Influence of Interfacial Structure upon Carbide Precipitation at Austenite : Ferrite Boundaries in an Fe-C-Mo Alloy

During conventional isothermal transformation of an Fe-0.11 pct C-1.95 pct Mo alloy, eutectoid decomposition occurs by the interphase boundary carbide precipitation and the fibrous carbide mechanisms at 770° to 825 °C. When proeutectoid ferrite is formed and then recrystallized within the α + γ region, and subsequently further transformed at 770° to 825 °C, however, both of these eutectoid decomposition mechanisms are rendered inoperative. Carbide precipitation occurs instead entirely as isolated particles. This result supports the deduction that carbide precipitation at austenite : ferrite boundaries can occur only when these boundaries are locally immobilized by a partially coherent interfacial structure. A general approach to explaining the development of planar and curved interphase boundary precipitation, fibrous structure, and pearlite is developed in terms of two crystallographic factors. Formerly Research Associate in the Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931 and the Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15213 Formerly Graduate Student, Michigan Technological University, and Visiting Graduate Student, Camegie-Mellon University Formerly Professor at Michigan Technological University     

The kinetics of isothermal and isovelocity pearlite growth in Cu-Al eutectoid alloy

Measurements of pearlite growth rate and inter lamellar spacing have been made on isothermally transformed specimens of Cu-11.8 wt pct Al eutectoid alloy. Growth rates which have been calculated on the assumption that volume diffusion in the parent β phase is the rate-controlling process show excellent agreement with the measured values. Directional control of eutectoid growth by translating specimens through steep temperature gradients resulted in the formation of aligned lamellar structures, and the effect of variations in imposed velocity and temperature gradient on alignment has been examined. A comparison of the kinetics of the two modes of transformation has shown that isothermal and isovelocity pearlite growth in this alloy may be readily interrelated. Analysis of the isovelocity growth data supports the conclusion that growth is controlled by volume diffusion. Formerly Graduate Student, Department of Metallurgy, University of Manchester     

Influence of interfacial structure upon carbide precipitation at austenite: Ferrite boundaries in an Fe-C-Mo alloy

During conventional isothermal transformation of an Fe-0.11 pct C-1.95 pct Mo alloy, eutectoid decomposition o°Curs by the interphase boundary carbide precipitation and the fibrous carbide Mechanisms at 770° to 825 °C. When proeutectoid ferrite is formed and then recrystallized within the α+ γ region, and subsequently further transformed at 770° to 825 °C, however, both of these eutectoid decomposition Mechanisms are rendered inoperative. Carbide precipitation o°Curs instead entirely as isolated particles. This result supports the deduction that carbide precipitation at austenite: ferrite boundaries can o°Cur only when these boundaries are locally immobilized by a partially coherent interfacial structure. A general approach to explaining the development of planar and curved interphase boundary precipitation, fibrous structure, and pearlite is developed in terM_s of two crystallographic factors. T. OBARA, formerly Research Associate in the Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931 and the Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15213. G. J. SHIFLET, formerly Graduate Student, Michigan Technological University, and Visiting Graduate Student, Carnegie-Mellon University. H-I AARONSON, formerly Professor at Michigan Technological University.     

On the decomposition of β phase in some rapidly quenched titanium-eutectoid alloys

The decomposition of the β phase in rapidly quenched Ti-2.8 at. pct Co, Ti-5.4 at. pct Ni, Ti-4.5 at. pct, and 5.5 at. pct Cu alloys has been investigated by electron microscopy. During rapid quenching, two compctitive phase transformations, namely martensitic and eutectoid transformation, have occurred, and the region of eutectoid transformation is extended due to the high cooling rates involved. The β phase decomposed into nonlamellar eutectoid product (bainite) having a globular morphology in Ti-2.8 pct Co and Ti-4.5 pct Cu (hypoeutectoid) alloys. In the near-eutectoid Ti-5.5 pct Cu alloy, the decomposition occurred by a lamellar (pearlite) type, whereas in Ti-5.4 pct Ni (hypereutectoid), both morphologies were observed. The interfaces between the proeutectoid α and the intermetallic compound in the nonlamellar type as well as between the proeutectoid α and the pearlite were often found to be partially coherent. These findings are in agreement with the Lee and Aaronson model proposed recently for the evolution of bainite and pearlite structures during the solid-state transformations of some titanium-eutectoid alloys. The evolution of the Ti₂Cu phase during rapid quenching involved the formation of a metastable phase closely related to an “ω-type” phase before the equilibrium phase formed. Further, the lamellar intermetallic compound Ti₂Cu was found to evolve by a sympathetic nucleation process. Evidence is established for the sympathetic nucleation of the proeutectoid a crystals formed during rapid quenching.     

Thermomechanical treatments of a 1050 pearlite steel

A study of the microstructures and mechanical properties of thermomechanically treated 1050 steel has been carried out. The materials were first transformed to fine and coarse lamellar pearlite structures. These samples were reduced 75 pct in thickness by cold rolling, then heat treated at 780 °C (up-quenching) or 650 °C (annealing) for different periods of time, followed by air-cooling. The mechanical properties were found to be vastly improved by suitable thermomechanical treatments. A significant improvement in yield strength was accompanied by moderate increase in ultimate tensile strength, and the elongations were maintained above 15 pct in 3 cm gage length. The optimal mechanical properties were developed between 30 and 120 seconds during 780 °C up-quenching for coarse pearlite samples and 100 to 400 seconds for both annealed coarse and fine pearlite specimens. In these instances the relatively long duration of heat treatment time shall facilitate the industrial processing of the steel. The improvement in mechanical properties was correlated with distinct microstructures in pearlite regions, which were fine subgrains (or grains) less than 1 μm in size and dispersed globular carbide particles along the subgrain (or grain) boundaries. Formerly with the Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan.     

An investigation of the fatigue and fracture behavior of mn-containing gamma titanium aluminides

The fatigue and fracture mechanisms in Ti-48Al-xMn (x = 1.4 to 2.0 at. pct) gamma-based titanium aluminide alloys are elucidated. Unlike most gamma alloys, which fail predominantly by transgranular fracture at room temperature, fracture in ternary Ti-48Al-xMn alloys is shown to occur mainly by intergranular failure. The incidence of intergranular failure increased with increasing annealing duration and temperature. Intergranular fracture is shown to occur as a result of the segregation of Mn to equiaxed and interlamellar boundaries. Annealing either above or below the eutectoid temperature results in the precipitation of α₂ particles. The reduction in the strength and toughness of ternary Mn-containing alloys is attributed to the combined effects of segregation and α₂ precipitation. A micromechanics framework is presented for the assessment of twin toughening mechanisms under monotonie and cyclic loading. Formerly Staff Scientist with General Electric Research and Development, Schenectady, NY 12301 Formerly Undergraduate Student, Department of Materials Science and Engineering, The Ohio State University     

Application of the double-shear theory of martensite crystallography to the β → α′ transformation in an U(Ga) alloy

The phenomenological double-shear theory of martensite crystallography has been applied to the β → α′ martensitic transformation in the U-1.6 at. pct Ga alloy. A correspondence matrix for the β → α′ transformation was derived from the experimentally determined β/α′ orientation relationship, and the double lattice invariant shear was considered as a combination of the principal slip (010) [100]_a with one of the minor slips in the α-uranium structure. The theoretical predictions of the habit plane are in good agreement with the experimental observations. Formerly Graduate Student, Ben-Gurion University of the Negev.     

Superplasticity in a thermomechanically processed High-Mg,Al-Mg alloy

Superplastic elongations in excess of 400 pct have been observed in tension testing at 573 K (300 °C) and strain rate έ= 2 × 10^-3 s^-1 for a thermomechanically processed Al-10.2 pct Mg-0.52 pct Mn alloy. The thermomechanical processing consists of solution treatment and hot working, followed by extensive warm rolling at 573 K (300 °C), a temperature below the solvus for Mg in the alloy. This processing results in a fine subgrain structure in conjunction with refined and homogeneously distributed β(Al₈Mg₅) and MnAl₆ precipitates. This structure does not statically recrystallize when annealed at 573 K (300 °C) but appears to recrystallize continuously during deformation at such a temperature and the resulting fine grain structure deforms with minimal cavitation. At temperatures above the Mg-solvus,e.g., 673 K (400 °C), recrystallization and growth occur readily resulting in rela tively coarser structures which deform superplastically but with extensive grain boundary sliding and cavitation. Formerly in Materials Group, Mechanical Engineering, Naval Postgraduate School Formerly Graduate Student in Mechanical Engineering, Naval Postgraduate School     

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     

Experimental study of the phase equilibria in the Fe-Mn-Al system

The isothermal sections with lower Al content at 800 ‡C, 900 ‡C, 1200 ‡C, and 1300 ‡C and two vertical sections of 4 wt pct Al and 8 wt pct Al in the Fe-Mn-AI system were determined by means of the diffusion couple technique, metallographic method, and differential thermal analysis (DTA). It is shown that the results of isothermal and vertical sections coincide with each other, and the twophase (α + γ) region and three-phase (α + γ + β) region trend toward the Fe-Mn side and Mn corner with the rise of temperature, respectively. The experimental results are in agreement with those calculated by the present authors. Formerly Graduate Student, Department of Materials Science and Engineering, Northeastern University     

The compctition between martensite and omega in quenched Ti-Nb alloys

A careful experimental study of the phase transformations which occur in annealed β phase Ti-Nb alloys during quenching has been completed. The compctition of the α″ and ω phases to form in alloys of 20 to 70 at. pct Nb was investigated as a function of quench rate and alloy composition. Particular attention was paid to the interstitial content and chemical homogeneity of the alloys. The martensitic α″ phase was found only in 20 and 25 at. pct Nb alloys, and then only using fast water quenches of ~300 °C/sec. Under slower quench conditions,e.g., ~0.3 to 3 °C/sec, ω phase precipitates were found in these alloys and in 30 and 35 at. pct Nb alloys. Evidence of “diffuse” ω phase precipitation was observed in alloys up to 50 at. pct Nb. Only alloys of 60 and 70 at. pct Nb were found to retain the single phaseβ structure upon quenching. These results constitute the first part of a study of the stable and metastable equilibria of the Ti-Nb alloy system. Formerly a Graduate Student in the Materials Science Program at the University of Wisconsin-Madison.     

An investigation of the generality of incomplete transformation to bainite in Fe-C-X alloys

The overall kinetics of the isothermal transformation of austenite to bainite and to pearlite in high-purity Fe-C-3 at. pct X alloys (X = Mn, Si, Ni, or Cu) containing 0.1 wt pct C and 0.4 wt pct C were investigated with quantitative metallography and transmission electron microscopy (TEM) to ascertain the presence or absence of the incomplete reaction phenomenon. The incomplete transformation of austenite to bainite was not observed in the Fe-C-Si, Fe-C-Ni, Fe-C-Cu, or Fe-0.4C-Mn alloys. It was found, however, in the Fe-0.1C-Mn alloy. Transmission electron microscopy results indicate that sympathetic nucleation of ferrite without carbide precipitation is a necessary but not a sufficient condition for the development of the incomplete reaction phenomenon. Transformation resumes following stasis in the low-carbon Fe-C-Mn alloy with the formation of a nodular bainite. The results support the view that the incomplete transformation of austenite to bainite is a characteristic of specific alloying elements and is not an inherent trait of the bainite reaction. Formerly Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University. Formerly Visiting Professor, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University. Formerly Undergraduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

Microstructure and its development in Cu-Al-Ni alloys

The microstructure of as-cast Cu-AI-Ni alloys, based on copper containing 9 to 10 wt pct Al and up to 5 wt pct Ni, has been examined. The development of the microstructure on continuous cooling has also been investigated. For alloys with 9.2 to 9.3 wt pct Al, and less than 1 wt pct Ni, the as-cast microstructure consists of proeutectoid α solid solution, α + γ₂ eutectoid, and martensitic β. If the nickel content is more than 2.5 wt pct, the α + γ₂ eutectoid is replaced by α + β ₂ ^′ eutectoid, and no martensitic β is observed in the as-cast alloys. The morphologies of the β ₂ ^′ and γ₂ eutectoid phases are similar; both have the Kurdjumov-Sachs (K-S) orientation relationship with the a phase. Two eutectoid reactions, involving β to α + γ₂ and β to α + β′₂, have been observed in an alloy containing 9.7 wt pct Al and 2.7 wt pct Ni. When both eutectoid reactions occur, the Nishiyama-Wassermann (N-W) orientation relationship exists between γ₂ or β ₂ ^′ and the α phase. During continuous cooling, proeutectoid α solid solution is the first phase to precipitate from the high-temperature β phase. The β to α + β ₂ ^′ eutectoid reaction starts at higher temperatures than the β to α + γ₂ reaction. Tempering of the as-cast alloys results in the elimination of the martensitic β. Y.S. SUN formerly Research Associate with the Manchester Materials Science Centre.     

The compctition between the alpha and omega phases in aged Ti-Nb alloys

The compctition between the stable α phase and metastable ω phase to precipitate in a metastableβ phase matrix was investigated in the present study. Four binary Ti-Ni alloys with compositions between 20 and 35 at. pct Nb were air cooled to room temperature from 1000 °C and then aged at temperatures between 300 and 500 °C. For aging temperatures of 400 °C and lower it was found that the quench before aging enabled ω phase precipitates to grow to the exclusion of α precipitates. When specimens were directly aged at 400 °C only α precipitates were observed. Precipitates which could not be identified using SAD were observed in specimens of the 30 and 35 at. pct Nb alloys. All precipitation reactions became more sluggish as the niobium content of an alloy was increased. The results reported here form the second part of a study of the stable and metastable equilibria of the Ti-Nb alloy system. Formerly a Graduate Student in the Materials Science Program at the University of Wisconsin-Madison     

Crystallography of grain boundary α precipitates in a β titanium alloy

The crystallography of α(hcp) precipitates formed on the β(bcc) matrix grain boundaries has been studied with transmission electron microscopy (TEM) in a Ti-15V-3Cr-3Sn-3Al alloy. The α precipitates have a near-Burgers orientation relationship with respect to at least one of the adjacent β grains. Among the possible 12 variants in this orientation relationship, the variant that [11•20]_α is parallel to the 〈111〉_β closest to the grain boundary plane tends to be preferred by the α precipitates. Additionally, further variant selections are made so as to minimize the deviation of orientation relationship with respect to the “opposite“ β grain from the Burgers one. Such rules in variant selection often result in the formation of precipitates with a single variant at a planar grain boundary. Prior small deformation of β matrix changes the variant of α precipitates at the deformed portion of grain boundary. It is considered that the stress field of dislocations in the slip bands intersecting with the boundary strongly affects the variants of α precipitates. Discussion of these results is based upon a classical nucleation theory. Formerly Graduate Student, Department of Materials Science and Engineering, Kyoto University Formerly Graduate Student, Department of Materials Science and Engineering, Kyoto University This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms of Nucleation and Growth in Solids,“ organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in Rosemont, Illinois.     

Transformations in α 2+γ titanium aluminide alloys containing molybdenum: Part II. Heat treatment

The response of as-cast structures of 12 alloys in the Ti-Al-Mo system containing 44 to 50 at. pct Al and 2 to 6 at. pct Mo to simple single step heat treatments in the temperature range 1373 to 1673 K is described. The microsegregation patterns present in the cast structure persist to a large extent after heat treatment, especially below 1673 K. However, tentative conclusions regarding phase equilibria in this temperature and composition range are drawn from the results. High-temperature equilibria are dominated by the β, α+β, and α+γ phase fields, while the β+γ phase field dominates equilibrium below 1473 K. Three major types of transformation behavior are observed: a massive α to γ transformation, which occurs within the α phase on quenching from 1673 and 1573 K in alloys centered around the 48 pct Al composition; a eutectoid transformation from α to B2+γ mixtures, which occurs at 1473 K and below in alloys centered around the 48Al-4Mo and 46Al-6Mo compositions; direct γ precipitation in β, which occurs primarily in the 44Al-6Mo composition at 1273 K and below; and finally growth of γ lamellae in α+γ lamellar structures with B2 precipitation on lamellar interfaces, which occurs over a broad range of alloy compositions and temperatures.     

The influence of thermomechanical processing variables on superplasticity in a High-Mg,Al-Mg alloy

The superplastic response of an Al-10.2 pct Mg-0.52 pct Mn alloy, warm rolled at 573 K (300 °C), may be enhanced by annealing at a lower temperature, 473 K (200 °C), prior to subsequent stressstrain testing at a warm temperature,e.g., 573 K (300 °C). This enhancement is attributed to recovery and precipitation during annealing effectively retarding continuous recrystallization and growth during subsequent deformation. The finer resultant structure more readily sustains superplastic flow processes. The warm-rolled structure, a refined subgrain structure in conjunction with fine β (Mg₅Al₈) and MnAl₆ precipitates, will statically recrystallize only upon heating to a temperature above the Mg solvus, such as 713 K (440 °C). Such recrystallization results in a relatively fine-grained material but also suppresses warm-temperature superplasticity; deformation at 573 K (300 °C) takes place by dislocation flow in the recrystallized material. At temperatures above the Mg solvus, both warm rolled and recrystallized materials exhibit superplastic elongations but cavitate extensively as a result of deforming by boundary sliding. Formerly with Materials Group, Mechanical Engineering Department, Naval Postgraduate School, Monterey, CA Formerly Graduate Student in Mechanical Engineering, Naval Postgraduate School, Monterey, CA     

Martensitic transformations in β Ag-cd alloys

Phase transformations have been studied inβ Ag-Cd alloys lying in the composition range 42.3 to 50.9 at. pct Cd. Thermal martensite with an orthorhombic structure was obtained on cooling below room temperature. The M_s temperature was found to change from —44° to — 137°C as the cadmium content changed from 44.2 at. pct Cd to 47.0 at. pct Cd. The martensite had a habit plane close to (133)β in good agreement with that calculated from the phenomenological theory assuming a (011) [0•11]β lattice-invariant shear. An fct martensite was obtained on moderate degrees of deformation at room temperature. This had a habit plane close to (110)β, again in good agreement with the phenomenological theory assuming a (110)[1•10]β lattice-invariant shear. On severe deformation a close-packed structure was obtained, this being fcc up to 45.5 at. pct Cd, hcp above 47.7 at. pct Cd, and a mixture of both fcc and hcp at 46.0 at. pct Cd. A spontaneous martensite with an fcc structure was found to occur along the thin edges of perforated specimens used for electron microscopy. A mechanism suggesting the course of the transformation has been proposed. Formerly Graduate Student, Department of Metallurgy, University of British Columbia, Vancouver, Canada.     

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     

Phase Equilibria and Interstitial Effects in the Ti-V-Mo Alloy System

The stability of theβ phase in the Ti-V, Ti-Mo, and Ti-V-Mo alloy systems was investi-gated, and theβ/α + β phase boundaries in these systems were determined in the range 300 to 600° C. The results indicate that Mo is more potent than V in stabilizing theβ phase with respect to α phase formation and in retarding the β → α reaction kinetics. It is shown that increasing the oxygen concentration in the alloys tends to enhance α phase formation in Mo-lean alloys (Mo contents < 15 wt pct), whereas it leads to the formation of an oxide phase in Mo-rich alloys (Mo contents ≥15 wt pct). Formerly Research Assistant, Department of Materials Science, University of Southern California