Effects of Strain State and Strain Rate on Deformation-Induced Transformation in 304 Stainless Steel: Part I. Magnetic Measurements and Mechanical Behavior

The γ→α transformation in 304 stainless steel can be induced by plastic deformation at room temperature. The kinetics of strain-induced transformations have been modeled recently by Olson and Cohen. We used magnetic techniques to monitor the progress of the γ→α transformation in 304 stainless steel sheet loaded in uniaxial and biaxial tension at both low (10^-3 per second) and high (10³ per second) strain rates. We found that using the von Mises effective strain criterion gives a reasonable correlation of transformation kinetics under general strain states. The principal effect of increased strain rate was observed at strains greater than 0.25. The temperature increase resulting from adiabatic heating was sufficient to suppress the γ→α transformation substantially at high rates. The consequences of the γ→α transformation on mechanical behavior were noted in uniaxial and biaxial tension. Uniaxial tension tests were conducted at temperatures ranging from 50 to -80°C. We found that both the strain hardening and transformation rates increased with decreasing temperature. However, the martensite transformation saturates at ≈85 pct volume fraction α. This can occur at strains less than 0.3 for conditions where the transformation is rapid. Once saturation occurs, the work hardening rate decreases rapidly and premature local plastic instability results. In biaxial tension, the same tendency toward plastic instability associated with high transformation rates provides a rationale for the low biaxial ductility of 304 stainless steel.     

Effect of heat treatment on the structure and properties of a high-nitrogen austenitic corrosion-resistant 03Kh20AG11N7M2 steel

The structure and mechanical and corrosion properties of a high-strength austenitic 03Kh20AG11N7M2 steel after quenching and aging at 500 and 800°C are analyzed. The phase composition of the steel and the mechanism of the decomposition of austenite during heat treatment are studied by electron-probe microanalysis and transmission electron microscopy. This steel is thermally stable upon heating to 800°C for 1 h and is stable to the γ → α and γ → ɛ martensitic transformation during deformation up to tensile strains leading to fracture. The homogeneous decomposition of a supersaturated γ solid solution at 500°C leads to the formation of disperse CrN nitrides, which increase the strength of the steel and insignificantly decrease its plasticity. In this case, the stress corrosion cracking resistance slightly decreases and the passivation of the steel increases in an corrosive medium without loading.     

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     

Precipitation-strengthened austenitic Fe−Mn−Ti alloys

The precipitation of intermetallic compounds in the Fe−20Mn−2Ti and Fe−28Mn−2Ti alloy systems has been investigated over the temperature range 700 to 900°C by hardness measurements, optical and scanning electron microscopy, and X-ray diffraction. In both systems only the equilibrium Laves phase was observed. The precipitate was identified as C14(MgZn₂) type hexagonal Laves phase with a chemical composition close to Fe₂ (Ti, Mn). In an as-annealed sample precipitation occurred in a heterogeneous manner, predominantly along grain boundaries. The effect of a cold deformation between the solution annealing and aging processes was also investigated. In addition to a high density of dislocations, martensitic phases were induced by deformation: a γ→∈ transformation occurred in the Fe−28Mn−2Ti alloy while a γ→α′ transformation was predominant in the Fe−20Mn−2Ti alloy. Subsequent aging was conducted at temperatures above theA _f . A large number of very fine precipitates formed randomly in the matrix after a short aging period. This cold work plus aging treatment resulted in an increase in yield strength. The enhancement of mechanical properties is due to the randomly distributed precipitates combined with the high defect density and fine substructure.     

In-situ phase mapping and direct observations of phase transformations during arc welding of 1045 steel

In-situ spatially resolved X-ray diffraction (SRXRD) experiments were performed during gas tung-sten arc (GTA) welding of AISI 1045 C-Mn steel. Ferrite (α) and austenite (γ) phases were identified and quantified in the weld heat-affected zone (HAZ) from the real time SRXRD data. The results were compiled with weld temperatures calculated using a coupled thermal fluids model to create a phase map of the HAZ. Kinetics of the α → γ transformation during weld heating and the reverse γ → α transformation during weld cooling were extracted from the map. Superheating as high as 250 °C above the A3 temperature was observed for the α → γ phase transformation to reach completion at locations near the fusion zone (FZ) boundary. The SRXRD experiments revealed that the newly created γ phase exists with two distinct lattice parameters, resulting from the inhomogeneous distribution of carbon and manganese in the starting pearlitic/ferritic microstructure. During cooling, the reverse γ → α phase transformation was shown to depend on the HAZ location. In the fine-grained region of the HAZ, the γ → α transformation begins near the A3 temperature and ends near the A1 temperature. In this region, where the cooling rates are below 40 °C/s, the transformation occurs by nucleation and growth of pearlite. In the coarse-grained region of the HAZ, the γ → α transformation requires 200 °C of undercooling for completion. This high degree of undercooling is caused by the large grains coupled with cooling rates in excess of 50 °C/s that result in a bainitic transformation mechanism.     

An investigation of transformation strengthening in precipitation-hardened Fe-Ni austenite

Ausaged Fe-33Ni-3Ti and Fe-34Ni-3Ti-0.5Al austenitic alloys were transformation strengthened through sequential martensite (α′ → γ) and reverse martensitic transfor-mations. An increase in yield strength of 20 to 30 ksi (130 to 210 MPa) was obtained, leading to alloy yield strengths near 180 ksi (1250 MPa) in the ausaged and transforma-tion strengthened condition. The presence of the γ′ precipitates formed on ausaging did not radically change the nature of the shear transformation nor the morphology and sub-structure of the transformation product. The presence of the γ′ precipitates did, how-ever, have the beneficial consequence that the α′→ γ reversion could be accomplished with slow heating rates without loss of transformation strengthening. The yield strength actually increased when a slow heating rate was used, a result attributed to the success-ful pinning of transformation induced defects by the γ′ precipitates and to the occurrence of additional precipitation during heating. The efficient pinning of transformation induced defects had the further consequence that the alloys could be annealed for long periods after reversion to austenite without significant loss of strength.     

Tensile deformation behavior of mechanically stabilized Fe-Mn austenite

The tensile deformation behavior of mechanically-stabilized austenite is investigated in Fe-Mn binary alloys. A 30 pct thickness reduction by rolling at 673 K (above the A_f temperature) largely suppresses the austenite (γ) to hcp epsilon martensite (ε) transformation in 17Mn and 25Mn steels. However, the deformation behavior of the mechanically stabilized austenite in the two alloys differs significantly. In 25Mn steel, the onset of plastic deformation is due to the stress-induced γ→ ε transformation and results in a positive temperature dependence of the yield strength. The uniform elongation is enhanced by the γ → ε transformation during deformation. In 17Mn steel, bccα′ martensite is deformation-induced along with e and a plateau region similar to Lüders band deformation appears at the beginning of the stress-strain curve. The mechanical stabilization of austenite also suppresses the intergranular fracture of 17Mn steel at low temperatures. M. STRUM, formerly Candidate for Ph.D. at the University of California at Berkeley     

The tempering of martensite in an Fe-1.5 pct N alloy

The tempering of Fe 1.5 pct N martensite has been studied at temperatures up to 300°C using X-ray and electron microscope techniques. Stage 1 decomposition occurs below 270°C by the general precipitation, resembling spinodal morphology, of fine τa^" (Fe₁₆ N₂) lamellae on 001 habit planes in both matrix and twin crystals of the partially 112 twinned martensite plates. Yet, gaged by changes in the X-ray spectrum, the reaction is discontinuous, the tetragonal martensite doublets decaying in intensity without change in their Bragg positions. The anomaly and the failure to detect by electron microscopy regions exhibiting fractional stages of the fine scale α^’ → α + α^" reaction is attributed to its occurrence at different times in different martensite (or parts of martensite) plates. It is believed that transformation occurs in this manner because the nucleation of coherent α^" plates is controlled by the prevailing internal stress field. Thus the time exponent “n” for the reaction decays from a normal value between 1 and 0.67 to less than 0.3 as stress relief by recovery dominates the more protracted stages of the reaction. Above 200°C the more stable nitride γ^’ (Fe₄N) forms at an increasing rate as plates on 012 habit planes, accompanied by marked softening.     

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.     

The ternary phase diagram,Fe-Ni-P

In order to provide the necessary phase equilibria data for understanding the development of the Widmanstatten pattern in iron meteorites, we have redetermined the Fe-Ni-P phase diagram from 0 to 100 pct Ni, 0 to 16.5 wt pct P, in the temperature range 1100° to 550°C. Long term heat treatments and 130 selected alloys were used. The electron microprobe was employed to measure the composition of the coexisting phases directly. We found that the fourphase reaction isotherm, where α+ liq ⇌ γ+ Ph, occurs at 1000° ± 5°C. Above this temperature the ternary fields α+ Ph + liq and α+ γ+ liq are stable and below 1000°C, the ternary fields ⇌+ γ + Ph and γ + Ph + liq are stable. Below 875°C a eutectic reaction, liq → γ + Ph, occurs at the Ni-P edge of the diagram. Altogether nineteen isotherms were determined in this study. The phase boundary compositions of the two-and three-phase fields are listed and are compared with the three binary diagrams. The α + γ + Ph field expands in area in each isotherm as the temperature decreases from 1000°C. Below 800°C the nickel content in all three phases increases with decreasing temperature. The phosphorus solubility in α and γ decreases from 2.7 and 1.4 wt pct at 1000°C to 0.25 and 0.08 wt pct at 550°C. The addition of phosphorus to binary Fe-Ni greatly affects the α/α + γ and γ/α + γ boundaries below 900°C. It stabilizes the α phase by increasing the solubility of nickel (α/α +γ boundary) and above 700°C, it decreases the stability field of the γ phase by decreasing the solubility of nickel(@#@ γ/α + γ boundary). However below 700°C, phosphorus reverses its role in γ and acts as a γ stabilizer, increasing the nickel solubility range. The addition of phosphorus to Fe-Ni caused significant changes in the nucleation and growth processes. Phosphorus contents of 0.1 wt pct or more allow the direct precipitation ofa from the parent γ phase by the reaction γ ⇌ α + γ. The growth rate of the α phase is substantially higher than that predicted from the binary diffusion coefficients. Formerly at Planetology Branch, Goddard Space Flight Center     

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%.     

Phase transformation in an Fe-10.1Al-28.6Mn-0.46C alloy

The microstructure of an (α + γ) duplex Fe-10.1Al-28.6Mn-0.46C alloy has been investigated by means of optical microscopy and transmission electron microscopy (TEM). In the as-quenched condition, extremely fine D0₃ particles could be observed within the ferrite phase. During the early stage of isothermal aging at 550 °C, the D0₃ particles grew rapidly, especially the D0₃ particles in the vicinity of the α/γ grain boundary. After prolonged aging at 550 °C, coarse K’-phase (Fe, Mn)₃AlC precipitates began to appear at the regions contiguous to the D0₃ particles, and —Mn precipitates occurred on the α/γ and α/α grain boundaries. Subsequently, the grain boundary β-Mn precipitates grew into the adjacent austenite grains accompanied by a γ→ α + β-Mn transition. When the alloy was aged at 650 °C for short times, coarse. K-phase precipitates were formed on the α/γ grain boundary. With increasing the aging time, the α/γ grain boundary migrated into the adjacent austenite grain, owing to the heterogeneous precipitation of the Mn-enrichedK phase on the grain boundary. However, the α/γ grain boundary migrated into the adjacent ferrite grain, even though coarse K-phase precipitates were also formed on the α/γ grain boundary in the specimen aged at 750 °C.     

Martensitic transformations in high-strength steels at aging

The effect of heat treatment and elastic stresses on the texture of maraging NiTi-steels is studied. The interruption of the decomposition of martensite at the early stages is shown to be accompanied by the γ → α transformation, which proceeds upon cooling from the aging temperature and under elastic (σ < σ_0.2) tensile stresses. The martensite has a crystallographic texture, which is caused by the evolution of hot-deformation texture as a result of quenching and decomposition of a supersaturated α solid solution.     

The morphology and substructure of martensite in maraging steels

Thin foil transmission electron microscopy, X-ray diffraction and dilatometric techniques have been used to study the martensitic γ → α transformation in three steels with nominal contents of 8 pct nickel and 0.2 pct beryllium and chromium contents of 12, 14 and 16 pct. In each case the martensite formed as laths with a habit plane close to {225}_γ. With increasing chromium content and increasing cooling rate greater numbers of the laths were observed to be internally twinned. Detailed analysis of the martensitic transformation suggested that the internally twinned laths are formed by a sequence of γ→ ε or faulted γ→ ά. The orientation relationships between the three phases γ, ε and α, determined from selected area diffraction analysis, corresponded to Kurdjumov-Sachs.     

The improvement of cryogenic mechanical properties of Fe-12 Mn and Fe-8 Mn alloy steels through thermal/mechanical treatments

An investigation has been made to improve the low temperature mechanical properties of Fe-8Mn and Fe-12Mn-0.2 Ti alloy steels. A reversion annealing heat treatment in the two-phase (α+ γ) region following cold working has been identified as an effective treatment. In an Fe-12Mn-0.2Ti alloy a promising combination of low temperature (-196°C) fracture toughness and yield strength was obtained by this method. The improvement of properties was attributed to the refinement of grain size and to the introduction of a uniform distribution of retained austenite (γ). It was also shown that an Fe-8Mn steel could be grain-refined by a purely thermal treatment because of its dislocated α martensitic structure and absence of ε martensite. As a result, a significant reduction of ductile to brittle transition temperature was obtained. formerly with the Lawrence Berkeley Laboratory, University of California.     

The martensite phases in 304 stainless steel

A detailed analysis of martensite transformations in 18/8 (304) stainless steel, utilizing transmission electron microscopy and diffraction in conjunction with X-ray and magnetization techniques, has established that the sequence of transformation is γ → ∈ → α. ε is a thermodynamically stable hcp phase whose formation is greatly enhanced as a result of plastic deformation. Comparison with the ε → α transformation in pure Fe-Mn alloys lends further support to the above sequence and suggests that a transformation line between ε and α in Fe-Cr-Ni alloys can be expected. In the 304 stainless steel used in this investigation, formation of α was induced only by plastic deformation and subsequent to formation of ε. Nucleation of α occurs heterogeneously at intersections of ε bands or where ε bands abut twin or grain boundaries (which represent unilaterally compressed regions). From electron diffraction, the Nishiyama relationship between γ and α phases appears to predominate at the start of the transformation, but then changes to that of Kurdjumov-Sachs. Based on these observations, a sequence of atom movements from the hcp structure to the bcc structure is proposed which has the basic geometric features of the martensitic transformation. Formerly with Department of Materials Science and Engineering, University of California, Berkeley, Calif.     

Effect of strain rate on the strain-induced <Emphasis Type="Italic">γ</Emphasis> → <Emphasis Type="Italic">α</Emphasis>′-martensite transformation and mechanical properties of austenitic stainless steels

The effect of strain rate on strain-induced γ → α′-martensite transformation and mechanical behavior of austenitic stainless steel grades EN 1.4318 (AISI 301LN) and EN 1.4301 (AISI 304) was studied at strain rates ranging between 3×10⁻⁴ and 200 s⁻¹. The most important effect of the strain rate was found to be the adiabatic heating that suppresses the strain-induced γ → α′ transformation. A correlation between the work-hardening rate and the rate of γ → α′ transformation was found. Therefore, the changes in the extent of the α′-martensite formation strongly affected the work-hardening rate and the ultimate tensile strength of the materials. Changes in the martensite formation and work-hardening rate affected also the ductility of the studied steels. Furthermore, it was shown that the square root of the α′-martensite fraction is a linear function of flow stress. This indicates that the formation of α′-martensite affects the stress by influencing the dislocation density of the austenite phase. Olson-Cohen analysis of the martensite measurement results did not indicate any effect of strain rate on shear band formation, which was contrary to the transmission electron microscopy (TEM) examinations. The β parameter decreased with increasing strain rate, which indicates a decrease in the chemical driving force of the α → α′ transformation.     

Thermomechanical strengthening of a γ′ precipitation-hardened nickel-base alloy

The advantages of using select thermomechanical processing for improving mechanical properties of γ′ strengthened nickel-base superalloys in the range room temperature to 1400°F are demonstrated in the present study. Based upon analysis of the deformation and precipitation behavior of this class of alloys, several processing conditions have been evaluated for the alloy Udimet 700. The working operation and resulting dislocation substructure are shown to be of critical importance. In the preferred treatment the alloy is warm worked in the two-phase γ-γ′ condition at a temperature below that of rapid recrystallization such that a polygonal substructure is formed on the scale of the γ′ interparticle spa cing. A thermomechanical treatment consisting of 1) solution heat treatment, 2) precipitation aging at 1950°F to form γ′, 3) warm working to 1.50∈ (92 pct RA) at 1950°F, plus 4) final aging at 1550° and 1400°F was found to be beneficial for Udimet 700. The resulting property improvements included large increases in ultimate and yield strength, creep resistance, and low and high cycle fatigue resistance. These were achieved with minimal loss in ductility. The mechanical properties characterized were intended to be broad in scope and no exhaustive attempt was made to optimize processing in order to achieve maximum improvement. However, the experimental results confirm the assessment that nickel-base superalloys can be effectively strengthened through controlled thermomechanical processing.     

Microstructure and mechanical properties of Ti-40 Wt Pct Ta (Ti-15 At. Pct Ta)

Of the β-isomorphous Ti-X alloy systems, Ti-Ta is one of the least studied. In the current work, the microstructure and mechanical properties of Ti-40 wt pct Ta (Ti-15 at. pct Ta) are investigated. Annealing at 810 °C produces a two-phase microstructure consisting of Ti-richa idiomorphs in a continuous Ta-rich β matrix; this suggests the β-transus temperature is higher than indicated by the most recently published phase diagram. Water quenching from 810 °C causes the β phase to partially transform to orthorhombic martensite (α), while furnace cooling yields secondarya The primary α formed isothermally remains unchanged in both cases. Subsequent aging causes transformation of the martensite to type 1a plus residual β, with a corresponding increase in strength and decrease in ductility. The maximum ductility (20 pct elongation) occurs in the water-quenched condition in which metastable β is retained. Analysis of the true stresstrue strain data suggests that transformation-induced plasticity may contribute to the enhanced ductility of the water-quenched material.     

A phenomenological study of the shape memory effect in polycrystalline uranium-niobium alloys

When uranium-niobium alloys containing between 13.9 and 17.9 at. pct Nb are quenched to room temperature from the BCC (γ) phase at elevated temperatures, diffusion-controlled precipitation of the equilibrium phases is prevented and martensitic transformations to transition phases occur instead. Dilatometry was used to detect transformation temperatures and with the help of X-ray diffraction analysis, a metastable phase diagram was established. At room temperature after quenching, alloys containing < 15.2 at. pct Nb were monoclinic (α″) and those with < 16.6 at. pct Nb were tetragonal (γ°). The deformation behavior and shape memory effects (SME) accompanying the reverse martensitic phase transformations in polycristalline specimens were surveyed and characterized phenomelogically. From uniaxial tensile tests at room temperature, macroscopic stress-strain parameters, associated with the reversible deformation modes in the α″ and γ° martensites, were defined and their composition and structural state dependencies delineated. A diffuse maximum manifested in the stress-strain diagrams was identified with the reversible strain limit, which varied inversely and continuously with composition. A concentration-independent value of 693 MPa was found for the plastic yield strength of the alloys. All the alloys exhibited heat-activated shape recovery but the degree depended on structural state and composition. The α″ alloys showed a much larger effect than γ° alloys. Shape recovery occurred in two stages in all alloys. The first stage of recovery accompanied martensite reversion but final reversion to the equilibrium y phase was not accomplished until much higher temperatures were reached. Rapid, low temperature aging reactions were thought to affect the finish of shape recovery and delay it to higher temperatures. Formerly with Metals and Ceramics Division, Oak Ridge National Laboratory