Influence of annealing treatment on the formation of nano/submicron grain size AISI 301 Austenitic stainless steels

Nano/submicron austenitic stainless steels have attracted increasing attention over the past few years due to fine structural control for tailoring engineering properties. At the nano/submicron grain scales, grain boundary strengthening can be significant, while ductility remains attractive. To achieve a nano/submicron grain size, metastable austenitic stainless steels are heavily cold-worked, and annealed to convert the deformation-induced martensite formed during cold rolling into austenite. The amount of reverted austenite is a function of annealing temperature. In this work, an AISI 301 metastable austenitic stainless steel is 90 pct cold-rolled and subsequently annealed at temperatures varying from 600 °C to 900 °C for a dwelling time of 30 minutes. The effects of annealing on the microstructure, average austenite grain size, martensite-to-austenite ratio, and carbide formation are determined. Analysis of the as-cold-rolled microstructure reveals that a 90 pct cold reduction produces a combination of lath type and dislocation cell-type martensitic structure. For the annealed samples, the average austenite grain size increases from 0.28 μm at 600 °C to 5.85 μm at 900 °C. On the other hand, the amount of reverted austenite exhibits a maximum at 750 °C, where austenite grains with an average grain size of 1.7 μm compose approximately 95 pct of the microstructure. Annealing temperatures above 750 °C show an increase in the amount of martensite. Upon annealing, (Fe, Cr, Mo)₂₃C₆ carbides form within the grains and at the grain boundaries.     

Effect of vanadium on structure-property relations of dual phase fe/mn/si/0.1c steels

The role of vanadium on the structure-property relations of dual phase Fe/Mn/Si/0.1C steels has been investigated. After intercritical annealing at 800 °C, the steels with and without V were either iced brine quenched or air cooled. The steels were also solution treated at 900 °C and subsequently air cooled. Although V modified the resultant microstructure, especially the morphology of carbides, the corresponding mechanical properties are not significantly affected by the modified microstructures. It is concluded that V is not beneficial to these dual phase low carbon steels when they are rapidly quenched (1000 °C/s) or air cooled (3 °C/s) after intercritical annealing.     

Microstructure and properties of Cu-C pseudoalloy films prepared by sputter deposition

The microstructure and properties of Cu-C pseudoalloy films prepared by R.F. magnetron sputtering have been investigated. As Cu and C are mutually immiscible, nonequilibrium supersaturated solid solutions of C in Cu with nanocrystalline microstructures were observed in as-deposited films. Upon heating of the films, three major transition events took place. Recovery occurred at ∼280 to 300 °C, while at ∼400 °C, crystallites started to growth and coalescence, due to the release of strain energies stored during deposition. Annealing at above 600 °C led to the occurrence of grain growth and altered the microstructure considerably. Although attempts have been made in this study, a possible annealing-induced phase separation could not be unambiguously identified. Yet, the fact of low twin densities and fine grain structures observed in the annealed films suggests that the extensive grain growth was impeded by the presence of carbon. Resistivity and hardness properties correlated well with the film microstructure and were governed by the impurity effect of carbon. Low-carbon Cu-C films yielded relatively low resistivity, attributable to the improved film microstructure. Hardness results indicated the strengthening of films was mainly due to fine structure, presence of carbon, and grain refinement by annealing twins.     

Microstructure Evolution during Recrystallization of Newly Developed Low Ni High Mn‐N Stainless Steels

Low cost stainless steels where nickel is replaced in a conventional Fe‐Cr‐Ni stainless steel by manganese and nitrogen were studied. In this work, three new steels based on the system (mass %) Fe‐18Cr‐15Mn‐2Ni‐2Mo‐XN were prepared and their microstructure after each treatment was evaluated by optical and scanning electron microscopy, and X‐ray diffraction. A good correlation between texture and microstructure evolution during annealing was established. A randomization of the texture during recrystallization of the austenite was observed. Recrystallization starts at temperatures above 850°C, and after annealing for 0.5 h at 900°C, the austenite is completely recrystallized, reaching the orientation density a value near 1. Precipitation of σ ‐ phase was observed in the samples annealed at temperatures ranging from 700 to 950°C.     

退火温度对441铁素体不锈钢组织性能的影响

为探索超纯铁素体不锈钢热轧板材在退火过程中组织和力学性能的演变,对441进行了900～1 050 ℃的退火试验,利用OM、SEM和TEM表征了441在退火过程中显微组织的变化规律,并通过拉伸试验和冲击试验研究了退火温度对力学性能的影响.结果表明,随着退火温度升高,轧制组织发生再结晶,且晶粒逐渐长大.退火后441热轧板材...     

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.     

The effect of annealing on the microstructure and mechanical properties of Cu-X microcomposites

The effect of annealing on the microstructure, texture, and room-temperature mechanical properties ofin situ processed copper-based microcomposites has been investigated. These copper microcomposites, containing 15 vol pct Nb, Cr, or Ta, were produced by rolling of cast material. Annealing was carried out in vacuum for 10 hours at 250 °C, 400 °C, and 650 °C. Evidence of microstructural coarsening was found even at the lowest annealing temperature. The through-thickness microstructure of the composites was examined by transmission electron microscopy both before and after the annealing treatments. Texture of the as-processed micro-composites was assessed using X-ray diffraction methods. The strength of the composites following annealing was found to scale with the melting point of the second component. This article is based on a presentation made in the symposium “High Performance Copper-Base Materials” as part of the 1991 TMS Annual Meeting, February 17–21, 1991, New Orleans, LA, under the auspices of the TMS Structural Materials Committee.     

Microstructural study of the titanium alloy Ti-15Mo-2.7Nb-3Al-0.2Si (TIMETAL 21S)

A relatively new titanium alloy, TIMETAL 21S (Ti-15Mo-2.7Nb-3Al-0.2Si-0.15O (in wt pct)), is a potential matrix material for advanced titanium matrix composites for elevated temperature use. In order to develop a perspective on the microstructural stability of this alloy, the influence of several commonly used heat treatments on the microstructure of TIMETAL 21S was studied using optical and transmission electron microscopy (TEM). Depending on the specific thermal treatment, a number of phases, includingα,ω- type, and silicide, can form in this alloy. It was found that both recrystallized and nonrecrystallized areas could be present in the microstructure of an annealed bulk alloy, but the microstructure of annealed sheet alloy was fully recrystallized. The mixed structure of the bulk alloy, developed as a result of inhomogeneous deformation, could not be removed by heat treatment alone at 900 °C. Athermalω-type phase formed in this alloy upon quenching from the solution treatment temperature (900 °C). Silicide precipitates were also found in the quenched sample. Thermal analysis was used to determine theβ transus and silicide solvus as close to 815 °C and 1025 °C, respectively. In solution-treated and quenched samples, a high-temperature aging at 600 °C resulted in the precipitation ofα phase. The precipitation reaction was slower in the recrystallized regions compared to the nonrecrystallized regions. During low-temperature aging (350 °C), the ellipsoidalω-type phase persisted in the recrystallized areas even after 100 hours, whereas a high density ofα precipitates developed in the nonrecrystallized areas within only 3 hours. The observed behavior in precipitation may be related to the influence of substructure in the nonrecrystallized areas, providing for an enhanced kinetics during aging. Theα precipitates (formed during continuous cooling from the solution treatment temperature, low-temperature aging, and high-temperature aging) always obeyed the Burgers orientation relationship. With respect to the microstructure, TIMETAL 21S is similar to other solute-lean, metastableβ titanium alloys.     

Effects of Annealing Temperature on Microstructure and Tensile Properties in Ferritic Lightweight Steels

An investigation was conducted into the effects of annealing temperature on microstructure and tensile properties of ferritic lightweight steels. Two steels were fabricated by varying the C content, and were annealed at 573 K to 1173 K (300 °C to 900 °C) for 1 hour. According to the microstructural analysis results, κ-carbides were formed at about 973 K (700 °C), which was confirmed by equilibrium phase diagrams calculated from a THERMO-CALC program. In the steel containing low carbon content, needle-shaped κ-carbides were homogeneously dispersed in the ferrite matrix, whereas bulky band-shaped martensites were distributed in the steel containing high carbon content. In the 973 K (700 °C)-annealed specimen of the steel containing high carbon content, deformation bands were formed throughout the specimen, while fine carbides were sufficiently deformed inside the deformation bands, thereby resulting in the greatest level of strength and ductility. These results indicated that the appropriate annealing treatment of steel containing high carbon content was useful for the improvement of both strength and ductility over steel containing low carbon content.     

The evolution of annealing textures in 90 Pct drawn copper wire

An electrolytic copper rod was drawn in 24 passes to a 90 pct reduction in area and subsequently annealed under various conditions. The global texture of the drawn wire, as measured by X-ray methods, showed a fiber texture approximated by a strong 〈111〉 and a weak 〈100〉 component. However, its microtexture, as measured by electron backscattered diffraction (EBSD), indicated that the major 〈111〉+minor 〈100〉 duplex fiber texture was dominant only in the center region, while a relatively diffuse texture developed with a somewhat higher density of orientations having a 〈11w〉//wire axis in the middle and surface regions. The inhomogeneous texture in the as-deformed wire gave rise to an inhomogeneous microstructure and texture after annealing. When annealed at 300 °C or 600 °C for 3 hours, the wire developed a duplex fiber texture consisting of major 〈100〉+minor 〈111〉 components in the center region, a strong 〈100〉 fiber texture in the middle region, and a weak texture consisting of 〈111〉 and 〈100〉 components with the 〈111〉 component being slightly stronger in the surface region. When the drawn wire was annealed at the high temperature of 700 °C, the texture at short annealing times was similar to that of the wire annealed at the lower temperatures of 300 °C and 600 °C for 3 hours, but prolonged annealing gave rise to a texture ranging from the 〈111〉 to 〈112〉 components due to abnormal grain-growth that started in the surface region. The recrystallization texture consisting of the major 〈100〉+minor 〈111〉 components was explained by the strain-energy-release maximization (SERM) model, in which the recrystallization texture is determined such that the absolute maximum principal stress direction due to dislocations in the deformed state is along the minimum elastic-modulus direction in recrystallized grains. On the other hand, the abnormal grain-growth texture was attributed to grain-boundary mobility differences between differently oriented grain.     

Microstructural evolution of a nanocrystalline Ti−47Al−3Cr alloy during annealing in the α+γ-phase field

Prealloyed, gas-atomized (GA) Ti−47Al−3Cr alloy powder, containing about 70 pct of the α₂ (Ti₃Al) phase and 30 pct of the γ (TiAl) phase, was fully amorphized by mechanical alloying. The amorphous phase was stable during heating to 600°C, but decomposed at higher temperatures, with an exothermic reaction peak at 624°C as the material transformed to a mixture of α₂ and γ and then to a fully γ structure at 722°C. A nanocrystalline compact with a mean grain size of 42 nm was obtained by hot isostatic pressing (HIP'ing) of the amorphous powder at 725°C. Isothermal annealing experiments were conducted in the two-phase α+γ field, at 1200°C, using holding times of 5, 10, 25, and 35 hours, followed by air cooling. The X-ray diffractometry and analytical transmission electron microscopy investigations carried out on annealed and air-cooled specimens revealed only the presence of the γ grains, which coarsened on annealing. Initially, the grains grew, followed by a saturation stage after annealing for 25 hours, with a saturation grain size of about 1 μm. This grain growth and saturation behavior can be described with a normal grain growth mechanism in which a permanent pinning force is taken into account. Twins formed in the γ grains as a result of annealing and air cooling and exhibited a common twinning plane of (111) with the matrix phase. The minimum γ grain size in which twinning occurred in the annealed specimens was determined to be 0.25 μm, which suggests that twinning is energetically unfavorable in the nanometer-sized grains. N. SRISUKHUMBOWORNCHAI, formerly Master's Student, IMAP, University of Idaho     

Formation of voids and dislocation loops in near-stoichiometric nial by aging at 700° to 900°c,and some effects on alloy properties

Dislocation loops and voids form spontaneously in stoichiometric NiAl annealed between 700° to 900°C. The former probably are formed by a punching mechanism from impurity particles, and the latter are frequently seen to have nucleated on particles. However, it is possible that both loops and voids are nucleated by vacancy/impurity complexes. Some of the loops act as dislocation sources during plastic deformation so that, for example, NiAl annealed at 700° to 900°C is softer and has greater ductility at room temperature, or has poorer creep resistance at higher temperatures, than material annealed at 1300°C. It is probable that small precipitate particles, possibly carbides and/or aluminum oxide form at 700° to 900°C but are in solution at higher temperatures. Their effect on mechanical properties is a form of precipitation softening.     

An analysis of deformation,temperature, and microstructure for hot extruded titanium alloy

During hot extrusion, the microstructure and resultant mechanical properties of materials are subjected to considerable change due to adiabatic local heat generation. In this work, strain, temperature distributions, and microstructural changes resulting from the hot extrusion of Ti-6A1-4V alloy were studied using visioplasticity methods, thermal calculations, and optical microscopy. The results were correlated to the microstructural behavior during hot deformation. Billets 62 mm in diameter were heated to either 950 °C(α + β region) or 1100 °C(β region) and extruded at the extrusion ratios of either 6 or 12. Visioplasticity calculations show that, in the deformation zone, strain is relatively high at the surface of the billet and gradually decreases with depth. Estimated strains of a bar extruded at 950 °C with the extrusion ratio of 12 are 3.5 at the surface and 2.5 at the center, respectively. But the estimated temperature at the surface is lower than that at the center. As a result, microstructures of the bar were bi-modal structure at the surface and acicular transformed structure at the center. A bar extruded at 950 °C with an extrusion ratio of 6 had all bi-modal microstructure. The strain distributions of bar extruded at 1100 °C were similar in nature to those at 950 °C. M. ISHII formerly with Hikari Research and Development Laboratory.     

Effect of Hot Rolling Temperature on the Structure and Magnetic Properties of High Permeability Non‐Oriented Silicon Steel

1.3%Si steel hot bands were produced by hot rolling in a Steckel mill with two entrance temperatures: 1000°C (γ rolling) and 910°C (α+γ rolling). Hot band samples were processed with and without hot band annealing (900°C for 30s), cold rolled to final thickness of 0.5mm and annealed in H₂‐25%N₂ at 900°C for 40s. The combination of high hot rolling temperature and hot band annealing resulted in lower core loss and higher permeability. Although no significant differences were observed on the hot band microstructures, after hot band annealing the γ rolled sample showed a larger grain size. The final results were attributed to the effect of the initial grain size prior to cold rolling on microstructure and texture after annealing.     

Ordering transformations and mechanical properties of Ti3Ai and Ti3Al-Nb alloys

Phase transformations and the kinetics of domain growth were studied in near stoichiometric Ti₃Al and in a similar alloy containing about 5 at. pct Nb (Cb). The alloys were quenched from the β and from the α+ β fields and were subsequently annealed in the α₂ field to study the ordering transformation. The critical temperature (T _c) for ordering was found to be between 1125 and 1150° for both alloys. When quenched from aboveT _c the microstructure of the stoichiometric compound contained massive martensite with small antiphase domains of average size 8 × 10^— μm. On annealing the quenched structures in the range 700 to 1000°, domain coalescence occurred, the domains growing approximately as the square root of the annealing time. The activation energy for the domain growth process was found to be 64.6 ± 6 Kcal/mole (2.68 ± 0.25 × 10⁵ J/mole). On quenching the alloy containing Nb the β transforms to a fine acicular martensite. On annealing, antiphase domain coalescence within the martensite plates and the simultaneous recrystallization of the martensite resulted in a fine subgrain structure even after annealing at 900° for up to 3 h. The mechanical properties and the fracture modes of the two alloys tested at 700° were correlated with the observed microstructural changes. The effects of Nb in this alloy are to slow the domain growth kinetics, to reduce the planarity of slip, and to increase nonbasal slip activity. Formerly NRC Research Associate in the Air Force Materials Laboratory, Wright-Patterson Air Force Base, OH     

The kinetics of static globularization of Ti-6Al-4V

The kinetics of the evolution of the lamellar-colony microstructure to an equiaxed morphology during heat treatment of a hot-worked, two-phase titanium alloy were established. For this purpose, the alpha/beta alloy Ti-6Al-4V was isothermally upset forged at 900 °C or 955 °C and subsequently annealed for times ranging from 0.5 to 100 hours. The degree of the breakup of alpha-phase lamellae into lower-aspect-ratio grains during static annealing was measured and related to the imposed strain estimated using finite-element analysis (FEA). The kinetics of the static globularization of the alpha phase were found to depend on the amount of strain and the annealing temperature but were not affected by the specific deformation temperature in the 900 °C to 955 °C range. These results demonstrated that deformation-induced dislocation substructure has a small effect on the static-globularization process. In addition, the relative globularization kinetics at 900 °C and 955 °C were rationalized in terms of classical coarsening theory.     

Recrystallization kinetics of an austenitic high-manganese steel subjected to severe plastic deformation

The evolution of the microstructure and the properties of an austenitic high-manganese steel subjected to severe deformation by cold rolling and subsequent recrystallization annealing is investigated. Cold rolling is accompanied by mechanical structural twinning and shear banding. The microhardness and microstructural analysis of annealed samples are used to study the recrystallization kinetics of the high-manganese steel. It is shown that large plastic deformation and subsequent annealing result in rapid development of recrystallization processes and the formation of an ultrafine-grained structure. A completely recrystallized structure with an average grain size of 0.64 μm forms after 30-min annealing at a temperature of 550°C. No significant structural changes are observed when the annealing time increases to 18 h, which indicates stability of the recrystallized microstructure. The steel cold rolled to 90% and annealed at 550°C for 30 min demonstrates very high strength properties: the yield strength and the tensile strength achieve 650 and 850MPa, respectively. The dependence of the strength properties of the steel on the grain size formed after rolling and recrystallization annealing is described by the Hall–Petch relation.     

Microstructural evolution of a nanocrystalline Ti-47Al-3Cr alloy during annealing in the α + γ-phase field

Prealloyed, gas-atomized (GA) Ti-47Al-3Cr alloy powder, containing about 70 pct of the α ₂ (Ti₃Al) phase and 30 pct of the γ (TiAl) phase, was fully amorphized by mechanical alloying. The amorphous phase was stable during heating to 600 °C, but decomposed at higher temperatures, with an exothermic reaction peak at 624 °C as the material transformed to a mixture of α ₂ and γ and then to a fully γ structure at 722 °C. A nanocrystalline compact with a mean grain size of 42 nm was obtained by hot isostatic pressing (HIP’ing) of the amorphous powder at 725 °C. Isothermal annealing experiments were conducted in the two-phase α+γ field, at 1200 °C, using holding times of 5, 10, 25, and 35 hours, followed by air cooling. The X-ray diffractometry and analytical transmission electron microscopy investigations carried out on annealed and air-cooled specimens revealed only the presence of the γ grains, which coarsened on annealing. Initially, the grains grew, followed by a saturation stage after annealing for 25 hours, with a saturation grain size of about 1 μm. This grain growth and saturation behavior can be described with a normal grain growth mechanism in which a permanent pinning force is taken into account. Twins formed in the γ grains as a result of annealing and air cooling and exhibited a common twinning plane of (111) with the matrix phase. The minimum γ grain size in which twinning occurred in the annealed specimens was determined to be 0.25 μm, which suggests that twinning is energetically unfavorable in the nanometer-sized grains.     

Microstructure and mechanical properties of sputter-deposited Cu1−x Ta x alloys

The microstructures and mechanical properties of a family of sputter-deposited Cu_1−x Ta _x (0<x<0.18) alloys have been investigated. The as-deposited microstructures for all film compositions consisted of a polycrystalline, face-centered-cubic (fcc) Cu matrix, with varying levels of Ta in solid solution, plus a very high density of discrete, 1 to 3 nm, fcc Ta particles. Decreased deposition temperature (−120 °C vs 100 °C) increased the level of Ta in solid solution. After annealing (900 °C for 1 hour) the as-deposited 6 at. pct Ta films, the Cu matrix grains remained submicron and the Ta particles remained fcc with no apparent particle coarsening. Additionally, the fcc Ta particles were found before and after annealing to be oriented identically with the Cu matrix and aligned on {111} and {100} habit planes. Annealing 17 at. pct Ta films at 900 °C for 1 hour resulted in the formation of body-centered-cubic (bcc) Ta particles (>50-nm diameter) in addition to the much smaller fcc Ta particles. Annealing the low and high Ta composition films at 900 °C for as long as 100 hours produced no observed change in either the Cu matrix grain size or the size and distribution of the fcc and bcc Ta particles. Microhardness and nanoindentation mechanical property evaluations of bulk hot-pressed materials indicated that the high strengths of the composites were unchanged, even after annealing for 100 hours at 900 °C.