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.”     

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.     

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.     

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.     

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).     

Hydride formation and decomposition in electrolytically charged metastable austenitic stainless steels

An investigation of phase transformations in hydrogen-charged metastable austenitic stainless steels was carried out. Solution-annealed, high-purity, ultralow-carbon Fel8Crl2Ni (305) and laboratory-heat Fel8Cr9Ni (304) stainless steels were examined. The steels were cathodically charged with hydrogen at 1, 10, and 100 mA/cm², at room temperature for 5 minutes to 32 hours, in an lN H₂SO₄ solution with 0.25 g/L of NaAsO₂ added as a hydrogen recombination poison. Changes in microstructure and hydrogen damage that resulted from charging and subsequent room-temperature aging were studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Hydrides from hydrogen charging (hep ε* in 305 SS and fcc γ* and hcp ε* in 304 SS) were observed. The evidence suggests the following mechanisms for hydride formation during charging: (1)γ → ε → ε* hydride and (2) γ → γ* hydride. These hydrides were found to be unstable and decomposed during room-temperature aging in air by the following suggested mechanisms: (1)ε* hydride (hcp) → expanded ε (hcp) phase →α′ (bcc) phase and (2) γ* hydride →γ phase. The transformation from ε* toα′, however, was incomplete, and a substantial fraction of ε was retained. A kinetics model for hydride decomposition and the accompanying phase transformation during aging is proposed.     

Phase transformations in Ti-6.8Mo-4.5Fe-1.5Al

Phase transformations during artificial and isothermal aging of Ti-6.8Mo-4.5Fe-1.5Al have been investigated over the temperature range from 300 °C to 750 °C utilizing hardness measurements, X-ray diffraction, optical microscopy, and electron microscopy. Artificial aging following solution treatment and water quenching initially involved growth of the athermal ω phase. This was followed by formation of the α phase, either in association with the ω phase, through homogeneous precipitation within the matrix, or through heterogeneous grain-boundary nucleation. Similarly, isothermal decomposition of the metastable β phase resulted in the precipitation of ω phase exhibiting an ellipsoidal morphology. While precipitation of ω was immediate at 345 °C, an incubation period was observed upon aging at 390 °C. Isothermal aging above this temperature involved direct precipitation of the α phase, either homogeneously within the β matrix or heterogeneously at β grain boundaries. The extent of homogeneous vs heterogeneous α nucleation during isothermal aging depended upon aging temperature; low aging temperatures promote homogeneous nucleation and higher aging temperatures promote α heterogeneous nucleation. Finally, continued aging resulted, independent of aging path, in coarsening and spheroidization of the α phase.     

Kinetics of the fcc to hcp phase transformation and the formation of martensite in pure cobalt

The experimental data on the kinetics of the fcc to hcp phase transformation in pure cobalt have been summarized (in Figs. 1 and 2) and analyzed in conjunction with the recent finding of a metastable 9R martensite structure by Kajiwara et al. [12,13]. From the observed transformation kinetics, it is predicted that there should exist another martensitic structure for cobalt in addition to the ε martensite and the 9R (ε″) martensite, i.e., there are three kinds of martensite. Each martensite has a separate M_s temperature, which is identified as 390 ± 5 °C (ε), 370 ± 8 °C (ε′) and 330 ±15 °C (ε″), respectively. The absence of this unidentified ε′ martensite in the ultra-fine particles of Kajiwara et al. does not exclude its existence in other forms of pure cobalt. Grain size and impurity effects on the transformation kinetics are also discussed and used to explain the experimental observations in both bulk and ultra-fine particle cobalt.     

Effect of heat-treatment conditions on the structure and physicomechanical and chemical properties of a Ni-Cr-Cu-Ti maraging steel

The effects of quenching (from 950°C or from 950 and 850°C) and the aging conditions on the structure, properties, and delayed fracture (DF) of 03Kh11N10M2DT maraging steel has been studied by dilatometry, X-ray diffraction, and fracture tests. The DF-crack growth rate is maximal after aging at 400°C irrespective of the quenching conditions, and the corrosion rate is maximal after aging at 350–400°C in the case of single quenching and at 350°C after double quenching. The kinetics and mechanism of the early stages of the decomposition of a supersaturated α solid solution are investigated by electrical-resistance measurements and transmission electron microscopy. In the state after single quenching, aging occurs in two stages at all isothermal heat treatments; in the state after double quenching, aging occurs in one stage at a time exponent n = 0.2 in the Johnson-Mehl equation. Upon aging at 400°C, the intermediate ordered Fe₃(Ni,Ti) phase with a complex cubic lattice precipitates, and the intermetallic compound Ni₃Ti precipitates upon subsequent aging. Moreover, copper-rich ε-phase precipitates form only in the case of single quenching. The substantial increase in the crack growth rate during DF with n < 0.2 is likely to be caused by the formation of Guinier-Preston zones enriched in nickel and titanium.     

Development of new methods for studying the decomposition in aging invars

N30K10T3 and N28K10T3 invar alloys are studied. After water quenching from 1150°C, they consist of supersaturated solid solutions, which can decompose in aging in the temperature range 500–700°C with the precipitation of intermetallic Ni₃Ti nanoparticles. It is shown that the decomposition can be controlled by measuring the magnetic (first harmonic amplitude, phase angle φ shift, magnetic susceptibility μ) and electric (electrical conductivity σ) parameters as functions of the isothermal holding time at various aging temperatures. The alloys are studied in the following three initial states: after quenching, phase transformation-induced hardening (γ → α → γ_pt), and cold (20°C) plastic deformation by 30%.     

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.     

Transformation and Precipitation Kinetics in 30Cr10Ni Duplex Stainless Steel

To improve the microstructure during casting, hot forming, and heat treatment of 30Cr10Ni duplex stainless steel, accurate data on the precipitation and transformation processes at high temperatures are needed. In this article, the precipitation and transformation processes at various aging times in the temperature range 873 K to 1573 K (600 °C to 1300 °C) were studied. The 30Cr10Ni ferrous alloy contains a relatively large amount of Cr, Ni, and C, which results in a complex microstructure. In addition to the ferrite, austenite, and sigma phase, the M₂₃C₆ and MC carbides were also observed in the microstructure. The precipitation of the sigma phase was observed after just 3 minutes of aging, and after 30 minutes of aging at approximately 1053 K (780 °C), its fraction exceeded 40 pct. An intensive austenite-to-ferrite transformation was observed above 1423 K (1150 °C). Optical microscopy, energy-dispersive X-ray spectroscopy (EDS), electron backscattered diffraction (EBSD), and X-ray diffraction (XRD), as well as micro-indentation hardness, hardness, impact toughness, and tensile tests, were carried out to evaluate the obtained microstructures of aged samples.     

Physical metallurgy of metastable bcc lanthanide-magnesium alloys for R = La,Gd, and Dy

Bcc La-Mg, Gd-Mg, and Dy-Mg alloys have been prepared by an ice water/acetone quench from liquid melts. Single-phase alloys could be retained in a window around the eutectoid composition: 13 to 22 at. pct Mg, 23.6 to 29 at. pct Mg, and 27 to 29 at. pct Mg for La, Gd, and Dy alloys, respectively. At the center of the windows, X-ray diffraction peaks are extremely sharp as in equilibrium bcc structures; however, as alloy composition is moved away from the eutectoid, line broadening is observed. Reversion of the bcc phase to the equilibrium micro-structure for R-Mg alloys (R = La, Gd, or Dy) has been characterized by differential thermal analysis (DTA) or differential scanning calorimetry (DSC) and isothermal annealing. La-Mg alloys revert directly to c~La (dhcp) + LaMg at about 350 °C when heated at 10 °C/min. In contrast, the Gd and Dy alloys revert by a two-step process: first, a transition to an intermediate distorted hcp phase between 300 °C and 400 °C and, second, the relaxation of this phase to αR (hcp) + RMg at about 490 °C when heated at 10 °/min. Isothermal annealing and high temperature X-ray diffraction confirm the nature of these reactions. Formerly Graduate Student, Ames Laboratory and Department of Materials Science and Engineering, Iowa State University Formerly Postdoctoral Associate, Ames Laboratory     

Kinetics and Equilibrium of Age-Induced Precipitation in Cu-4 At. Pct Ti Binary Alloy

Transformation kinetics and phase equilibrium of metastable and stable precipitates in age-hardenable Cu-4 at. pct Ti binary alloy have been investigated by monitoring the microstructural evolution during isothermal aging at temperatures between 693 K (420 °C) and 973 K (700 °C). The microstructure of the supersaturated solid solution evolves in four stages: compositional modulation due to spinodal decomposition, continuous precipitation of the needle-shaped metastable β′-Cu₄Ti with a tetragonal structure, discontinuous precipitation of cellular components containing stable β-Cu₄Ti lamellae with an orthorhombic structure, and eventually precipitation saturation at equilibrium. In specimens aged below 923 K (650 °C), the stable β-Cu₄Ti phase is produced only due to the cellular reaction, whereas it can be also directly obtained from the intergranular needle-shaped β′-Cu₄Ti precipitates in specimens aged at 973 K (700 °C). The precipitation kinetics and phase equilibrium observed for the specimens aged between 693 K (420 °C) and 973 K (700 °C) were characterized in accordance with a time–temperature–transformation (TTT) diagram and a Cu-Ti partial phase diagram, which were utilized to determine the alloy microstructure, strength, and electrical conductivity.

     

Transformation squence in a Cu-Al-Ni shape memory alloy at elevated temperatures

Precipitation sequences in a Cu-14 pct Al-4 pct Ni (wt pct) shape memory alloy were studied by means of transmission electron diffraction and microscopy as well as X-ray microanalysis techniques. On aging thin foil specimens up to 550 °C in the electron microscope, an as-quenched sample having a mixture of 2H-type and D0₃-type metastable structures transformed to the stable simple cubic γ₂ phase at or above 450 °C. The remaining matrix either showed precipitates of the fcc α-phase on prolonged annealing at 500 to 550 °C for a longer period, or transformed to martensite on cooling below theM _s temperature (~150 °C).     

In-Situ High-Energy X-ray Diffraction Study of Austenite Decomposition During Rapid Cooling and Isothermal Holding in Two HSLA Steels

In-situ high-energy X-ray diffraction experiments with high temporal resolution during rapid cooling (280 °C s⁻¹) and isothermal heat treatments (at 450 °C, 500 °C, and 550 °C for 30 minutes) were performed to study austenite decomposition in two commercial high-strength low-alloy steels. The rapid phase transformations occurring in these types of steels are investigated for the first time in-situ, aiding a detailed analysis of the austenite decomposition kinetics. For the low hardenability steel with main composition Fe-0.08C-1.7Mn-0.403Si-0.303Cr in weight percent, austenite decomposition to polygonal ferrite and bainite occurs already during the initial cooling. However, for the high hardenability steel with main composition Fe-0.08C-1.79Mn-0.182Si-0.757Cr-0.094Mo in weight percent, the austenite decomposition kinetics is retarded, chiefly by the Mo addition, and therefore mainly bainitic transformation occurs during isothermal holding; the bainitic transformation rate at the isothermal holding is clearly enhanced by lowered temperature from 550 °C to 500 °C and 450 °C. During prolonged isothermal holding, carbide formation leads to decreased austenite carbon content and promotes continued bainitic ferrite formation. Moreover, at prolonged isothermal holding at higher temperatures some degenerate pearlite form.

     

Crystallization of an Fe-Ni base amorphous alloy

The crystallization of an amorphous Fe-Ni base alloy was studied in a dynamic heating mode from room temperature to 700°C and during isothermal annealing at 400°C. Differential scanning calorimetry, X-ray diffraction, transmission electron microscopy, and hardness measurement were used to characterize the crystallization process under two heating conditions. In the dynamic heating condition, structural relaxation or atomic regrouping was thought to occur belowT _c. AboveT _c, crystallization occurred spontaneously and four crystalline phases were formed. The number of phases and the relative amount of these phases varied with the heating temperature. At a higher temperature, recrystallization occurred which resulted in grain growth. The final matrix phase was observed to coexist with other phases after crystallization. In the isothermal heating condition, it was found that the transformation of the alloy from amorphous state to crystalline state was through the nucleation and growth process. The first crystallization steps were via the formation of metastable phases. The final matrix phase than nucleated from the existing metastable phases. Hardness measurements in both heating conditions indicated that the alloy attained its peak hardness immediately after complete crystallization.     

Technological application of the martensitic transformation of some austenitic stainless steels

The final heat treatment of austenitic stainless steels of types X 5 CrNi 18 9 (1.4301) and X 2 CrNi 18 10 (1.4306) normally is annealing at 1050°C and subsequent water quenching. The resulting structure is of a metastable fcc-type. Plastic deformation, especially at low temperatures, causes martensitic transformation of these metastable structures. The transformation is accompanied by a substantial flow stress increase. This strengthening mechanism should be used in practice, e.g. to save weight. The deformed structure consists of tetragonal α′-martensite, austenite and hcp ε-martensite. Whereas α′-martensite increases continuously with deformation, the content of ε-martensite reaches a maximum value at about 5% plastic strain at 77 K. The hcp phase is only detectable by means of X-ray analysis, whilst α′-martensite can be determined quantitatively by saturation magnetisation measurement. The flow stress increase during low temperature deformation of metastable austenitic stainless steels is based on normal work-hardening by dislocation accumulation, in addition to a distinct amount of work-hardening due to martensitic transformation. Analysis of the work-hardening behaviour in the range of stable deformation (T > M_D) can be used to predict the amount of normal work-hardening when deformation is performed in the instable temperature regime. Separation of the flow stress contributions according to the procedure described above enables the possible savings in weight to be predicted when using cryogenically stretched instable austenitic steels in comparison with stable grades deformed under the same conditions.     

Effect of heat and thermomechanical treatments on the linear thermal expansion coefficient of an N30K10T3 invar

The N30K10T3 invar that has a temperature of the onset of martensite transformation of austenite M _s ≈ −80°C and a Curie point θ_C ≈ 200°C after water-quenching from 1150°C is studied. The decomposition of a supersaturated solid solution is shown to substantially influence the linear thermal expansion coefficient. The alloy is studied in the following three initial states: after quenching, after phase transformation-induced hardening (γ → α_m → γ_p.h), and after cold (20°C) plastic deformation by 30%.