after sensitization heat treatment (aging) at 900 °C. The contents of Mo and W in the three alloys were 7 wt pct Mo, (6 wt pct Mo + 2 wt pct W) and (5 wt pct Mo + 5 wt pct W), respectively. While σ and x were the main secondary phases found in the W-free alloy, replacement of Mo by W was found to promote the formation of Laves-related phases with high Mo + W content. The complex crystallographic nature of Laves-related precipitates was exemplified through the formation of intergrowing C14 Laves, μ, and C phases, all with closely related crystal structures. There was no difference in chemical composition between the three phases. Prolonged aging resulted in intragranular precipitation of different intermetallic phases, as well as formation of nitrogen bearing phases, π and Cr₂N, adjacent to previously formed intermetallic precipitates. The content of Mo + W was found to decrease with increasing aging time for all secondary phases.