A transmission electron microscopy study of the mechanisms of strengthening in heat-treated Co-Cr-Mo-C alloys

Microstructures produced in the Co-Cr-Mo-C alloy H.S.21 were observed by transmission electron microscopy in cast specimens following solutionizing at 1230°C and aging at 650°C and in low-carbon wrought specimens following solutionizing and aging at 650°C and 750°C. In all cases, aging was found to promote the formation of fcc stacking faults and to cause an initial martensitic transformation from the fcc phase to a heavily faulted hep structure. Precipitate formation was observed in hcp areas of the cast material after 20 h at 650°C and in hcp areas of wrought material after 20 h at 750°C. Prolonged aging at 750°C produced a transformation in the hcp structure of wrought specimens, with a relatively fault-free structure replacing the heavily faulted martensitic form. Interruption of fcc slip by both fcc stacking faults and bands of hcp phase was found to be the principal strengthening mechanism activated by aging. Precipitate formation in the hcp plays an increasingly significant role as aging time is increased. This microstructural information is used to explain the observed tensile properties of these alloys after the heat treatments mentioned.     

Formation of hcp martensite during the isothermal aging of an fcc Co-27Cr-5Mo-0.05C orthopedic implant alloy

The evolution of the microstructure of a Co-27Cr-5Mo-0.05C alloy was investigated during isothermal aging between 650 °C and 950 °C. The main structural change observed as a result of aging was an fcc (metastable)→hcp isothermal martensitic transformation. The relationships between transformation, temperature, and time for this phase transition were determined using two different techniques: (1) room-temperature X-ray diffraction on samples aged after quenching from 1150 °C to 25 °C and (2) high-temperature in situ X-ray diffraction on samples cooled at 50 °C/min from 1150 °C to the aging temperature. The results show that the intermediate water quench significantly retards the kinetics of the phase transition by up to one order of magnitude in time. In addition, it was found that the grain size of the metastable fcc phase prior to aging does not affect the kinetics of the transformation. Age hardening resulting from this transformation varies linearly with the amount of hcp phase formed during the isothermal treatment and does not depend on the aging temperature. It is suggested that local plastic deformation, due to thermal and transformation stresses induced by quenching, reduces the number of hcp martensite embryos formed in the metastable fcc phase. This effect decreases the number of nucleation sites available for the fcc→hcp transformation during isothermal aging and leads to the slower transformation rates observed in water-quenched material.     

Phase transformations in a wrought Co-Cr-Mo-C alloy

The effect of heat treatment on microstructure has been studied in a Co-Cr-Mo-C alloy using transmission electron microscopy. Isothermal aging treatments at 750 °C were found to promote a two stage fcc → hcp transformation, coincident with a discontinuous precipitation of M₂₃C₆ carbides. The variation in morphology of the carbides associated with the fcc → hcp transition is discussed in terms of the nature of the fcc/hcp interface.     

Effect of fcc-hcp phase transformation produced by isothermal aging on the corrosion resistance of a Co-27Cr-5Mo-0.05C alloy

The corrosion resistance of two-phase (fcc-hcp) Co-27Cr-5Mo-0.05C alloys produced by isothermal aging at 800 °C was studied using potentiostatic polarization tests in Ringer’s solution. Critical pitting potentials were estimated from the potentiostatic polarization curves and were found superior to that exhibited by the conventional ASTM-F75 cast alloy used for the manufacture of orthopedic implants. Formation of suitable distributions of hcp embryos (incoherent twin boundaries and stacking faults) prior to and during the early stages of aging required for isothermal fcc-hcp transformation led to a relative reduction of the corrosion resistance of two-phase alloys. However, once the transformation proceeded rapidly, between 4 and 8 hours of aging, the elimination of lattice defects caused a reduction of the dissolution rates and the breakdown potential became nearly independent of the relative amounts of fcc and hcp phases present in the microstructure. This behavior was due to the uniform chemical composition of the two-phase alloys. Concurrent work has shown that the hardness and yield strength of a 50 pct hcp alloy are increased by at least 30 pct without undue ductility losses. Therefore, the results of the present article suggest that these materials are excellent candidates for the manufacture of orthopedic implant devices requiring higher strength than provided by conventional ASTM-F75 materials.     

Development of athermal and isothermalε-martensite in atomized Co-Cr-Mo-C implant alloy powders

In this work, CoCr-Mo compacted powders were sintered at 900°C to 1300°C for 1 to 2 hours and conditions for total carbide dissolution in fcc cobalt were determined. Accordingly, it was found that sintering at temperatures between 900°C to 1100°C led to removal of the dendritic structure and to carbide precipitation at the grain boundaries (gbs), as well as in the bulk. Moreover, recrystallization and grain growth were always found to occur during powder sintering. At temperatures above 1100°C, no carbide precipitation occurred indicating that carbides were not stable at these temperatures. Hence, compact powders were annealed at 1150°C to promote the development of a single-phase fcc solid solution. This was followed by rapid cooling to room temperature and then aging at 800°C for 0 to 18 hours. Rapid cooling from 1150°C promoted the development of up to 64 pct athermal ε-martensite through the face-centered cubic (fcc) → hexagonal crystal structure (hcp) martensitic transformation. The athermal martensite was associated with the development of a network of parallel arrays of fine straight transgranular markings within the fcc matrix. Moreover, aging at 800°C for 15 hours led to the development of 100 pct isothermal hcp ε-martensite. From the experimental outcome, it is evident that isothermal ε-martensite is the most stable form of the hcp Co phase. Apparently, during aging at 800°C, the excess defects expected in athermal martensite are removed by thermally activated processes and by the development of isothermal ε-martensite, which has the appearance of “pearlite.”     

The influence of heat treatment on the structure and properties of a near-α titanium alloy

The microstructure and tensile properties of a near-α titanium alloy, IMI-829 (Ti-6.1 wt pct Al-3.2 wt pct Zr-3.3 wt pct Sn-0.5 wt pct Mo-1 wt pct Nb-0.32 wt pct Si) have been studied after solutionizing (and no subsequent aging) at two different temperatures separately, one above the β transus (1050 °C) and another below the β transus (975 °C) followed by various cooling rates (furnace, air, oil, or water). While 1050 °C treatment resulted in coarse Widmanstätten structures on furnace or air cooling, fine Widmanstätten structure on oil quenching and martensitic structure on water quenching, 975 °C treatment produced duplex microstructures consisting of equiaxed alpha and partially transformed beta phases. Transmission electron microscopy studies revealed the morphology, size, and distribution of the α, β, and martensite phases and also the presence of small ellipsoidal suicide particles and an interface phase with fcc structure at almost all α-β interfaces. The oil quenched structure from 1050 °C has been found to be a mixture of fine Widmanstätten α coexisting with martensite laths and retained beta at the lath boundaries. Silicides with hcp structure of about 0.4 μm size were observed in specimens solution treated at 975 °C. The interface phase is seen in all slowly-cooled specimens. The YS and UTS are superior for 975 °C treatment compared to 1050 °C treatment after water quenching or oil quenching. The tensile ductility values are superior for any cooling rate after 975 °C solution treatment as compared to 1050 °C solution treatment. The specimens failed in tension diagonally by shear after 1050 °C treatment and by cup and cone fracture after 975 °C treatment. In all cases fracture has taken place by microvoid coalescence and in most cases, along the α-β boundaries.     

Effects of electron irradiation on precipitation in Ni-Al alloys

Binary Ni-Al alloys have been irradiatedin situ by high voltage electron microscopy (650 kV) in order to investigate the effects on precipitation. Nucleation and growth of dis-location loops are observed during irradiation in the temperature range 20 to 650°C. In solution-treated specimens, large faulted loops form during irradiation at 500°C. Con-trast analysis shows that loops are Frank-type with l/3 〈HI〉 Burgers vectors and in-terstitial in character. At 650°C and above, loop formation and growth are not observed once γ′ precipitates have formed. In preaged specimens, much smaller loops form dur-ing irradiations at below 500°C. At 500°C and above, no loops are observed in specimens containing large volume fractions of precipitates. The spatial distribution of loops ap-pears rather uniform regardless of the presence of γ′ precipitates except when large in-coherent precipitates exist. The growth of γ′ precipitates is enhanced by irradiation in the temperature range 300 to 750°C. Precipitate coarsening under irradiation follows radiation-enhanced diffusion kinetics. Quantitative analysis demonstrates that the radia-tion-enhanced diffusion coefficient responsible for coarsening of γ′ is independent of the temperature between 300 and 650°C. This result is in agreement with theoretical pre-dictions for the case in which irradiation-produced point defects are lost by their anneal-ing at fixed sinks. Formerly with Department of Metallurgy and Materials Science, Case Western Reserve University, is now with Glidden Metals Group, SCM Corpora-tion, Cleveland, OH 44106 This paper is based on a presentation made at a symposium on “Radiation Induced Atomic Rearrangements in Ordering and Clustering Alloys” held at the annual meeting of the AIME, Atlanta, Georgia, March 7 to 8, 1977, under the sponsorship of the Physical Metallurgy and Nuclear Metallurgy Committees of The Metallurgical Society of AIME.     

Kinetics of the fcc → hcp Phase Transformation in Cu-Ge Solid Solutions Upon Isothermal Aging

The kinetics of the ζ-phase formation from a supersaturated α-Cu(Ge) solid solution (i.e., transformation from the fcc crystal structure to the hcp crystal structure) containing 10.8 at. pct Ge [at isothermal temperatures of 573 K, 613 K, and 653 K (300 °C, 340 °C, and 380 °C)] were studied by X-ray diffraction (XRD) for phase fraction determination. Both in situ and ex situ annealing experiments were performed. The transformation kinetics were modeled on the basis of a versatile modular model. The transformation kinetics complied with a site-saturation nucleation mode and strongly anisotropic interface-controlled growth mode in association with a corresponding impingement mode: diffusion of Ge (towards the stacking faults, SFs) does not control the transformation rate. Transmission electron microscopy (TEM) investigations showed that segregation of Ge at the stacking faults (SFs) takes place (relatively fast) prior to the structural transformation (fcc → hcp).     

A critical assessment of the mechanistic aspects in HAYNES 188 during low-cycle fatigue in the range 25 °C to 1000 °C

The low-cycle fatigue (LCF) behavior of a wrought cobalt-base superalloy, Haynes 188, has been investigated over a range of temperatures between 25 °C and 1000 °C employing a triangular waveform and a constant strain amplitude of ±0.4 pct. Correlations between macroscopic cyclic deformation and fatigue life with the various microstructural phenomena were enabled through scanning electron microscopy (SEM) and transmission electron microscopy (TEM), detailing the crack initiation and propagation modes, deformation substructure, and carbide precipitation. Cyclic stress response varied as a complex function of temperature. Dynamic strain aging (DSA) was found to occur over a wide temperature range between 300 °C and 750 °C. In the DSA domain, the alloy exhibited marked cyclic hardening with a pronounced maximum at 650 °C. Dynamic strain aging has been documented through the occurrence of serrated yielding, inverse temperature dependence of maximum cyclic stress, and cyclic inelastic strain developed at half of the fatigue life. Additionally, the alloy also displayed a negative strain rate sensitivity of cyclic stress in the DSA regime. These macroscopic features in the DSA domain were accompanied by the substructure comprised of coplanar distribution of dislocations associated with the formation of pileups, stacking faults, and very high dislocation density. Toward the end of the DSA domain, dislocation pinning by M₂₃C₆ precipitates occurred predominantly. The deformation behavior below and above the DSA domain has also been investigated in detail. The temperature dependence of LCF life showed a maximum at ≈300 °C. The drastic reduction in life between 300 °C and 850 °C has been ascribed primarily to the deleterious effects of DSA on crack initiation and propagation, while the lower life at temperatures less than 200 °C has been attributed to the combined influence of low ductility and larger cyclic response stress.     

Molecular-dynamic simulations of martensitic transformation of cobalt

With a potential-energy function of Co described by the embedded-atom method (EAM), molecular-dynamics (MD) simulations were performed for a series of initial fcc configurations with different types of dislocations or preset hcp embryos. The gliding process of a Shockley dislocation on a closepacked plane has been observed, which starts from the origin of the dislocation and proceeds at a high speed of 280 m/s toward a certain direction. An atom which has been swept by the dislocation line was detected to contribute a displacement close to the Burgers vector of a Shockley dislocation. It is in this way that a new stacking sequence is produced and an hcp lamella grows in the fcc structure. A similar gliding process has been observed in the case where an intrinsic stacking fault is preexisting in the fcc structure. The transformation is, again, toward forming a local hcp region. These results prove that a special dislocation in the fcc structure can act as an embryo of the hcp, as described in many dislocation mechanisms of the martensitic transformation. The fcc → hcp phase-tranformation process of Co has been further reproduced by a simulation initiated from an fcc/hcp two-phase configuration. It yields a single hcp crystal as the final transformed product.     

Morphology and structure of bainite plates in the β’ phase of an Ag-45 at. pct Cd alloy

The morphology and structure of bainite plates which formed in the ordered bcc β’ phase of a Ag-45 at. pct Cd alloy at temperatures 160 to 300°C were studied by X-ray diffraction and transmission electron microscopy. Initially, the plates formed with a 3R stacking fault modulation of the fcc structure and contained a high density of random stacking faults. The stacking faults annealed out during a prolonged isothermal treatment, the structure gradually changing to a regular fcc. The orientation relationship between the bcc matrix and the fcc bainite was as follows: [1•11]_b 0.7 deg from [0•11]_f, [110]_b 1.1 deg from [l00]_f and [011]_b 4.3 deg from the stacking fault plane pole [111]_f. The habit plane of the bainite plates, determined by two surface trace analysis, was close to (3, 11, 12)_b. The surface relief of the plates, examined by interference microscopy, was in the form of a simple tilt indicating an invariant plane strain transformation. The features of the transformation agreed with the predictions of the Bowles-MacKenzie theory of martensite formation. It was concluded that the morphology, structure and other characteristics of the freshly formed bainite plates were consistent with their formation by a thermally activated martensitic process. formerly Research Associate, Department of Metallurgy, University of British Columbia, Vancouver, B.C., Canada.     

Formation of Reversed Austenite in High Temperature Tempering Process and its Effect on Mechanical Properties of Cr15 Super Martensitic Stainless Steel Alloyed with Copper

Formation mechanism of the reversed austenite of Cr15 super martensitic stainless steel (SMSS) alloyed with copper after high temperature tempering was investigated by means of thermo‐calc software, transmission electron microscope (TEM), and X‐ray diffraction (XRD). The mechanical properties of the SMSS were also tested. The experimental results show that the reversed austenite with low dislocation density is formed at high temperature tempering processing. The transformation of the martensite to reversed austenite is a diffused phase transformation, and the growth of the reversed austenite is closely related to the diffusion process of Ni. The bulk reversed austenite with large amount of stacking faults is formed with the increase of the tempering temperature. The volume fraction of reversed austenite increases at first and then decreases with increasing tempering temperature, and the maximum amount of the reversed austenite is obtained at 650°C. The reversed austenite is unstable at the tempering temperature above 650°C and the martensitic phase transformation will occur at the following cooling process. The mechanical properties of Cr15 super martensitic stainless steel are significantly influenced by the volume fraction of reversed austenite.     

Kinetics of σ-phase formation in a strain aging hard magnetic Fe-33% Cr-12% Co-2% Cu alloy

The formation of σ-phase in aging hard magnet alloy Fe-33% Cr-12% Co-2% Cu, which was obtained by quenching from 1150°C into water and annealing in the temperature range 650–800°C, has been investigated. It was found that the maximum rate of the σ-phase formation was at 750°C.     

High-resolution transmission electron microscopy investigation of the face-centered cubic/hexagonal close-packed martensite transformation in Co-31.8 wt pct Ni alloy: Part 2. Plate intersections,extended defects,and nucleation mechanisms

The face-centered cubic/hexagonal close-packed (fcc/hcp) martensite phase transformation in a Co-31.8 wt pct Ni alloy was studied by high-resolution transmission electron microscopy (HRTEM). The HRTEM was used to study the structure and properties of intersections between martensite plates and other defects observed in the alloy such as stacking fault tetrahedra (SFT) and Z-type defects. The HRTEM was also used to attempt to determine if various proposed mechanisms for the fcc/hcp martensite transformation were operating. There is evidence to suggest that the reflection mechanism proposed by Bollmann and the dipole mechanism proposed by Hirth are active in the fcc/hcp martensitic transformation, although the evidence is not completely certain in either case. Growth of the hcp phase by a four- or six-plane mechanism as proposed by Mahajanet al. is possible in theory but was not observed in this study. Transformation by previously proposed pole mechanisms was also not observed in this study, although evidence for a new type of pole mechanism was found. The formation of SFT along the fcc/hcp martensite interface was observed to occur by the cross-slip of Shockley partial dislocations out of the fcc/hcp interface onto conjugate fcc matrix planes, followed by further cross-slip to form the SFT, as previously observed for grain boundaries in fcc alloys.     

The mechanisms and temperature dependence of superlattice stacking fault formation in the single-crystal superalloy PWA 1480

Deformation microstructures in PWA 1480 nickel-base superalloy single crystals were studied in the range of 20 °C to 1100 °C. Similar to previous investigations, superlattice stacking faults were observed after slow strain rate deformation at temperatures between 700 °C and 950 °C. Unlike previous studies, a high density of superlattice stacking faults was observed after deformation at 200 °C and below. The mechanisms of fault formation in the two temperature regimes were different. In the range of 700 °C to 950 °C, single isolated superlattice-intrinsic stacking faults (SISFs) were produced by the decomposition of an a/2(110) matrix dislocation in the γ/γ′ interface. The a/3(112) partial shears the particle, while the a/6(112) Shockley remains in the interface. At 200 °C and below, a high density of faults was produced on closely spaced parallel planes. The most common feature after deformation in this range is the faulted loop, which is most often observed to be a superlattice-extrinsic stacking fault (SESF). These low-temperature faults, along with their temperature dependence, were quite similar to those observed in single-phase Ll₂2 materials. The available evidence suggests that the low-temperature faults were produced by the dissociation of an a<11> unit superdislocation into a pair of a/3<112> partials. The temperature dependence of the faulting (at low temperatures) was modeled by linear isotropic elasticity, and the results suggest that the SISF energy increases significantly from 20 °C to 400 °C. Multiplanar, overlapping superlattice faults were analyzed with respect to bond violations. This analysis suggested that an antiphase boundary (APB) on top of an SISF has a very high fault energy, similar to that of the complex stacking fault. Therefore, the presence of SISF loops on glide planes promotes further dissociation by the SISF scheme instead of the APB scheme and explains the high density of SESFs and microtwins observed in the deformation structures.     

Reverse Martensitic Transformation and Resulting Microstructure in a Cold Rolled Metastable Austenitic Stainless Steel

The reverse martensitic transformation in cold‐rolled metastable austenitic stainless steel has been investigated via heat treatments performed for various temperatures and times. The microstructural evolution was evaluated by differential scanning calorimetry, X‐ray diffraction and microscopy. Upon heat treatment, both diffusionless and diffusion‐controlled mechanisms determine the final microstructure. The diffusion reversion from α′‐martensite to austenite was found to be activated at about 450°C and the shear reversion is activated at higher temperatures with A_f′ ~600°C. The resulting microstructure for isothermal heat treatment at 650°C was austenitic, which inherits the α′‐martensite lath morphology and is highly faulted. For isothermal heat treatments at temperatures above 700°C the faulted austenite was able to recrystallize and new austenite grains with a low defect density were formed. In addition, carbo‐nitride precipitation was observed for samples heat treated at these temperatures, which leads to an increasing M_s‐temperature and new α′‐martensite formation upon cooling.     

Microstructures and superplastic behavior of eutectic Fe-C and Ni-Cr white cast irons produced by rapid solidification

Superplastic behavior of two commercial grade white cast irons, eutectic Fe-C and Ni-Cr white cast irons, was investigated at intermediate temperatures (650 to 750 °C). For this purpose, rapidly solidified powders of the cast irons were fully consolidated by compaction and rolling at about 650 °C. The volume fractions of cementite in the eutectic cast iron and in the Ni-Cr cast iron were 64 pct and 51 pct, respectively, and both cast irons consisted of fine equiaxed grains of cementite (1 to 2 μm) and ferrite (0.5 to 2 μm). The cast iron compacts exhibited high strain-rate sensitivity (strain-rate-sensitivity exponent of 0.35 to 0.46) and high tensile ductility (total elongation of 150 pct to 210 pct) at strain rates of 10^-4 to 10^-3 s^-1 and at 650 °C to 750 °C. Microstructure evaluations were made by TEM, SEM, and optical microscopy methods. The equiaxed grains in the as-compacted samples remained unchanged even after large tensile deformation. It is concluded that grain boundary sliding (e.g., along cementite grain boundaries in the case of the eutectic cast iron) is the principal mode of plastic deformation in both cast irons during superplastic testing conditions. Formerly with the Department of Materials Science and Engineering, Stanford University Formerly Visiting Scholar, Department of Materials Science and Engineering, Stanford University     

Microstructure and properties of spray-deposited 2014+15 vol pct SiC particulate-reinforced metal matrix composite

A metal matrix composite (MMC) of 2014 aluminum alloy reinforced with 15 vol pct SiC particulate was produced by the spray-forming-deposition process. The as-deposited preform revealed a high density and a homogeneous reinforcement distribution. Reactive products were not found on interfaces between the reinforcement and the matrix. Compared to the control alloy, the composite showed accelerated aging after solutionizing at 502 °C, while aging was retarded after solutionizing at 475 °C. Analysis indicated that the activation energy was almost the same for the aging process after different solutionizing treatments. This suggested that while the thermal barrier for the aging process was the same, other factors affecting the aging process should be considered. For example, the effective concentration of the precipitate forming elements possibly decreased after incompletely solutionizing at 475 °C. After heat treatment, the composite showed a tensile strength similar to the control alloy. The wear resistance of the composite improved considerably. The aging behavior of the composite was also studied using the nanoindentation technique. Steep gradient distribution of elastic modulus and hardness around the reinforcement SiC particulate was observed. Theoretical analysis showed that this could be attributed to the gradient distribution of precipitates, resulting from a gradient distribution of dislocation density around the SiC particulates caused by residual thermal misfit stresses.     

Part I. The microstructural evolution in Ti-Al-Nb O+Bcc orthorhombic alloys

Phase transformations and the resulting microstructural evolution of near-Ti₂AlNb and Ti-12Al-38Nb O+bcc orthorhombic alloys were investigated. For the near-Ti₂AlNb alloys, the processing temperatures were below the bcc transus, while, for Ti-12Al-38Nb, the processing temperature was supertransus. Phase evolution studies showed that these alloys contain several constituent phases, namely, bcc, O, and α ₂; when present, the latter was in small quantities compared to the other phases. The transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray investigations of samples that were solutionized and water quenched were used to estimate the phase fields, and a pseudobinary diagram based on Ti=50 at. pct was modified. The aging-transformation behavior was studied in detail. For solutionizing temperatures between 875 °C and the bcc transus, the phase composition and volume fraction of the near-Ti₂AlNb alloys adjusted through relative size changes of the equiaxed B2, O, and α ₂ grains. The aging behavior followed three distinct transformation modes, dependent on the solutionizing and aging temperatures. Widmanstatten formation was observed when a new phase evolved from a parent phase. Thus, Widmanstatten O phase precipitated within the B2 phase for supertransus fully B2 microstructures, as well as for substransus α ₂+B2 microstructures. Similarly, Widmanstatten B2 phase can form from a fully O microstructure, a transformation that has not been observed before. In the case of equiaxed O+B2 solutionized and water-quenched microstructures, Widmanstatten O-phase formation occurred only below 875 °C. For the subtransus-solutionized and water-quenched microstructures, a second aging transformation mode, cellular precipitation, was dominant below 750 °C. This involved formation of coarse and lenticular O phase that grew into the prior B2 grains from the grain boundaries. A third transformation mode involved composition-invariant transformation, where the fully B2 supertransus-solutionized and water-quenched microstructure transformed to a fully O microstructure at 650 °C. This microstructure reprecipitated B2 phase out of the O phase with continued aging time. For Ti-12Al-38Nb, Widmanstatten O precipitation remained the only transformation mode. It is shown that subtransus processing offers flexibility in controlling microstructures through postprocessing heat treatments.