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.     

Interdiffusion and intrinsic diffusion in the Ni AI (δ) phase of the Al-Ni system

Interdiffusion coefficients at 950 to 1150°C and the ratio of intrinsic diffusion coefficients at 1100°C were measured as functions of composition in the NiAl (δ) phase of the Al-Ni system, using a vapor-solid technique. Diffusivity values were also obtained for the Ni₃Al (∈)and Ni (Al) solid solution (ζ) phases from 950 to 1150°C. The interdiffusion coefficient in NiAl (δ) varies several orders of magnitude over the δ phase field with a deep minimum in the diffusivity-composition curve at 48 to 49 at. pct Al. The ratio of intrinsic diffusion coefficients, D_ni/D_aI, in the δ phase also varies with composition from a value of 3 to 3.5 below 50 at. pct Al to 0.1 or less above 50 at. pct Al. Formerly Research Assistant, Department of Mate-rials Science, SUNY at Stony Brook, New York.     

Interdiffusional effects between tungsten fibers and an iron-nickel-base alloy

Tungsten fibers in the INCOLOY* 903 alloy were annealed for over 100 hours at 1038 °C and 1200 °C. It was found that interdiffusion results in the formation of a reaction zone. SEM-EDS probe analysis showed that the chemistries across this zone were constant, suggesting the zone was a compound phase. The composition of the compound was estimated to be that of a μ-type phase. The local chemistry (in atomic percent) at the reaction zone/alloy matrix interface was found to be approximately 8 pct W, 1.2 pct Nb, 40 pct Fe, 14 pct Co, and 36 pct Ni. In addition, recrystallization was observed in both the remaining tungsten fiber and the nearby INCOLOY 903 matrix after annealing at 1200 °C, but not at 1038 °C. The results of this study suggest that reaction zone growth kinetics can be minimized by the reduction of Co and Fe and the increase of W in the matrix alloy.     

Cosintering of Powder Injection Molding Parts Made from Ultrafine WC-Co and 316L Stainless Steel Powders for Fabrication of Novel Composite Structures

Sintering response and phase formation during sintering of WC-Co/316L stainless steel composites produced by assembling of powder injection molding (PIM) parts were studied. It is shown that during cosintering a significant mismatch strain (>4 pct) is developed in the temperature range of 1080 °C to 1350 °C. This mismatch strain induces biaxial stresses at the interface, leading to interface delamination. Experimental results revealed that sintering at a heating rate of 20 K/min could be used to decrease the mismatch strain to <2 pct. Meanwhile, WC is decomposed at the contact area and the diffusion of C and Co into the iron lattice results in the formation of a liquid and MC and M₆C carbides at 1220 °C. Spreading of the liquid accelerates the reaction, affecting the dimensional stability of the PIM parts. To prevent the reaction, surface oxidation of the cemented carbide followed by hydrogen reduction during sintering was examined. Although the amount of mismatch strain increased, formation of a metallic interface consisting of a W-Co alloy (45 to 50 at. pct Co) and a Co-rich iron alloy (18 at. pct Co) prevented the decomposition of WC and melt formation. It is also shown that the deposition of a thin Ni layer after thermal debinding decreases the mismatch stresses through melt formation, although interlayer diffusion causes pore-band formation close to the steel part.     

Tracer diffusion of59Fe and51Cr in Fe-17 Wt Pet Cr-12 Wt Pet Ni austenitic alloy

The volume and grain-boundary diffusion of⁵⁹Fe and⁵¹Cr have been studied in an austenitic iron alloy containing 17 wt pct Cr and 12 wt pct Ni. The diffusivities in this alloy of these two tracers and⁶³Ni are compared with their diffusivities in pure iron and in other austenitic stainless steels. For volume diffusion at any particular temperature in the present alloy, Cr is the most rapid while Ni is the slowest, and all three tracers diffuse slower than that reported for pure iron or for other austenitic stainless steels. For grain-boundary transport, Fe diffuses most rapidly above 850°C and Ni diffuses most rapidly below that temperature. The activation energies for both volume and grain-bounary diffusion obey the relationshipQ _Ni <Q _Cr <Q _Fe. Formerly Presidential Intern in the Metals and Ceramics Division, Oak Ridge National Laboratory     

The interaction of reaction-bonded silicon carbide and inconel 600 with a nickel-based brazing alloy

The objective of the present research was to join reaction-bonded silicon carbide (RBSC) to INCONEL 600 (a nickel-based superalloy) for use in advanced heat engine applications using either direct brazing or composite interlayer joining. Direct brazing experiments employed American Welding Society (AWS) BNi-5, a commercial nickel-based brazing alloy, as a filler material; composite interlayers consisted of intimate mixtures of α-SiC and BNi-5 powders. Both methods resulted in the liquid filler metal forming a Ni-Si liquid with the free Si in the RBSC, which, in turn, reacted vigorously with the SiC component of the RBSC to form low melting point constituents in both starting materials and Cr carbides at the metal-ceramic interface. Using solution thermodynamics, it was shown that a Ni-Si liquid of greater than 60 at. pct Ni will decompose a-SiC at the experimental brazing temperature of 1200 ‡C; these calculations are consistent with the experimentally observed composition profiles and reaction morphology within the ceramic. It was concluded that the joining of RBSC to INCONEL 600 using a nickel-based brazing alloy is not feasible due to the inevitability of the filler metal reacting with the ceramic, degrading the high-temperature properties of the base materials.     

Nitrogen solubility in complex liquid Fe−Cr−Ni alloys

Measurements of nitrogen solubility were performed in a series of liquid iron-chromium-nickel alloys near the composition of the commercial superalloy INCOLOY^* 800 (I:21 pct Cr, 33 pct Ni, bal. Fe). This work was carried out at 1450 to 1600°C and up to one atmosphere of nitrogen gas pressure. Sieverts' law was obeyed by nitrogen in all the alloys. Changes were observed in the compositions of all the melts studied, mainly due to the chromium loss by volatilization. These changes necessitated nitrogen solubility measurements in a series of alloys immediately surrounding alloy I. The experimental results have been prepared as a regression polynomial equation for the logarithm of nitrogen solubility as a function of temperature and reported weight percentages of chromium and nickel in the alloys. The standard Gibbs free energy of nitrogen solution in a region around alloy I is given as ΓG ^o=−58,700+48.20T Joules/g atom N. At 1600°C, the temperature coefficient of nitrogen solubility in I is −2.65×10⁻⁴ pct N/K. The shape of a portion, of the nitrogen solubility surface for the Fe−Cr−Ni system near to alloy I at 1600°C is defined.     

Optimization of composition and heat treatment of age-hardened Pt-Al-Cr-Ni alloys

The objective of this work was to produce an alloy showing a microstructure similar to Ni-base superalloys, but with Pt as base metal. The Pt-base alloys with various contents of Al, Cr, and Ni were arc melted. Solution heat treatments at 1450 °C followed by water quenching lead to single-phase alloys. Ageing at 1000 °C resulted in the precipitation of Ll₂ ordered particles. An alloy with 11 at. pct Al, 3 at. pct Cr, 6 at. pct Ni, and Pt balance shows cuboidal precipitates with edge lengths of 200 to 500 nm along with a volume fraction of 23 pct and a lattice misfit of −0.1 pct. Aging at 1100 °C leads to coarsening of precipitates. Volume fraction and morphology of the precipitates were investigated by scanning electron microscopy and optical microscopy. X-ray diffraction as well as transmission electron microscopy (TEM) were applied to verify the crystal structure.     

Optimization of composition and heat treatment of age-hardened Pt−Al−Cr−Ni alloys

The objective of this work was to produce an alloy showing a microstructure similar to Ni-base superalloys, but with Pt as base metal. The Pt-base alloys with various contents of Al, Cr, and Ni were are melted. Solution heat treatments at 1450 °C followed by water quenching lead to single-phase alloys. Ageing at 1000 °C resulted in the precipitation of L1₂ ordered particles. An alloy with 11 at. pct Al, 3 at. pct Cr, 6 at. pct Ni, and Pt balance shows cuboidal precipitates with edge lengths of 200 to 500 nm along with a volume fraction of 23 pct and a lattice misfit of −0.1 pct. Aging at 1100 °C leads to coarsening of precipitates. Volume fraction and morphology of the precipitates were investigated by scanning electron microscopy and optical microscopy. X-ray diffraction as well as transmission electron microscopy (TEM) were applied to verity the crystal structure. M. Huller, formerly with Metallic Materials, University Bayreuth, D-95440 Bayreuth, Germany     

High temperature phase chemistries and solidification mode prediction in nitrogen-strengthened austenitic stainless steels

Nitronic 50 and Nitronic 50W, two nitrogen-strengthened stainless steels, were heat treated over a wide range of temperatures, and the compositions of the ferrite and austenite at each temperature were measured with analytical electron microscopy techniques. The compositional data were used to generate the (γ + δ phase field on a 58 pct Fe vertical section. Volume fractions of ferrite and austenite were calculated from phase chemistries and compared with volume fractions determined from optical micrographs. Weld solidification modes were predicted by reference to the Cr and Ni contents of each alloy, and the results were compared with predictions based on the ratios of calculated Cr and Ni equivalents for the alloys. Nitronic 50, which contained ferrite and austenite at the solidus temperature of 1370 °C, solidified through the eutectic triangle, and the weld microstructure was similar to that of austenitic-ferritic solidification. Nitronic 50W was totally ferritic at 1340 °C and solidified as primary delta ferrite. During heat treatments, Nitronic 50 and Nitronic 50W precipitated secondary phases, notably Z-phase (NbCrN), sigma phase, and stringered phases rich in Mn and Cr.     

Structural superplasticity in a fine-grained eutectic intermetallic NiAl−Cr alloy

A rapidly solidified and thermomechanically processed fine-grained eutectic NiAl−Cr alloy of the composition Ni₃₃Al₃₃Cr₃₄ (at, pct) exhibits structural superplasticity in the temperature regime from 900°C to 1000°C at strain rates ranging from 10⁻⁵ to 10⁻³ s⁻¹. The material consists of a B2-ordered intermetallic NiAl(Cr) solid solution matrix containing a fine dispersion of bcc chromium. A high strain-rate-sensitivity exponent of m=0.55 was achieved in strain-rate-change tests at strain rates of about 10⁻⁴ s⁻¹. Maximum uniform elongations up to 350 pct engineering strain were recorded in superplastic strain to failure tests. Activation energy analysis of superplastic flow was performed in order to establish the diffusion-controlled dislocation accommodation process of grain boundary sliding. An activation energy of Q _c=288±15 kJ/mole was determined. This value is comparable with the activation energy of 290 kJ/mole for lattice diffusion of nickel and for ⁶³Ni tracer selfdiffusion in B2-ordered NiAl. The principal deformation mechanism of superplastic flow in this material is grain-boundary sliding accommodated by dislocation climb controlled by lattice diffusion, which is typical for class II solid-solution alloys. Failure in superplastically strained tensile samples of the fine-grained eutectic alloy occurred by cavitation formations along NiAl‖‖Cr interfaces.     

Short-range order effects on the tensile behavior of a nickel-base alloy

A nickel base weld filler metal alloy with nominal composition of 67 pct Ni, 20 pct Cr, 3 pct Mn, 3 pct Fe, and 2.5 pct Nb (Cb) is used to make austenitic-ferritic dissimilar metal joints. Tensile properties were determined for this alloy over the range 25 to 732°C at strain-rates of 3×10⁻⁶ and 3×10⁻⁴/s. Above about 450°C, both the yield strength and the ultimate tensile strength in the low strain-rate tests showed significant increases over the strengths at the higher strain-rate. The enhanced values for the yield strength persisted to the highest test temperature (732°C), whereas the ultimate tensile strength for the low strain-rate fell below the curve for the higher strain-rate at about 600°C. Above 600°C, the ultimate tensile strength dropped off rapidly and at 677°C approached the yield strength (i.e., the uniform elongation dropped to less than 1 pct). The strain-rate effects have been attributed to “K-state” formation, an effect that investigators have attributed to short range order in other Ni−Cr base alloys.     

Chemistry and distribution of phases produced by solid state sic/nicrai reaction

Aspects of the solid state reaction between SiC and a model superalloy consisting of Ni-20 at. pct Cr-10 at. pct Al at 1150 °C are studied in detail using analytical electron microscopy. The metal reaction zone formed on the metal side of the original metal/ceramic interface is found to consist of a complex mixture of four phases: γ′-Ni₃Al, β-NiAl, α-Cr, and Ξ, a ternary Ni-Si-Al phase. The chemistry of each phase is determined using X-ray spectroscopy in the analytical electron microscope. The effect of silicon on the γ —γ′ microstructure of the model superalloy is described, and is shown to alter the chemistry, morphology, and size of the γ′ phase. The γ-γ′ lattice parameter mismatch is calculated from the chemistry data, and changes in this mismatch caused by silicon correlate well with γ′ morphology changes. Finally, phase equilibria concepts are used to explain the presence and distribution of phases in the metal reaction zone. Department of Materials Science and Engineering, and during the course of this work was employed at the General Electric Corporate Research and Development Center as part of the Cornell University Engineering Cooperative Program.     

Processing and Characterization of Cu-Al-Ni Shape Memory Alloy Strips Prepared from Elemental Powders <Emphasis Type="Italic">via</Emphasis> a Novel Powder Metallurgy Route

In the present work, Cu-Al-Ni shape memory alloy strips were prepared successfully from premixed elemental Cu, Al, and Ni powders in the ratio 82:14:4 (wt pct) by a novel processing route consisting of preparing powder preforms, sintering, and unsheathed hot rolling of the sintered preforms. Subsequently, the hot rolled strips were homogenized. The as-rolled strips consisted of two phases—α and β′. A postconsolidation homogenization of the hot rolled strips was carried out at 1173 K (900 °C) for different time periods. It has been shown that a homogenization period of 4 hours was sufficient to achieve a single-phase material consisting of only martensitic phase. It also has been shown that the 4-hour homogenized and quenched Cu-Al-Ni shape memory alloy strips primarily consisted of self-accommodated β′ martensite plates, which are necessary for realizing shape memory effect (SME). The finished hot rolled Cu-Al-Ni strips had a fracture strength of 476 MPa, coupled with 2.5 pct elongation. The shape memory tests showed almost 100 pct recovery after 10 thermomechanical cycles in the hot rolled strips at 1 pct and 2 pct prestrain level.     

An Investigation of the Massive Transformation from Ferrite to Austenite in Laser-Welded Mo-Bearing Stainless Steels

A series of 31 Mo-bearing stainless steel compositions with Mo contents ranging from 0 to 10 wt pct and exhibiting primary δ-ferrite solidification were analyzed over a range of laser welding conditions to evaluate the effect of composition and cooling rate on the solid-state transformation to γ-austenite. Alloys exhibiting this microstructural development sequence are of particular interest to the welding community because of their reduced susceptibility to solidification cracking and the potential reduction of microsegregation (which can affect corrosion resistance), all while harnessing the high toughness of γ-austenite. Alloys were created using the arc button melting process, and laser welds were prepared on each alloy at constant power and travel speeds ranging from 4.2 to 42 mm/s. The cooling rates of these processes were estimated to range from 10 K (°C)/s for arc buttons to 10⁵ K (°C)/s for the fastest laser welds. No shift in solidification mode from primary δ-ferrite to primary γ-austenite was observed in the range of compositions or welding conditions studied. Metastable microstructural features were observed in many laser weld fusion zones, as well as a massive transformation from δ-ferrite to γ-austenite. Evidence of epitaxial massive growth without nucleation was also found when intercellular γ-austenite was already present from a solidification reaction. The resulting single-phase γ-austenite in both cases exhibited a homogenous distribution of Mo, Cr, Ni, and Fe at nominal levels.     

Effect of heat treatment on the microstructure,tensile properties,and fracture behavior of permanent mold Al-10 wt pct Si-0.6 wt pct Mg/SiC/10p composite castings

The present study was undertaken to investigate the effect of solution treatment (in the temperature range 520 °C to 550 °C) and artificial aging (in the temperature range 140 °C to 180 °C) on the variation in the microstructure, tensile properties, and fracture mechanisms of Al-10 wt pct Si-0.6 wt pct Mg/SiC/10_p composite castings. In the as-cast condition, the SiC particles are observed to act as nucleation sites for the eutectic Si particles. Increasing the solution temperature results in faster homogenization of the microstructure. Effect of solution temperature on tensile properties is evident only during the first 4 hours, after which hardly any difference is observed on increasing the solution temperature from 520 °C to 550 °C. The tensile properties vary significantly with aging time and temperature, with typical yield strength (YS), ultimate tensile strength (UTS), and percent elongation (EL) values of ∼300 MPa, ∼330 MPa, and ∼1.4 pct in the underaged condition, ∼330 MPa, ∼360 MPa, and ∼0.65 pct in the peakaged condition, and ∼323 MPa, ∼330 MPa, and ∼0.8 pct in the overaged condition. Prolonged solution treatment at 550 °C for 24 hours results in a slight improvement in the ductility of the aged test bars. The fracture surfaces exhibit a dimple morphology and cleavage of the SiC particles, the extent of SiC cracking increasing with increasing tensile strength and reaching a maximum in the overaged condition. Microvoids act as nucleation sites for the formation of secondary cracks that promote severe cracking of the SiC particles. A detailed discussion of the fracture mechanism is given.     

Kinetics of Reverse Transformation from Pearlite to Austenite in an Fe-0.6 Mass pct C Alloy and the Effects of Alloying Elements

Substitutional alloying effects on reversion kinetics from pearlite structure at 1073 K (800 °C) in an Fe-0.6 mass pct C binary alloy and Fe-0.6C-1 or 2 mass pct M (M = Mn, Si, Cr) ternary alloys were studied. Reverse transformation in the Fe-0.6C binary alloy at 1073 K (800 °C) was finished after holding for approximately 5.5 seconds. The reversion kinetics was accelerated slightly by the addition of Mn but retarded by the addition of Si or Cr. The difference of acceleration effects by the addition of the 1 and 2 mass pct Mn is small, whereas the retardation effect becomes more significant by increasing the amount of addition of Si or Cr. It is clarified from the thermodynamic viewpoint of carbon diffusion that austenite can grow without partitioning of Mn or Si in the Mn- or Si-added alloys. On the one hand, austenite growth is controlled by the carbon diffusion, whereas the addition of them affects carbon activity gradient, resulting in changes in reversion kinetics. On the other hand, thermodynamic calculation implies that the long-range diffusion of Cr is necessary for austenite growth in the Cr-added alloys. It is proposed that austenite growth from pearlite in the Cr-added alloys is controlled by the diffusion of Cr along austenite/pearlite interface.     

Study of the porosity produced in an aluminum alloy matrix composite due to a T6 heat treatment

In the present article, the reactions and phase transformations occurring between the fibers and the matrix during the T6 thermal treatment of an industrial composite are analyzed. The composite (taken from prototype diesel piston heads) consisted of an aluminum alloy conforming to AFNOR AS12UNG (Al-12 pct Si-1 pct Mg-1 pct Cu-1 pct Ni) that was reinforced with SAFFIL δ-alumina short fibers. Different heat-treating temperatures, holding times, and heating rates were considered. As a result of all these treatments, small to medium size (between 1 to 10 μm) holes were observed at the matrixfiber interfaces, although the amount and size varied depending on the heat-treating conditions. This porosity was attributed to the volume contraction associated with the reaction between the silica contained in the preform (colloidal silica is the binder for the alumina fibres) and the magnesium present in the aluminum alloy to produce MgO and Si during the solution heating treatment. This local volume contraction is higher than 28 pct.     

Experimental observations on the nucleation and growth of δ′ (Al3Li) in dilute Al-Li alloys

The influence of nucleation temperature and pre-aging thermal history on δ′ (Al₃Li) precipitate distributions was studied in a binary Al-8.2 at. pct Li alloy. The distribution of δ′ in the microstructure is found to be homogeneous and insensitive to variations in pre-aging thermal history when nucleation occurs at large undercoolings (ΔT > 90 °C). When nucleation occurs at low undercoolings (90 °C > gDT > 40 °C), nucleation is homogeneous but the precipitate number density is influenced by preaging thermal history. At very small undercoolings (ΔT < 40 °C) nucleation next to dislocations is favored unless large numbers of excess vacancies are present, in which case homogeneous nucleation can be more prevalent than nucleation next to dislocations. The early stages of δ′ growth are seen to be consistent with bulk diffusion control. Enhanced growth rates are observed when the material contains a higher concentration of excess vacancies. The retention of this enhanced growth for long periods of time suggests that vacancies can be trapped within the structure for long times despite their mobility. Morphological instabilities are found to develop during δ′ particle growth. These instabilities occur at very small particle sizes and cause the normally spherical particles to tend toward a dendritic morphology.