High Strength and High Ductility of Ultrafine-Grained,Interstitial-Free Steel Produced by ECAE and Annealing

Interstitial-free steel (IF steel) underwent severe plastic deformation by equal-channel angular extrusion/pressing (ECAE/P) to improve its strength, and then it was annealed to achieve a good strength-ductility balance. The coarse-grained microstructure of IF steel was refined down to the submicron level after eight-pass ECAE. The ultrafine-grained (UFG) microstructure with high dislocation density brought about substantially improved strength but limited tensile ductility. The limited ductility was attributed to the small, uniform elongation caused by early plastic instability. The annealing at temperatures below 723 K (450 °C) for 1 hour did not lead to remarkable softening, whereas annealing at temperatures up to 923 K (650 °C) resulted in complete softening depending on the development of recrystallization. Therefore, the temperature of approximately 923 K (650 °C) can be considered as a critical recrystallization temperature for UFG IF steel. The annealing at 873 K (600 °C) for different time intervals resulted in different stress–strain response. Uniform tensile elongation increased at the expense of strength with annealing time intervals. After annealing at 873 K (600 °C) for 60 minutes, the yield strength, tensile strength, uniform elongation, and total elongation were found to be 320 MPa, 485 MPa, 15.1 pct, and 33.7 pct, respectively, showing the better combination of strength and ductility compared with cold-rolled samples.     

Nanograined/Ultrafine-Grained Structure and Tensile Deformation Behavior of Shear Phase Reversion-Induced 301 Austenitic Stainless Steel

We describe here an electron microscopy study of shear reversion-induced nanograined/ultrafine-grained (NG/UFG) structure and evolution of tensile strained microstructure in metastable type 301 austenitic stainless steel. The NG/UFG structure with grain size in the range of 200 to 500 nm was obtained by severe cold deformation and controlled annealing in the narrow temperature range of 973 to 1073 K (700 to 800 °C). The different stages of annealing involve the following: (a) transformation of strain-induced martensite to highly dislocated lath-type austenite, (b) formation of dislocation-cell structure and transformation to recovered austenite structure with defect-free subgrains, and (c) coalescence of subgrains to form a NG/UFG structure concomitant with a completely recrystallized structure, and consistent with martensitic shear-type phase reversion mechanism. The optimized cold working and annealing treatment resulted in NG/UFG material with a high yield strength (~1000 MPa) and high ductility (~30 pct) combination. Multiple deformation mechanisms were identified from postmortem electron microscopy examination of tensile strained NG/UFG 301 austenitic stainless steel and include dislocation glide and twinning. The evidence of heterogeneous nucleation of overlapping stacking faults and partial dislocations points toward deformation     

Microstructure and Mechanical Properties of Oxide-Dispersion Strengthened Al6063 Alloy with Ultra-Fine Grain Structure

The microstructure and mechanical properties of the ultra-fine grained (UFG) Al6063 alloy reinforced with nanometric aluminum oxide nanoparticles (25 nm) were investigated and compared with the coarse-grained (CG) Al6063 alloy (~2 μm). The UFG materials were prepared by mechanical alloying (MA) under high-purity Ar and Ar-5 vol pct O₂ atmospheres followed by hot powder extrusion (HPE). The CG alloy was produced by HPE of the gas-atomized Al6063 powder without applying MA. Electron backscatter diffraction under scanning electron microscopy together with transmission electron microscopy studies revealed that the microstructure of the milled powders after HPE consisted of ultra-fine grains (>100 nm) surrounded by nanostructured grains (<100 nm), revealing the formation of a bimodal grain structure. The grain size distribution was in the range of 20 to 850 nm with an average of 360 and 300 nm for Ar and Ar-5 pct O₂ atmospheres, respectively. The amount of oxide particles formed by reactive mechanical alloying under the Ar/O₂ atmosphere was ~0.8 vol pct, whereas the particles were almost uniformly distributed throughout the aluminum matrix. The UFG materials exhibited significant improvement in the hardness and yield strength with an absence of strain hardening behavior compared with CG material. The fracture surfaces showed a ductile fracture mode for both CG and UFG Al6063, in which the dimple size was related to the grain structure. A mixture of ductile–brittle fracture mode was observed for the UFG alloy containing 0.8 vol pct Al₂O₃ particles. The tensile behavior was described based on the formation of nonequilibrium grain boundaries with high internal stress and dislocation-based models.     

Effect of Annealing on Thermal Stability, Precipitate Evolution, and Mechanical Properties of Cryorolled Al 7075 Alloy

The effect of annealing on microstructural stability, precipitate evolution, and mechanical properties of cryorolled (CR) Al 7075 alloy was investigated in the present work employing hardness measurements, tensile test, X-ray diffraction (XRD), differential scanning calorimetry (DSC), electron backscattered diffraction (EBSD), and transmission electron microscopy (TEM). The solution-treated bulk Al 7075 alloy was subjected to cryorolling to produce fine grain structures and, subsequently, annealing treatment to investigate its thermal stability. The recrystallization of CR Al 7075 alloys started at an annealing temperature of 423 K (150 °C) and completed at an annealing temperature of 523 K (250 °C). The CR Al 7075 alloys with ultrafine-grained microstructure are thermally stable up to 623 K (350 °C). Within the range of 523 K to 623 K (250 °C to 350 °C), the size of small η phase particles and AlZr₃ dispersoids lies within 300 nm. These small precipitate particles pin the grain boundaries due to the Zener pinning effect, which suppresses grain growth. The hardness and tensile strength of the CR Al 7075 alloys was reduced during the annealing treatment from 423 K to 523 K (150 °C to 250 °C) and subsequently it remains constant.     

Achieving High Strength and High Ductility in Friction Stir-Processed Cast Magnesium Alloy

Friction stir processing (FSP) is emerging as an effective tool for microstructural modification and property enhancement. As-cast AZ91 magnesium alloy was friction stir processed with one-pass and two-pass to examine the influence of processing conditions on microstructural evolution and corresponding mechanical properties. Grain refinement accompanied with development of strong basal texture was observed for both processing conditions. Ultrafine-grained (UFG) AZ91 was achieved under two-pass FSP with fine precipitates distributed on the grain boundary. The processed UFG AZ91 exhibited a high tensile strength of ~435 MPa (117 pct improvement) and tensile fracture elongation of ~23 pct. The promising combination of strength and ductility is attributed to the elimination of casting porosity, and high density of fine precipitates in an UFG structure with quite low dislocation density. The effects of grain size, precipitate, and texture on deformation behavior have been discussed.     

Improving the mechanical properties of 2024 Al alloy by cryo rolling

Ultra fine grained (1 micron size) materials usually exhibit more strength. Most of the approaches to refine microstructure lead to decrease in ductility. Cryo rolling is a successful technique; samples are rolled at cryogenic temperature, to improve strength of an age hardenable alloy with minimum loss in ductility. Aging after cryo rolling ensures good strength and ductility due to bimodal structure and nano sized precipitation of S’ phase. Al 2024 alloy are partially solutionised to retain some T-phase particles, which are very effective in accumulating dislocations during cryo-rolling, and in turn promoted the precipitation of Al₂CuMg precipitates with a size of 10–40 nm. The nano sized Al₂CuMg precipitates and bimodal grain structure leads to simultaneous increases in strength and ductility.     

Influence of Process Parameters on the Mechanical Behavior of an Ultrafine-Grained Al Alloy

Aluminum alloys with nanocrystalline (NC) and ultrafine grain (UFG) size are of interest because of their strengths that are typically 30 pct greater than conventionally processed alloys of the same composition. In this study, UFG AA 5083 plate was prepared by quasi-isostatic (QI) forging of cryomilled powder, and the microstructure and mechanical behavior was investigated and compared with the behavior of coarse-grained AA 5083. Forging parameters were adjusted in an effort to strengthen the UFG material while retaining some tensile ductility. Different forging parameters were employed on three plates, with approximate dimensions of 254 mm diameter and 19 mm thickness. The overarching goal of the current effort was to increase strength through minimized grain growth during processing while maintaining ductility by breaking up prior particle boundaries (PPBs) with high forging pressures. Mechanical tests revealed that strength increased inversely with grain size, whereas ductility for some of the experimental materials was preserved at the level of the conventional alloy. The application of the Hall-Petch relationship to the materials was studied and is discussed in detail with consideration given to strengthening mechanisms other than grain size, including dispersion (Orowan), solid solution, and dislocation strengthening.     

A comparative study on mechanical properties of ultrafine-grained Al 6061 and Al 6063 alloys processed by cryorolling

The present work has been focused to investigate the mechanical behavior and microstructural characteristics of cryorolled Al 6063 and Al 6061 alloys. Hardness and tensile tests of the cryorolled Al alloys were carried out to understand its deformation behavior. SEM/EBSD was used to characterise the microstructures of cryorolled Al alloys and observed the formation of ultrafine-grained microstructures in the materials due to severe plastic strain induced during cryorolling. XRD was used to analyse the formation of different phases during cryorolling of the Al alloys. It is evident from the present study that UFG Al alloys exhibit higher hardness and strength when compared to the bulk Al alloys due to the grain size, higher dislocation density and precipitation hardening effect. The cryorolled Al 6061 alloys exhibit higher tensile strength (346 MPa) and hardness (120 H_v) as compared to Al 6063 alloys (Tensile strength: 240MPa and Hardness: 96.5 H_v) in the present investigation. The deformation mechanisms of UFG Al alloys contributing to their enhanced strength are discussed.     

Characteristics on microstructure and mechanical performances of 6111Al influenced by Ce-containing precipitates

In this work,the effects of Ce addition(0,0.1 wt%,0.3 wt%,0.5 wt%and 0.7 wt%)on the evolution of microstructure and mechanical properties of 6111 Al alloy and strengthening mechanism of 6111 Al-Ce alloy were systematically investigated by a polarizing microscope,a scanning electronic microscope,an energy dispersive spectroscope and a high-resolution transmission electron microscope.The results indicate that with 0.3 wt%Ce addition,theα-Al grains show the equiaxed crystal morphology with the average size decreasing from 137 to 57μm and numerous small AlCeSi phases with lump-like or platelike morphology are distributed closely along the grain boundary.The peak yield strength,ultimate tensile strength and elongation of 6111 Al-Ce alloy reach to 279 MPa,316 MPa and 12.1%,respectively,which is attributed to the grain refinement strengthening and the formation of nanosized Al11Ce3 precipitates.Eventually,this investigation gives us instructive suggestion to prepare the new kind of aluminum alloy with high strength and high ductility.     

Influence of Al on the Microstructural Evolution and Mechanical Behavior of Low-Carbon,Manganese Transformation-Induced-Plasticity Steel

Microstructural design with an Al addition is suggested for low-carbon, manganese transformation-induced-plasticity (Mn TRIP) steel for application in the continuous-annealing process. With an Al content of 1 mass pct, the competition between the recrystallization of the cold-rolled microstructure and the austenite formation cannot be avoided during intercritical annealing, and the recrystallization of the deformed matrix does not proceed effectively. The addition of 3 mass pct Al, however, allows nearly complete recrystallization of the deformed microstructure by providing a dual-phase cold-rolled structure consisting of ferrite and martensite and by suppressing excessive austenite formation at a higher annealing temperature. An optimized annealing condition results in the room-temperature stability of the intercritical austenite in Mn TRIP steel containing 3 mass pct Al, permitting persistent transformation to martensite during tensile deformation. The alloy presents an excellent strength-ductility balance combining a tensile strength of approximately 1 GPa with a total elongation over 25 pct, which is comparable to that of Mn TRIP steel subjected to batch-type annealing.     

The effect of heat treatment on the properties and structure of molybdenum and vanadium dual-phase steels

A systematic study was made of the effect of the heat treating parameters,(i.e., temperature, time, and cooling rate) on the properties and structure of molybdenum and vanadium bearing dual-phase steels. The volume percent martensite was found to be the major structural factor that controls the strength and ductility of these steels. The relationship between strength and ductility was independent of alloy addition for the alloys studied. Annealing temperature was shown to be very important in these alloys, especially at high quench rates. The molybdenum alloy exhibited better hardenability than the vanadium alloy for equivalent heat treating conditions. Therefore, for a given set of annealing conditions the molybdenum alloy generally had the highest tensile strength and lowest total elongation. A minimum in the 0.2 pct yield strength was found at a specific volume fraction martensite. The increase in yield strength at the lowest volume fraction studied can be related to a jog or discontinuity in the stress-strain curve during tensile testing. This jog was found to be the result of the lack of a sufficient amount of free dislocations. The causes of this deficiency of the dislocations may be: 1) an insufficient amount of transformed martensite, 2) a large martensite interparticle spacing, 3) dynamic recovery of dislocations during cooling, and 4) pinning of dislocations by precipitates during cooling.     

Impact of Aging Temperature on the Performance of a Nickel-Iron-Based Superalloy

In this study, the effects of aging temperature on the microstructure and properties of a nickel-iron (Ni-Fe)-based superalloy were investigated. On the one hand, owing to the increase in the size and particle spacing of Ni3Al (γ′) precipitate, long-term aging induced a significant drop in the alloy strength. Moreover, the increasing aging temperature from 700 °C to 750 °C further induced more than 75 MPa decline in the alloy yield strength. Furthermore, it led to a decrease in the critical stress because of dynamic recrystallization. On the other hand, the long-term aging increased the alloy’s ductility. The crack propagation along the grain boundary was inhibited, because of the decreasing grain boundary brittleness. Although the grain boundary precipitates changed from carbide to γ′ when the aging temperature increased, a distinct change in the alloy’s ductility was not observed. The transmission electron microscopy results showed that both precipitates were sheared by the grain boundary during the alloy deformation. These results confirm that aging temperature has less effect on alloy’s ductility.     

The effect of sheet processing on the microstructure, tensile, and creep behavior of INCONEL alloy 718

The grain size, grain boundary character distribution (GBCD), creep, and tensile behavior of INCONEL alloy 718 (IN 718) were characterized to identify processing-microstructure-property relationships. The alloy was sequentially cold rolled (CR) to 0, 10, 20, 30, 40, 60, and 80 pct followed by annealing at temperatures between 954 °C and 1050 °C and the traditional aging schedule used for this alloy. In addition, this alloy can be superplastically formed (IN 718SPF) to a significantly finer grain size and the corresponding microstructure and mechanical behavior were evaluated. The creep behavior was evaluated in the applied stress (σ _a ) range of 300 to 758 MPa and the temperature range of 638 °C to 670 °C. Constant-load tensile creep experiments were used to measure the values of the steady-state creep rate and the consecutive load reduction method was used to determine the values of backstress (σ₀). The values for the effective stress exponent and activation energy suggested that the transition between the rate-controlling creep mechanisms was dependent on effective stresses (σ _e =σ _a σ₀) and the transition occurred at σ _e ≅ 135 MPa. The 10 to 40 pct CR samples exhibited the greatest 650 °C strength, while IN 718SPF exhibited the greatest room-temperature (RT) tensile strength (>1550 MPa) and ductility (ε _f >16 pct). After the 954 °C annealing treatment, the 20 pct CR and 30 pct CR microstructures exhibited the most attractive combination of elevated-temperature tensile and creep strength, while the most severely cold-rolled materials exhibited the poorest elevated-temperature properties. After the 1050 °C annealing treatment, the IN 718SPF material exhibited the greatest backstress and best creep resistance. Electron backscattered diffraction was performed to identify the GBCD as a function of CR and annealing. The data indicated that annealing above 1010 °C increased the grain size and resulted in a greater fraction of twin boundaries, which in turn increased the fraction of coincident site lattice boundaries. This result is discussed in light of the potential to grain boundary engineer this alloy. INCONEL is a registered trademark of Special Metals Corp., Huntington, WV. This article is based on a presentation made in the symposium entitled “Processing and Properties of Structural Materials,” which occurred during the Fall TMS meeting in Chicago, Illinois, November 9–12, 2003, under the auspices of the Structural Materials Committee.     

Modeling of the Precipitation Kinetics During Aging a Predeformed Fe-Cu Alloy

A model is developed to describe the microstructure evolution during aging a Fe-Cu alloy. The precipitation process during aging a predeformed Fe-1.5 wt pct Cu is calculated and the kinetic details of aging are discussed. The model is satisfactorily tested by comparing with the reported experimental results. The numerical results demonstrate that the nucleation of the precipitates occurs on dislocations first. However, the maximum nucleation rate of the precipitates in the matrix is much higher than that on dislocations and it is not affected by the appearance of dislocations. Only a small fraction of precipitates locate on dislocations immediately after nucleation. The particles in the matrix dissolve preferentially during coarsening, and most of the remained precipitates locate on dislocations in an overaged sample. Dislocations have little effect on the number density and average radius of the precipitates in the late stage of aging, although they promote the beginning of precipitation in the early stage of aging. The heating rate of the specimen almost does not affect the summit of the particle number density as well as its corresponding mean particle radius.     

Thermal stability in bulk cryomilled ultrafine-grained 5083 Al alloy

Thermal stability in bulk ultrafine-grained (UFG) 5083 Al that was processed by gas atomization followed by cryomilling, consolidation, and extrusion, and that exhibited an average grain size of 305 nm, was investigated in the temperature range of 473 to 673 K (0.55 to 0.79 T _m , where T _m is the melting temperature of the material) for different annealing times. Appreciable grain growth was observed at temperatures > 573 K, whereas there was limited grain growth at temperatures < 573 K even after long annealing times. The values of the grain growth exponent, n, deduced from the grain growth data were higher than the value of 2 predicted from elementary grain growth theories. The discrepancy was attributed to the operation of strong pinning forces on boundaries during the annealing treatment. An examination of the microstructure of the alloy suggests that the origin of the pinning forces is most likely related to the presence of dispersion particles, which are mostly introduced during cryomilling. Two-grain growth regimes were identified: the low-temperature region (<573 K) and the high-temperature region (>573 K). For temperatures lower than 573 K, the activation energy of 25 ± 5 kJ/mol was determined. It is suggested that this low activation energy represents the energy for the reordering of grain boundaries in the UFG material. For temperatures higher than 573 K, an activation energy of 124 ± 5 kJ/mol was measured. This value of activation energy, 124 ± 5 kJ/mol, lies between that for grain boundary diffusion and lattice diffusion in analogous aluminum polycrystalline systems. The results show that the strength and ductility of bulk UFG 5083 Al, as obtained from tensile tests, correlate well with substructural changes introduced in the alloy by the annealing treatment.     

On the Significance of Nature of Strain-Induced Martensite on Phase-Reversion-Induced Nanograined/Ultrafine-Grained Austenitic Stainless Steel

We recently described the reversal of strain-induced martensite to the parent austenite phase in the attempt to produce nanograins/ultrafine grains via controlled annealing of heavily cold-worked metastable austenite. The phase-reversion-induced microstructure consisted of nanocrystalline (d < 100 nm), ultrafine (d ≈ 100 to 500 nm), and submicron (d ≈ 500 to 1000 nm) grains and was characterized by high strength (800 to 1000 MPa)–high ductility (30 to 40 pct) combination, which was a function of cold deformation and temperature-time annealing sequence.[1] In this article, we demonstrate that the success of the approach in obtaining nanograined/ultrafine-grained (NG/UFG) structure depends on the predominance of dislocation-cell–type structure in the severely deformed martensite. Electron microscopy and selected area electron diffraction analysis indicated that with an increase in the degree of cold deformation there is transformation of lath martensite to dislocation-cell–type martensite, which is a necessary prerequisite to obtain phase-reversion-induced NG/UFG austenite. The transformation of lath-type to dislocation-cell–type martensite involves refinement of packet and lath size and break up of lath structure. Based on detailed and systematic electron microscopy study of cold-deformed metastable austenite (~45 to 80 pct deformation) and constant temperature-time annealing sequence, when the phase reversion kinetics is rapid, our hypothesis is that the maximization of dislocation-cell–type structure in lieu of lath-type facilitates NG/UFG structure with a high strength–high ductility combination. Interestingly, the yield strength follows the Hall–Petch relation in the NG/UFG regime for the investigated austenitic stainless steel.     

Characterization of retrogression and reaging behavior of 8090 Al-Li-Cu-Mg-Zr alloy

An 8090 Al-Li-Cu-Mg-Zr alloy in the peak-aged (T8) temper was subjected to retrogression treatment at temperatures above and below the δ′ (Al₃Li) solvus line and immediately reaged to various tempers. Retrogression and reaging (RRA) behavior is characterized by hardness testing, tensile testing, transmission electron microscopy (TEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and electrochemical polarization studies. Retrogression of the T8 temper alloy causes dissolution primarily of δ′ (Al₃Li) precipitates into solid solution that results in a decrease of hardness and tensile strength and an increase of ductility of the alloy. Reaging of the retrogressed state causes reprecipitation of the δ′ precipitates in the matrix resulting in the restoration of strength and ductility properties. Retrogression and reaging to the peak-aged temper, designated at T77 temper, has been found to retain the strength of the conventional T8 temper, but with the gross aging time in the RRA temper almost twice that of the conventional T8 temper, the microstructure of the RRA temper approaches that of the overaged (T7) temper. Thus, RRA treatment contributes to an improvement of stress corrosion cracking (SCC) resistance over the conventional T8 temper while retaining the mechanical properties of T8 temper.     

Influence of Aging and Thermomechanical Treatments on the Mechanical Properties of a Nanocluster-Strengthened Ferritic Steel

This study investigated the effect of aging and thermomechanical treatments on the mechanical properties of a nanocluster-strengthened ferritic steel, Fe-1.5Mn-2.5Cu-4.0Ni-1.0Al (wt pct). The effect of thermomechanical treatments on the microhardness and tensile properties were measured at room temperature and correlated with microstructural features. Cu-rich precipitates were characterized by transmission electron microscopy and were found to coarsen slowly during long-time aging. The microhardness measurements indicate a typical precipitation hardening behavior during aging at 773 K (500 °C). Tensile tests showed that thermomechanical treatments can improve the mechanical strength and ductility of the nanocluster-strengthened ferritic steel significantly compared with those without the treatments. Fractography results indicated that the high yield strength resulted from precipitation hardening makes the steel more susceptible to grain-boundary decohesion, which can be suppressed by grain refinement. Atmosphere adsorption and diffusion along grain boundaries were found to intensify brittle intergranular fracture, and this embrittlement can be avoided by vacuum heat treatment.     

Microstructure and Mechanical Properties of a High-Strength Ti-4Al-2Fe-3Cu Alloy Fabricated by Sintering and Hot Extrusion

A Ti-4Al-2Fe-3Cu (wt pct) alloy containing only low-cost alloying elements was fabricated by vacuum sintering a blend of TiH₂, Al, Fe, and Cu powders at 1200 °C for 1 hour followed by hot extrusion at the same temperature. The as-extruded alloy exhibited a microstructure consisting of mainly α/β lamellar colonies and Ti₂Cu as a minor phase. The average colony size and lamella thickness were 118 and 12 µm, respectively, and Fe and Cu were predominantly distributed in the β lamellae. The as-extruded alloy had a high tensile yield strength (YS) and ultimate tensile strength (UTS) of 1248 and 1270 MPa, respectively, but a limited ductility (elongation to fracture: 2.3 pct). Annealing at 750 °C for 4 hour caused the average colony size and lamella thickness of the alloy to increase to 145 and 17 µm, respectively, and the volume fraction of the β phase decreased with the annealing. These microstructural changes resulted in a slight decrease of the YS and UTS to 1221 and 1253 MPa, but a clear increase of the ductility with the elongation to fracture reaching 4 pct. This work demonstrates that a combination of relatively low-temperature vacuum sintering, hot extrusion, and annealing can be effectively utilized to fabricate a low-cost Ti-4Al-2Fe-3Cu alloy with high strength and appreciable tensile ductility.

     

Mechanical Behaviors of Ultrafine-Grained 301 Austenitic Stainless Steel Produced by Equal-Channel Angular Pressing

The technique of equal-channel angular pressing (ECAP) was used to refine the microstructure of an AISI 301 austenitic stainless steel (SS). An ultrafine-grained (UFG) microstructure consisting mainly of austenite and a few martensite was achieved in 301 steel after ECAP processing for four passes at 523 K (250 °C). By submitting the as-ECAP rods to annealing treatment in the temperature range from 853 K to 893 K (580 °C to 620 °C) for 60 minutes, fully austenitic microstructures with grain sizes of 210 to 310 nm were obtained. The uniaxial tensile tests indicated that UFG 301 austenitic SS had an excellent combination of high yield strength (>1.0 GPa) and high elongation-to-fracture (>30 pct). The tensile stress–strain curves exhibited distinct yielding peak followed by obvious Lüders deformation. Measurements showed that Lüders elongation increased with an increase in strength as well as a decrease in grain size. The microstructural changes in ultrafine austenite grains during tensile deformation were tracked by X-ray diffraction and transmission electron microscope. It was found that the strain-induced phase transformation from austenite to martensite took place soon after plastic deformation. The transformation rate with strain and the maximum strain-induced martensite were promoted significantly by ultrafine austenite grains. The enhanced martensitic transformation provided extra strain-hardening ability to sustain the propagation of Lüders bands and large uniform plastic deformation. During tensile deformation, the Lüders bands and martensitic transformation interacted with each other and made great contribution to the excellent mechanical properties in UFG austenitic SS.