Phase transformation in an Fe-9.0AI-29.5Mn-1.2Si alloy

The microstructure of an (α + γ) duplex Fe-9.0Al-29.5Mn-l.2Si alloy has been investigated by means of transmission electron microscopy. In the as-quenched condition, extremely fine D0₃ particles were formed within the ferrite matrix by a continuous ordering transition during quenching. After being aged at 550 °C, the extremely fine D0₃ particles existing in the as-quenched specimen grew preferentially along (100) directions. With increasing the aging time at 550 °C, a (Si, Mn)-rich phase (designated as “L phase”) began to appear at the regions contiguous to the D0₃ particles. The L phase has never been observed in various Fe-Al-Mn, Fe-Al-Si, Fe-Mn-Si, and Mn-Al-Si alloy systems before. When the as-quenched specimen was aged at temperatures ranging from 550 °C to 950 °C, the phase transformation sequence occurring within the (α + D0₃) region as the aging temperature increases was found to be (α + D0₃ + L phase) → (α + D0₃ + A13 β-Mn)→ (B2 + D0₃ + A13 β-Mn)→ (B2 + A13β-Mn)→ (α + A13 β-Mn)→ (α +γ)→α.     

Phase transformations in an Fe-7.8Al-29.5Mn-1.5Si-1.05C alloy

Phase transformations in an Fe-7.8Al-29.5Mn-l.5Si-1.05C alloy have been investigated by means of optical microscopy and transmission electron microscopy. In the as-quenched condition, a high density of fine (Fe,Mn)₃AlC carbides could be observed within the austenite matrix. When the as-quenched alloy was aged at temperatures ranging from 550 °C to 825 °C, aγ → coarse (Fe,Mn)₃AlC carbide + DO₃ reaction occurred by a cellular precipitation on theγ/γ grain boundaries and twin boundaries. Both of the observations are quite different from those observed by other workers in Fe-Al-Mn-C alloys. In their studies, it was found that the as-quenched microstructure was austenite phase(γ), and (Fe,Mn)₃AlC carbides could only be observed within the austenite matrix in the aged alloys. In addition, aγ →α (ferrite) + coarse (Fe,Mn)₃AlC carbide reaction or aγ →α + coarse (Fe,Mn)₃AlC carbide +β-Mn reaction was found to occur on theγ/γ grain boundary in the aged Fe-Al-Mn-C alloys.     

Decomposition of Fe-Ni martensite: Implications for the low-temperature (≤500 °C) Fe-Ni phase diagram

The low-temperature (<500 °C) decomposition of Fe-Ni martensite was studied by aging martensitic Fe-Ni alloys at temperatures between 300 °C and 450 °C and by measuring the composition of the matrix and precipitate phases using the analytical electron microscope (AEM). For aging treatments between 300 °C and 450 °C, lath martensite in 15 and 25 wt pct Ni alloys decomposed with γ [face-centered cubic (fcc)] precipitates forming intergranularly, and plate martensite in 30 wt pct Ni alloys decomposed with γ (fcc) precipitates forming intragranularly. The habit plane for the intragranular precipitates is {111}_fcc parallel to one of the {110}_bcc planes in the martensite. The compositions of the γ intergranular and intragranular precipitates lie between 48 and 58 wt pct Ni and generally increase in Ni content with decreasing aging temperature. Diffusion gradients are observed in the matrix α [body-centered cubic (bcc)] with decreasing Ni contents close to the martensite grain boundaries and matrix/precipitate boundaries. The Ni composition of the matrix α phase in decomposed martensite is significantly higher than the equilibrium value of 4 to 5 wt pct Ni, suggesting that precipitate growth in Fe-Ni martensite is partially interface reaction controlled at low temperatures (<500 °C). The results of the experimental studies modify the γ/α + γ phase boundary in the present low-temperature Fe-Ni phase diagram and establish the eutectoid reaction in the temperature range between 400 °C and 450 °C. Formerly Research Assistant, Department of Materials Science and Engineering, Lehigh University     

Effect of Si addition on the microstructure of an Fe-8.0AI-29.0Mn-0.90C alloy

The microstructures of two alloys with Fe-8.0Al-29.0Mn-0.90C, one without Si and one with 1.5 wt pct Si, have been investigated by means of transmission electron microscopy (TEM). In the as-quenched condition, the alloy without Si is single-phase austenite; however, some discrete particles along the austenite grain boundaries can be observed in the Si-bearing alloy. The discrete particles have a mixture of (α + D0₃) phases, indicating that the Si addition enhances the formation of (α + D0₃) phases. This result is in disagreement with those reported by other research on Fe-Al-Mn-Si-C alloys. Transmission electron microscopy examinations reveal that the (α + D0₃) phases are formed by an ordering transition during quenching. When the quenched specimen is aged at temperatures ranging from 450 °C to 1050 °C, the phase transformation sequence occurring within the (α + D0₃) region as the temperature increases is found to be D0₃→ (D0₃ +K phase) → B2 → α.     

Orientation relationship between β-Mn and L21 matrix in a Cu2MnAl alloy

In the as-quenched condition, the microstructure of the Cu₂MnAl alloy was L2₁ phase containing extremely fine L-J precipitates. This result is different from that reported by other workers in the as-quenched Cu₂MnAl alloy. When the as-quenched alloy was aged at 350 °C, γ-brass precipitates started to appear within the L2₁ matrix. The orientation relationship between the γ-brass and the L2₁ matrix was determined to be cubic to cubic. This result is consistent with that observed by other workers in the aged Cu-Mn-Al alloy. When the alloy was aged at 460 °C, the γ-brass precipitates disappeared and platelike β -Mn precipitates occurred within the L2₁ matrix. As the aging temperature was increased to 560 °C, the morphology of the β-Mn precipitates changed from platelike to granular shape. Electron diffraction examinations indicated that in spite of the morphology change the same orientation relationship between the β-Mn and the L2₁ matrix is maintained, and it could be best stated as follows:

Effect of Aging on the Fracture Behavior of Lean Duplex Stainless Steels

The influence of aging in the range of 550 °C to 850 °C for 5 to 120 minutes on the impact fracture behavior of 2101 and 2304 lean duplex stainless steels (DSS) was investigated in the present study. The 2304 steel displayed ductile behavior irrespective of aging conditions. In contrast, the 2101 steel displayed a ductile behavior only in the case of aging for 5 minutes at 550 °C and 650 °C, whereas in all other cases, it fractured in a brittle manner. The brittle fracture behavior of the 2101 steel has been attributed to the precipitation of small black particles at the α/α and α/γ grain boundaries (nitrides), which form paths for easy crack propagation. In the 2304 steel, such particles precipitated at 750 °C and 850 °C, but they were located inside the austenitic grains because of the formation of secondary austenite. They therefore did not embrittle the steel. The larger Ni content of the 2304 steel favored the formation of the secondary austenite that is absent in the 2101 steel.     

Precipitation in a Cu-30 Pct Ni-1 Pct Nb alloy

Decomposition of a Cu-30 pct Ni-1 pct Nb alloy on aging in the range of 866 K (600°C) to 1073 K (800°C) was investigated. The initial decomposition, concomitant with age hardening, occurred through the precipitation of body centered tetragonal metastable Ni₃Nb-γ” precipitates on the 100 matrix planes. Equilibrium orthorhombicβ phase formed either through a grain boundary cellular reaction at low temperature (≤973 K (700°C)) or as Widmanstaettenplatelets on the 1ll planes at higher temperatures (≥1073 K (800°C)) with the following crystallographic relationship: (0l0)_β//111_γ [100]β//[1•11]_γ. Based on the observations, a schematic transformation sequence is presented.     

The precipitation behavior of a Zr-2.5 wt pct Nb alloy

The morphology, crystallography, and nature of precipitates in a quenched and aged Zr-2.5 wt pct Nb alloy has been studied by transmission electron microscopy. The needle-shaped matrix precipitates and equiaxed twin boundary nucleated precipitates produced by aging at 500 °C were the equilibrium Nb-rich β₂ phase. On aging at 600 °C, the matrix precipitation was a mixture of β₂ needles and coarse metastable Zr-rich β₁ particles, while only β₁ particles were found at twin boundaries. The growth direction of the needle-shaped particles, 6.6 deg to 8.2 deg from (1-100)_h, and their orientation relationship can be predicted by an invariant line strain model. The β₁ precipitates have the Burgers orientation relationship. The formation of metastable β₁ and stable β₂ particles is considered from the free energy approach of Menon, Banerjee, and Krishnan.     

Isothermal transformations in an Fe-9 pct Ni alloy

Isothermal transformation from austenite in an Fe-9.14 pct Ni alloy has been studied by optical metallography and examination by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In the temperature range 565 °C and 545 °C, massive ferrite (α _q ) forms first at prior austenite grain boundaries, followed by Widmanst?tten ferrite (α _W ) growing from this grain boundary ferrite. Between 495 °C and 535 °C, Widmanst?tten ferrite is thought to grow directly from the austenite grain boundaries. Both these transformations do not go to completion and reasons for this are discussed. These composition invariant transformations occur below T ₀ in the two-phase field (α+γ). Previous work on the same alloy showed that transformation occurred to α _q > and α _W on furnace cooling, while analytical TEM showed an increase of Ni at the massive ferrite grain boundaries, indicating local partitioning of Ni at the transformation interface. An Fe-3.47 pct Ni alloy transformed to equiaxed ferrite at 707 °C ±5 °C inside the single-phase field on air cooling. This is in agreement with data from other sources, although equiaxed ferrite in Fe-C alloys forms in the two-phase region. The application of theories of growth of two types of massive transformation by Hillert and his colleagues are discussed. This article is based on a presentation made at the symposium entitled “The Mechanisms of the Massive Transformation,” a part of the Fall 2000 TMS Meeting held October 16–19, 2000, in St. Louis, Missouri, under the auspices of the ASM Phase Transformations Committee.     

Deformation characteristics of age hardened Ti-6Al-4V

A commercial Ti−6Al−4V alloy with an equiaxed grain shape was investigated after solution annealing at 810°C and after aging at 550 and 350°C. Age hardening at both temperatures produced significant increases in Young's modulus and yield strength. Finely dispersed α₂(Ti₃Al) precipitates formed within the α phase upon aging at 550°C, but not when aging at 350°C. However, there is evidence of order, probably of oxygen, in the α grains of specimens which were aged at 350°C. The formation of the ordered Ti₃Al precipitates at 550°C and the occurrence of oxygen ordering at 350°C can account for the increases in Young's modulus and yield strength. since January 1977 with General Electric Co., Lighting Research Division, Nela Park, Cleveland, OH. KANAY GAZIOGLU, formerly with DFVLR, is deceased.     

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).     

Simulation of ferrite growth in continuously cooled low-carbon iron alloys

The growth of a planar ferrite (α): austenite (γ) boundary in low-carbon iron and Fe-Mn alloys continuously cooled from austenite through the (α+γ) two-phase field and the α single-phase field was simulated by incorporating carbon diffusion in austenite, intrinsic boundary mobility, and the drag of an alloying element. At a very high cooling rate (≥ 10³ °C/s), the width of the carbon diffusion spike in austenite approaches the limit at which spikes are viable, so that the growth of ferrite in which carbon is not partitioned can occur even above the α solvus. In this context, the upper limiting temperature of partitionless growth of ferrite is the T ₀ temperature. In the presence of drag of an alloying element, e.g., Mn, both carbon-partitioned and partitionless growth of ferrite begins to occur at finite undercoolings from the Ae ₃, T ₀, or α-solvus temperature, at which the driving force for transformation exceeds the drag force. The intrinsic mobility of the α:γ boundary may play a significant role at an extremely high cooling rate (≥10⁵ °C/s). This article is based on a presentation made at the symposium entitled “The Mechanisms of the Massive Transformation,” a part of the Fall 2000 TMS Meeting held October 16–19, 2000, in St. Louis, Missouri, under the auspices of the ASM Phase Transformations Committee.     

Structure and properties of thermal-mechanically treated 304 stainless steel

Mechanical and thermal-mechanical treatments of 304 stainless steel enables yield strengths of over 200,000 psi to be obtained with elongations better than 10 pct. Electron microscopy, X-ray, and magnetic techniques show that during deformation, strain induced γ → ∈ → α transformation occurs with further thermal nucleation of α achieved by aging up to 400°C. The yield strength is linearly proportional to the amount of ° irrespective of the treatment used to form α. The yield strength is given by α_y = 225f + 48.65 ksi, where ƒ is the volume fraction of martensite. Softening occurs by aging at 500°C and above due to a decrease in percent α which may occur by renucleation of γ. The system is an unusual form of composite strengthening; hard martensite particles are formed within the austenite, and the percent α (and thereby the mechanical properties), can be controlled by the mechanical/thermal-mechanical processing. Formerly with the Department of Materials Science and Engineering, University of California, Berkeley, Calif.     

A fine γ′+α cellular structure in Fe-37.3 wt pct Ni-3.6 wt pct Al-3.3 wt pct Ti-0.2 wt pct C and its influence on high-temperature tensile properties

Fe-37.3 wt pct Ni-3.6 wt pct Al-3.3 wt pct Ti-0.2 wt pct C alloy, which reveals an excellent combination of high strength and good elongation endowed by formation of homogeneously dispersed fine γ′ precipitates in the matrix during aging at 823 K, has been investigated by means of transmission electron and optical microscopies, electron diffractions, and tensile tests. The influence of unique γ′+α cellular products on the mechanical properties has also been studied. Because of low elastic mismatch between the austenitic γ matrix and isomorphic γ′ precipitate phases, the homogeneously distributed precipitate particles, which formed at the early stage of aging, were observed to persist even after long-term aging. After very lengthy aging, the fine γ′ phase particles were changed to coarser γ′ lamellae at the grain boundary reaction front, which were alternately arranged with fine α lamellae that were estimated to have been transformed from the austenite-stabilizing-solute(Ni, C)-depleted γ lamellae. The fine duplex γ′+α cellular product did not affect deleteriously the room-temperature tensile properties of the alloy. However, the cellular structure was observed to cause the grain boundary embrittlement of the aged alloy at elevated temperatures higher than 681 K.     

Microstructure and high-temperature tensile deformation of tiai(si) alloys made from elemental powders

Two ternary TiAl-based alloys with chemical compositions of Ti-46.4 at. pct Al-1.4 at. pct Si (Si poor) and Ti-45 at. pct Al-2.7 at. pct Si (Si rich), which were prepared by reaction powder processing, have been investigated. Both alloys consist of the intermetallic compounds y-TiAl, α₂-Ti₃Al, and ξ-Ti₅(Si, Al)₃. The microstructure can be described as a duplex structure(i.e., lamellar γ/α₂ regions distributed in γ matrix) containing ξ precipitates. The higher Si content leads to a larger amount of ξ precipitates and a finer y grain size in the Si-rich alloy. The tensile properties of both alloys depend on test temperature. At room temperature and 700 °C, the tensile properties of the Si-poor alloy are better than those of the Si-rich alloy. At 900 °C, the opposite is true. Examinations of tensile deformed specimens reveal ξ-Ti₅(Si, Al)₃ particle debonding and particle cracking at lower test temperatures. At 900 °C, nucleation of voids and microcracks along lamellar grain boundaries and evidence for recovery and dynamic recrystallization were observed. Due to these processes, the alloys can tolerate ξ-Ti₅(Si, Al)₃ particles at high temperature, where the positive effect of grain refinement on both strength and ductility can be utilized.     

Metastable and equilibrium phases in dilute V-N alloys

Alloys of vanadium containing 0 to 10 at. pct nitrogen were prepared and aged at temperatures from 300 to 950°C. The nitrogen solvus line was determined and the discontinuities are discussed in terms of the phases coexisting with the α-phase at various temperatures. Aging temperatures greater than 550°C produced only V₃N in equilibrium with the α-phase. Aging at 550°C and below resulted in a sequence of transformations beginning with the precipitation of V₁₆N from supersaturated α-phase. This transformation was followed by the decomposition of the V₁₆N into two other metastable phases, termed V_xN and V_γN. Longer aging times at 550°C resulted in dissolution of VγN and resultant large discs of V*N precipitate. Further aging yielded V₃N. Except for the α-phase, the phases involved in this sequence have nitrogen atoms which are ordered. The crystal structures of both the vanadium atoms and the nitrogen atoms in V_xN and VγN are presented and discussed. VN is shown to be bet while VγN is bcc with respect to the vanadium atoms. The morphology of the precipitates is described and discussed in terms of their crystal structures. Formerly Assistant Professor, Department of Mechanical Engineering, Union College Formerly Graduate Student, Department of Mechanical Engineering, Union College Formerly student in Department of Physics, Union College     

Determination of the Fe-Ni and Fe-Ni-P phase diagrams at low temperatures (700 to 300 °C)

The α + γ two-phase fields of the Fe-Ni and Fe-Ni (P saturated) phase diagrams have been determined in the composition range 0 to 60 wt pet Ni and in the temperature range 700 to 300 °C. The solubility of Ni in (FeNi)₃P was measured in the same temperature range. Homogeneous alloys were austenitized and quenched to form α₂, martensite, then heat treated to formα (ferrite) + γ (austenite). The compositions of the α and γ phases were determined with electron microprobe and scanning transmission electron microscope techniques. Retrograde solubility for the α/(α + γ) solvus line was demonstrated exper-imentally. P was shown to significantly decrease the size of the α + γ two-phase field. The maximum solubility of Ni in α is 6.1 ± 0.5 wt pct at 475 °C and 7.8± 0.5 wt pct at 450 °C in the Fe-Ni and Fe-Ni (P saturated) phase diagrams, respectively. The solubility of Ni in α is 4.2 ± 0.5 wt pct Ni and 4.9 ± 0.5 wt pct Ni at 300 °C in the Fe-Ni and Fe-Ni (P saturated) phase diagrams. Ternary Fe-Ni-P isothermal sections were constructed between 700 and 300 °C. Formerly Research Assistant in Department of Metallurgy & Materials Engineering, Lehigh University, Bethlehem, PA.     

Structure-property relations in a metastable β Ti alloy containing Si

The hardness response, tensile behavior, and phase transformations occurring in a quenched and aged metastable β phase Ti-30 at. pct V-l at. pct Si alloy have been inves-tigated. Upon aging at 570°C, as-quenched samples show a broad hardness peak which is associated with the formation of rod-like, hexagonal (Ti,V)_xSi_y transition phase precipi-tates. The equilibrium silicide is observed upon aging at 570°C in the form of faceted, tetragonal particles. A loss of tensile ductility and a transition to intergranular fracture occurs after extended aging at 570°C and is related to Si segregation to the grain bound-aries. Comparing the behavior of Ti-30V to that of Ti-30V-lSi shows that the presence of Si strongly retards α-phase formation. However, a substantial age hardening re-sponse still occurs upon aging at 450°C, especially after prior cold work (the yield strength increases from 635 to 982 MPa). This hardening response is combined with a retention of a ductile, transgranular fracture even after extended aging at 450°C. Aging first at 570°C followed by aging at 450°C results in an increase in the volume fraction of α-phase formed but a subsequent decrease in ductility and hardness response upon aging at 450°C. These results are discussed in terms of the structure/property relationships which result from the influence of Si on the formation of, a) (Ti.V)_xSi_y precipitates, b) the equilibrium silicide, and c) the α-phase.