Quantitative Analysis of Microstructural Constituents in Welded Transformation-Induced-Plasticity Steels

A quantitative analysis of retained austenite and nonmetallic inclusions in gas tungsten arc (GTA)–welded aluminum-containing transformation-induced-plasticity (TRIP) steels is presented. The amount of retained austenite in the heat-affected and fusion zones of welded aluminum-containing TRIP steel with different base metal austenite fractions has been measured by magnetic saturation measurements, to study the effect of weld thermal cycles on the stabilization of austenite. It is found that for base metals containing 3 to 14 pct of austenite, 4 to 13 pct of austenite is found in the heat-affected zones and 6 to 10 pct in the fusion zones. The decomposition kinetics of retained austenite in the base metal and welded samples was also studied by thermomagnetic measurements. The decomposition kinetics of the austenite in the fusion zone is found to be slower compared to that in the base metal. Thermomagnetic measurements indicated the formation of ferromagnetic ε carbides above 290 °C and paramagnetic η(ε′) transient iron carbides at approximately 400 °C due to the decomposition of austenite during heating.     

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     

Microstructural dependence of Fe-high Mn tensile behavior

The tensile properties of Fe-high Mn (16 to 36 wt pct Mn) binary alloys were examined in detail at temperatures from 77 to 553 K. The Mn content dependence of the deformation and fracture behavior in this alloy system has been clarified by placing special emphasis on the starting microstructure and its change during deformation. In general, the intrusion of hcp epsilon martensite (ε) into austenite (γ) significantly increases the work hardening rate in these alloys by creating strong barriers to further plastic flow. Due to the resulting high work hardening rates, large amounts of e lead to high flow stresses and low ductility. Alloys of 16 to 20 wt pct Mn are of particular interest. While these alloys are thermally stable with respect to bcc α’ martensite formation, 16 to 20 wt pct Mn alloys undergo a deformation induced ε →α’ transformation. The martensitic transformation plays two contrasting roles. The stress-induced ε→ α’ transformation decreases the initial work hardening rate by reducing locally high internal stress. However, the work hardening rate increases as the accumulated α’ laths become obstacles against succeeding plastic flow. These rather complicated microstructural effects result in a stress-strain curve of anomolous shape. Since both the M_s and M_d temperatures for both the ε and α’-martensite transformations are strongly dependent on the Mn content, characteristic relationships between the tensile behavior and the Mn content of each alloy are observed.     

Nonequilibrium phases in rapidly quenched Fe-Al-C ternary alloys

Nonequilibrium phases of austenite(Y), ordered austenite (γ′) and hcp epsilon (ε) have been found in Fe-Al-C ternary alloys quenched rapidly from the melt. The formation ranges of these single phases are 2 to 6 pct Al and 1.8 to 2.1 pct C for the 7 phase, 6 to 12 pct Al and 1.7 to 2.1 pct C for the γ′ phase and 2 to 5 pct Al and above 4 pct C for the e phase. The lattice parameter varies from 0.361 to 0.365 nm for the γ phase and from 0.361 to 0.367 nm for the γ′ phase with increasing carbon and aluminum contents and is abouta = 0.264 nm andc = 0.434 nm for the e phase. Among these non-equilibrium phases, the austenite is so ductile that no crack is observed even after closely contacted bending test. The austenite phase has fine subgrains of 0.1 to 0.4 μm diam and the Vickers hardness, yield strength and tensile fracture strength are about 360 DPN, 940 MPa and 995 MPa, respectively, for Fe-4.0 pct Al-2.0 pct C alloy. Thus, due to relatively high hardness and strength combined with good ductility, the nonequilibrium austenite found in Fe-Al-C system is attractive as high-strength materials whose useful dimensions may be limited by critical rapid cooling rates. Formerly Graduate Student of Tohoku University, Formerly Graduate Student of Tohoku University     

Metallography of bainitic transformation in silicon containing steels

The formation of carbide in lower bainite was studied in two silicon containing carbon steels by transmission electron microscopy and diffraction techniques. Epsilon carbide was identified in the low temperature isothermally transformed bainite structure. The crystallographic relationship between epsilon carbide and bainitic ferrite was found to follow the Jack orientation relationship,viz, (0001)ε l l(011)α, (101l)ε l 1(101)α. The cementite observed in lower bainite was in the shape of small platelets and obeyed the Isaichev orientation relationship with the bainitic ferrite,viz, (010) cl 1(1-11)α, (103) cl 1 (011)α. Direct evidence showing the sequence of carbide formation from aus-tenite in bainite has also been obtained. Based on the observations and all the crystallo-graphical features, it is strongly suggested that in silicon containing steels the bainitic carbide precipitated directly from austenite instead of from ferrite at the austenite/fer-rite interface as has been proposed by Kinsman and Aaronson (Ref. 1). The uniformity of the carbide distribution is thus envisaged to be the outcome of precipitation at the aus-tenite-ferrite interphase boundary. DER-HUNG HUANG, formerly with the Department of Materials Science and Mineral Engineering, University of California     

Kinetics of strain-induced fcc→hcp martensitic transformation

The kinetics of the strain-induced γ (fcc)→ε (hcp) transformationi.e. the amount of phase transformationvs applied strain were determined by density measurements at various temperatures. The transformation curve has a sigmoidal shape and approaches saturation below 100 pct transformation. Assuming that ε-platelets form from stacking faults, the volume fraction can be expressed as an implicit function of strain. The saturation value is constant and can be evaluated from quantitative metallography. The approach to saturation is determined by only one temperature-dependent parameter related to the stacking fault energy. Good agreement with experimental results was obtained. The model was also applied to transformation kinetics after a prestrain inducing both slip and twinning. The prestrain stabilizes austenite with respect to the strain-induced transformation through a block-refining of austenite by the substructure. In addition the nucleation is enhanced through the introduction of stacking faults. This effect vanishes at high applied strains but causes the shape of the transformation curve to become parabolic. It is concluded that decreasing the size of the ε platelets provides a simple means for reducing the temperature dependence of the transformation kinetics.     

Tempering of Iron-Carbon-Nitrogen martensites

The tempering behavior of ternary FeCN martensitic specimens, with a total amount of interstitials of about 5.5 at. pct and carbon and nitrogen contents between about 1.5 and 3.9 at. pct, was investigated in the temperature range 110 to 830 K. Analysis of the corresponding changes in crystalline structure (X-ray diffraction), volume (dilatometry), and hardness and enthalpy (calorimetry) revealed that the following processes occurred: (a) martensitic transformation of retained austenite between 110 and 200 K; (b) redistribution of interstitials in martensite up to 370 K; (c) formation of nitrogen containing α′ precipitates and carbon containing ε/η) precipitates between 370 and 450 K; (d) conversion of α′ nitride into γ′ nitride and coarsening of ε/η carbide between 450 and 560 K; (e) decomposition of retained austenite above 540 K; and (f) conversion of ε/η carbide into cementite above 570 K. Significant precipitation of carbon and nitrogen together, as nitrocarbides or carbonitrides, was not observed. From a comparison with the tempering behavior of binary FeC and FeN alloys of similar interstitial content, it was concluded for the ternary FeCN alloy that the transformation of the transition nitride (α′) into the “equilibrium” nitride (γ′) was advanced and that the precipitation of the transition carbide(ε/η) and its conversion into the equilibrium carbide (cementite) and the decomposition of retained austenite were retarded. Formerly Graduate Student, Laboratory of Metallurgy, Delft University of Technology.     

Role of Si in Improving the Shape Recovery of FeMnSiCrNi Shape Memory Alloys

The effect of Si addition on the microstructure and shape recovery of FeMnSiCrNi shape memory alloys has been studied. The microstructural observations revealed that in these alloys the microstructure remains single-phase austenite (γ) up to 6 pct Si and, beyond that, becomes two-phase γ + δ ferrite. The Fe₅Ni₃Si₂ type intermetallic phase starts appearing in the microstructure after 7 pct Si and makes these alloys brittle. Silicon addition does not affect the transformation temperature and mechanical properties of the γ phase until 6 pct, though the amount of shape recovery is observed to increase monotonically. Alloys having more than 6 pct Si show poor recovery due to the formation of δ-ferrite. The shape memory effect (SME) in these alloys is essentially due to the γ to stress-induced ε martensite transformation, and the extent of recovery is proportional to the amount of stress-induced ε martensite. Alloys containing less than 4 pct and more than 6 pct Si exhibit poor recovery due to the formation of stress-induced α′ martensite through γ-ε-α′ transformation and the large volume fraction of δ-ferrite, respectively. Silicon addition decreases the stacking fault energy (SFE) and the shear modulus of these alloys and results in easy nucleation of stress-induced ε martensite; consequently, the amount of shape recovery is enhanced. The amount of athermal ε martensite formed during cooling is also observed to decrease with the increase in Si.     

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.     

Driving force for γ→ε martensitic transformation and stacking fault energy of γ in Fe-Mn binary system

A regular solution model for the difference of the chemical free energy between γ and ε phases during γ→ε martensitic transformation in the Fe-Mn binary system has been reexamined and partly modified based on many articles concerning the M _s and A _s temperatures of Fe-Mn alloys. Using the regular solution model, the measured M _s temperatures, and a thermodynamic model for the stacking fault energy (SFE) of austenite (γ), the driving force for γ→ε martensitic transformation, and the SFE of γ have been calculated. The driving force for γ→ε martensitic transformation increases linearly from − 68 to − 120 J/mole with increasing Mn content from 16 to 24 wt pct. The SFE of γ decreases to approximately 13 at. pct Mn and then increases with increasing Mn content, which is in better agreement with Schumann’s result rather than Volosevich et al.’s result.     

Hydrogen-related phase transformations in austenitic stainless steels

The effect of hydrogen and stress (strain) on the stability of the austenite phase in stainless steels was investigated. Hydrogen was introduced by severe cathodic charging and by elevated temperature equilibration with high pressure H₂ gas. Using X-ray diffraction and magnetic techniques, the behavior of two “stable” type AISI310 steels and an “unstable” type AISI304 steel was studied during charging and during the outgassing period following charging. Transformation from the fcc γ phase to an expanded fcc phase, γ*, and to the hcp ε phase occurred during cathodic charging. Reversion of the γ* and e phases to the original γ structure and formation of the bcc α structure were examined, and the kinetics of these processes was studied. The γ* phase was shown to be ferromagnetic with a subambient Curie temperature. The γ⇆ε phase transition was studied after hydrogen charging in high pressure gas, as was the formation of a during outgassing. These results are interpreted as effects of hydrogen and stress (strain) on the stability of the various phases. A proposed psuedo-binary phase diagram for the metal-hydrogen system was proposed to account for the formation of the γ* phase. The relation of these phase changes to hydrogen embrittlement and stress corrosion cracking of stainless steel is discussed.     

Study of microstructure of low-temperature plasma-nitrided AISI 304 stainless steel

The microstructure of the low-temperature plasma-nitrided layer on AISI 304 austenitic stainless steel was studied by transmission electron microscopy (TEM). The results show that the surface of the layer consists of a supersaturated solid solution (γ′_N) based on the γ′-Fe₄N phase whose electron diffraction pattern (EDP) has a strong diffuse scattering effect resulting from supersaturating nitrogen (above 20 at. pct) and 〈110〉 streaks arising from matrix elastic strain due to the formation of paired or clustered Cr-N. The latter is due to the N above the 20 at. pct γ′-Fe₄N-phase value and leads to a lattice parameter that is greater than that of the γ′-Fe₄N phase. The subsurface of the layer is composed of a supersaturated solid solution based on γ-austenite, which is an expanded austenite, γ _N. Its morphology shows the basketweave or “tweedlike” contrast consisting of so-called stacking fault precipitates having twin relationships with the matrix whose EDP shows diffuse scattering streaks with certain directions. The ε martensite transformation was observed in the subsurface of the layer. The increase in stacking faults compared with the original stainless steel and formation of ε martensite in the subsurface of the layer indicate that nitrogen lowers the stacking fault energy of austenite.     

Effects of thermal cycling on the kinetics of the γ → ε martensitic transformation in an Fe-17 wt pct Mn alloy

The thermal cycling of an Fe-17 wt pct Mn alloy between 303 and 573 K was performed to investigate the effects of thermal cycling on the kinetics of the γ → ε martensitic transformation in detail and to explain the previous, contrasting results of the change in the amount of ε martensite at room temperature with thermal cycling. It was observed that the shape of the γ → ε martensitic transformation curve (volume fraction vs temperature) changed gradually from a C to an S curve with an increasing number of thermal cycles. The amount of ε martensite of an Fe-17 wt pct Mn alloy at room temperature increased with thermal cycling, in spite of the decrease in the martensitic start (M _s) temperature. This is due to the increase in transformation kinetics of ε martensite at numerous nucleation sites introduced in the austenite during thermal cycling.     

Experimental investigation of the thermodynamics of the Fe-Ti-C austenite and the solubility of titanium carbide

The thermodynamics of Fe-Ti-C austenite has been investigated in the temperature range of 1273 to 1473 K using a dynamic gas equilibrium technique. Hydrogen-methane mixtures have been used for fixing carbon potentials, and the carbon contents have been determined as dynamic weight changesvia a sensitive Cahn microbalance. The effect of titanium-carbon interaction in austenite has been observed (1) as a minimum in the carbide solubility curve, (2) as increases in carbon content due to titanium additions at constant carbon activity, and (3) as the variation of solubility limit of the carbide with carbon content at high carbon contents. The results on the isoactivity measurements in the ternary Fe-Ti-C austenite have been analyzed using the modified Wagner formalism. The ternary interaction parameter ε _C ^Ti been quantitatively related to the solubility minimum and the relative increase in carbon content at constant carbon activity. The variation of the solubility limit of the carbide with carbon content has been described using an additional term to the classical solubility relationships. This additional term is related to the self-and cross-interaction of carbon. Using the solubility relations, the dissolution free energy of body-centered cubic (bcc) Ti in face-centered cubic (fcc) Fe has also been determined. Very good agreement between the thermodynamic calculations and the experimental results was found. K. BALASUBRAMANIAN, Formerly with McMaster University,     

Critical Strengths for Slip Events in Nanocrystalline Metals: Predictions of Quantized Crystal Plasticity Simulations

This article studies how the monotonic and cyclic stress-strain response of nanocrystalline (NC) metals is affected by the grain-to-grain distribution of critical strengths (τ _c ) for slip events, as well as plastic predeformation (ε _pre ^p ). This is accomplished via finite element simulations that capture large jumps in plastic strain from dislocation slip events—a process referred to as quantized crystal plasticity (QCP).[1] The QCP simulations show that τ _c and ε _pre ^p significantly alter the monotonic and cyclic response at small strain, but only τ _c affects the response at large strain. These features are exploited to systematically infer the τ _c and ε _pre ^p characteristics that best fit experimental data for electrodeposited (ED) NC Ni. Key outcomes are the following: (1) the τ _c distribution is truncated, with an abrupt onset of slip events at a critical stress; (2) ε _pre ^p  = −0.4 pct, signifying precompression; (3) there is reverse slip bias, meaning that reverse slip events are easier than forward events; and (4) highly inhomogeneous residual stress states can be enhanced or reduced by tensile deformation, depending on ε _pre ^p .     

Martensitic Transformation During Cold Rolling Deformation of an Austenitic Fe-26Mn-0.14C Alloy

A high-Mn austenitic steel was deformed in cold rolling to study the martensitic transformation and microstructure using X-ray diffraction and electron backscatter diffraction. Despite heavy deformation of 70 pct reduction (1.2 true strain), α′-martensite could not be induced in this alloy, but about 90 pct of the austenite transformed to ε-martensite. However, a small fraction (~4 pct) of α′-martensite could be observed when the same alloy was subjected to low strain compression tests in a Gleeble simulator. The stability of ε-martensite was attributed to the increase in stacking fault energy of the steel, expected to be more than 20 mJ/m² because of the increase in temperature during the cold rolling deformation.     

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.     

The effect of predeformation and heat treatment on microstructure of Al-7.0% Si-0.45% Mg alloy cast through SIMA route

The effect of warm working, hot working and heat treatment conditions on microstructure of Al-7.0%Si-0.45%Mg alloys were investigated in strain induced melt activation (SIMA) process. Predeformation of 30%, 40% and 50% was done by hot working. The hot working has been carried out at 380°C. The samples of various deformations were kept at 580°C, 590°C, 600°C with varied soaking time for 10, 20, 30 min respectively. It was found that increased predeformation reduced the soaking time to obtain globular α Al grains. The shape factor and metallography were done on SIMA processed Al-7.0%Si-0.45%Mg alloys. Those were compared with as cast Al-7.0%Si-0.45%Mg alloy and it was observed that strain induced predeformation and subsequently melt activation has caused the globular microstructure of the alloy.