Structural Superplasticity at High Strain Rates of Super Duplex Stainless Steel Fe‐25Cr‐7Ni‐3Mo‐0.3N

The fine‐grained super duplex stainless steel Fe‐25Cr‐7Ni‐3Mo‐0.3N consisting of two phases (δ‐ferrite/austenite) exhibits structural super‐plasticity at higher strain rates of ? ≈ 10^?2s^?1 in the temperature range between 975 and 1100°C. The equiaxed microstructure with an average grain size of was produced by thermomechanical processing. Maximum strain‐rate‐sensitivity exponents of m ≈ 0.5 and elongations to failure of more than 500% were achieved. From thermal activation analysis an activation energy for superplastic flow of Q = 310 ± 20 kJ/mole was derived. The superplastic behaviour at higher strain rates is quantitatively described by a deformation model where grain or interphase boundary sliding is accommodated by sequential steps of dislocation glide and climb. The high strain‐rate‐sensitivity exponent and the observed dislocation density indicate that dislocation climb in the slightly solid solution strengthened austenite is the rate controlling step for superplastic flow. The deformation mechanism reveals that the investigated super duplex stainless steel exhibits superplastic behaviour that is typical for class II solid solution alloys.     

热变形及热处理过程中TC17钛合金组织与取向的关联性

为了进一步研究热压缩及热处理过程对组织及取向变化的关联性, 通过对TC17进行热压缩变形及后续热处理, 利用光学显微镜和背散射电子衍射等分析方法, 结合晶粒尺寸、织构分布图、极图以及反极图, 研究变形后及热处理后的TC17的组织结构、晶粒尺寸的变化和取向的演变规律以及两者之间的关联性.结果表明: 随着变形温度升高, 初生α相含量大幅减小, 尺寸减小, 大部分α相晶粒分散分布, 且位于高温β相晶粒的三叉晶界上; 热处理后, α相和β相组织特征清晰, 界限明显, 初生α相依旧存在, 且趋于等轴化, 亚稳定β相发生转变, 形成片层状β转变组织; 热变形使α相织构极密度值减小, 且随之温度增加, α相织构极密度值也变小; 热变形后的α相已不存在明显的强织构, 热变形对α相晶粒的取向影响较大, 很明显的改善了其取向的均匀性; 热变形同样使β相织构极密度值减小, 但效果不明显.β相仍存在取向集中现象, 取向均匀性相对较差.      

High-Temperature Mechanical Behavior of Extruded Mg-Y-Zn Alloy Containing LPSO Phases

The high-temperature mechanical behavior of extruded Mg_97?3x Y_2x Zn _x (at. pct) alloys is evaluated from 473 K to 673 K (200 °C to 400 °C). The microstructure of the extruded alloys is characterized by Long Period Stacking Ordered structure (LPSO) elongated particles within the magnesium matrix. At low temperature and high strain rates, their creep behavior shows a high stress exponent (n = 11) and high activation energy. Alloys behave as a metal matrix composite where the magnesium matrix transfers part of its load to the LPSO phase. At high-temperature and/or low stresses, creep is controlled by nonbasal dislocation slip. At intermediate and high strain rates at 673 K (400 °C) and at intermediate strain rates between 623 K and 673 K (350 °C and 400 °C), the extruded alloys show superplastic deformation with elongations to failure higher than 200 pct. Cracking of coarse LPSO second-phase particles and their subsequent distribution in the magnesium matrix take place during superplastic deformation, preventing magnesium grain growth.     

Mechanisms of phase and structural transformations and texture formation in titanium alloy sheet semiproducts

Inverse pole figures are used to analyze texture formation in sheet semiproducts made from titanium alloys of various classes. The role of the mechanisms of phase and structural transformations in texture formation has been revealed. A VT6 alloy is used as an example to study the texture formation during hot, warm, and cold rolling of high-alloy α + β titanium alloys. Hot rolling followed by cooling is shown to form the β → α transformation texture. Warm or cold rolling of α + β titanium alloys leads to the (0001)〈hkio〉 deformation texture of the α phase, and subsequent annealing in the two-phase field results in the transformation texture with a strictly determined orientation distribution of α-phase crystallites due to the shear mechanism of α-phase nucleation. Specifically, the 〈10\(\overline 1 \)0〉 orientation is parallel to the rolling direction, the [0001] direction is parallel to the transverse direction, and the {11\(\overline 2 \)0} planes are parallel to the rolling plane. Thermohydrogen treatment in combination with rolling is shown to form nano-and submicrocrystalline structures with a virtually textureless α phase in VT6 sheet semiproducts. As a result, the anisotropy of the mechanical properties of the sheet semiproducts produced by this new combined hydrogen technology decreases sixfold.     

Microstructural stability of thermal-mechanically pretreated type 316 austenitic stainless steel

Studies of the microstructural stability of Type 316 austenitic stainless steel were performed for a wide range of thermal-mechanical pretreatments in the limited aging temperature range of 550° to 760°C. The pretreatments were selected in order to investigate the effects of varying solution treatment temperature, amount of cold reduction by rolling, initial grain size, and initial precipitate distribution. Large variations in both phase stability and recrystallization behavior can be effected by appropriate pretreatments. Cold work accelerates precipitation of M₂₃C₆ carbide and the intermetallic compounds (Laves, χ, and σ phases). Both the amount and kinetics of σ phase formation are especially enhanced by recrystallization occurring in the aging temperature range. It is suggested that this occurs due to ready σ nucleation at slowly moving (recrystallizing) grain boundaries together with enhanced growth rates due to diffusion along the boundary. Fine grain size enhances phase instability by providing additional nucleating sites and decreased diffusion paths for precipitate forming elements, but in the grain size range studied (ASTM No. 3.5 to No. 13) the effect is not as significant as the effect of cold work, particularly when recrystallization occurs during the aging treatment. Fine grain size and pretreatments which precipitate the carbides prior to the final cold working step enhance recrystallization kinetics relative to solution treated and cold-worked materials. This is apparently due to stabilization of the cold-worked substructure in the solution treated samples by precipitation of carbide and Laves phases on the dislocations and stacking faults.     

Influence of the second phase on the room-temperature tensile and creep deformation mechanisms of α-β titanium alloys: Part I. Tensile deformation

The effects of α and β phase interactions on the room-temperature tensile and creep deformation behavior of α + β titanium alloys with Widmanst?tten microstructures were studied using Ti-6.0 wt pct Mn and Ti-8.1 wt pct V as the model two-phase alloy systems. This article, Part I, deals with tensile deformation. It was found that when the α phase is present as thin (<10-μm) plates in the α + β alloys, significant twinning occurs. No significant twinning was observed in single-phase alloys with the same chemistry and similar grain size. Additionally, the β phase of Ti-8.1 V deforms by stress-induced hexagonal martensite (α′), while only twinning occurs in the single-phase β alloy with the same chemistry. Twinning in the α phase in association with stress-induced martensite (SIM) in the β phase was observed for the first time in a two-phase titanium alloy. This behavior is explained in terms of a number of factors including elastic interaction stresses between the α and β phases, coherency between the α phase and hexagonal martensite, and β phase stability.     

Influence of the second phase on the room-temperature tensile and creep deformation mechanisms of α-β titanium alloys, part II: Creep deformation

This work focuses on the effect of the second phase on the ambient temperature creep deformation mechanisms of titanium alloys, using Ti-6.0 wt pct Mn and Ti-8.1 wt pct V with Widmanstätten microstructures as the model systems. In Part I it was observed that the presence of a second phase can affect the tensile deformation behavior. Likewise, the creep deformation mechanisms of the two-phase alloys differ from the mechanisms of single-phase alloys. These α-β deformation mechanisms include twinning in fine grains of the α phase and stress-induced hexagonal martensite in the β phase of Ti-8.1 V. This is the first time that twinning in the α phase and stress-induced martensite in the β phase are reported as creep deformation mechanisms in an α-β titanium alloy. Several factors, including elastic interaction effects, shear stress due to deformation products in adjacent phases, and the stability of the β phase, affect the creep deformation mechanisms in these alloys. Models for the time-dependent growth of martensite are suggested. In addition, the difference between tensile and creep deformation in regard to accumulation of stresses to reach the critical stresses is described.     

Effect of the phase composition and structure of titanium alloys on their interaction with nitrogen during low-temperature ion nitriding

The interaction of nitrogen with commercial titanium alloys having a single-phase α (VT1-0, VT5 alloys) or β (TS6 alloy) structure and a two-phase α + β structure with various contents of the β phase (VT20, VT6, VT3-1, VT23, VT22) is studied during ion nitriding at 550 and 600°C. The initial phase composition, the degree of phase alloying, and the structure have been shown to substantially affect the length of the diffusion zone having an α_N structure and the amounts of the Ti₂N (ε phase) and TiN (δ phase) surface nitrides. A group of alloys containing up to 30% β phase in an α + β structure is separated. These alloys are of interest as a basis for designing surface gradient structure materials using surface nitrogen alloying upon low-temperature ion nitriding.     

Dislocation structures in single-crystal tungsten and tungsten alloys

Deformation of tungsten single crystals as a function of strain, temperature, and alloying was studied by transmission electron microscopy. Single crystals oriented for (?101)[lll] slip were grown by electron beam zone refining. Compression specimens of tungsten, W-l and 3 pct Re and W-l and 3 pct Ta were deformed to 2 pct strain at 150°, 300°, and 590°K (0.04, 0.08, and 0.16T _m). Specimens were also strained to 0.5 and 5.0 pct strain at 300°K. Transmission microscopy revealed that the dislocation substructures in single-crystal tungsten are similar to substructures in other refractory metals when compared on a homologous temperature basis. At temperatures greater than 0.1T _m, the substructure is characterized primarily by edge dipoles. At temperatures less than 0.1T _m, long screw dislocations lying parallel to the primary [111] slip direction characterize the substructure. Rhenium additions to tungsten promote formation of edge dipoles at temperatures of 300° and 150°K and increase dislocation density at all three temperatures. In addition, dislocations consistent with (1?12)[?111] slip were observed in the W-Re single crystals after deformation at 150°K. Tantalum additions had a lesser effect on the dislocation substructure compared to rhenium additions. The W-l and 3 pct Ta alloys exhibited higher dislocation densities than unalloyed tungsten after similar strains and, at 150°K, W-3 pct Ta contained a few dislocations consistent with (1?12)[?111] slip. It is concluded that the reduction in ductile-brittle transition temperature of poly crystalline tungsten containing dilute rhenium additions, 1 to 5 pct, can be attributed to an increase in dislocation mobility at temperatures less than 0.1 T_m.     

Superplasticity and residual tensile properties of a microduplex copper-nickel-zinc alloy

Structural superplasticity in two phase alloys of the copper-nickel-zinc system (nominal composition in wt. pct Cu-15Ni-38 Zn-0.2 Mn) occurs over a wide range of strain rates in the temperature range 850 to 1050∮F (454 to 565°). The upper temperature limit for super-plastic behavior in this system is determined by the reversion of the fine-grained two-phase structure to a single phase structure in which extensive grain growth is possible. Residual room temperature tensile properties and microstructure of the microduplex alloy after superplastic straining have been studied as a function of test temperature and total super-plastic strain. At test temperatures sufficiently removed from the phase transformation temperature, the high tensile properties and fine microstructure of the starting material are essentially retained after superplastic strains approaching 200 pct. In the immediate vicinity of the phase transformation temperature, rapid degradation of the microduplex structure occurs during superplastic deformation with a consequent severe degradation of the residual room temperature tensile properties. Formerly with The International Nickel Company, is now with Gulf Energy and Environmental Systems, Materials Science Department, P. O. Box 608, San Diego, Calif. 92112.     

Microstructural examination of straininduced phase transformations in selected zirconium alloys

Abstract

This paper describes the investigation of the stability, with respect to deformation by rolling at ambient temperatures, of the metastable β phase of zirconium alloys containing molybdenum, vanadium, niobium, tin and aluminum. The stability of the alloys has been analyzed in terms of a stability diagram originally proposed for isomorphous titanium alloys. Molybdenum was found to be a very potent {3stabilizer as was vanadium. Niobium appears to be the least effective of the β stabilizers investigated. Aluminum and tin act as α stabilizers: aluminum has a far greater effect than tin. Of the alloys investigated only a limited number showed any evidence of a strain-induced transformation of the metastable β phase to a martensitic α _D ′ phase. An alloy containing Zr-0.8 wt% Mo-2 wt% V-2 wt% Nb 2 wt% Sn was the most promising of these alloys and an investigation of its mechanical properties is proposed.

Résumé

Nous avons étudié la stabilité, lors du laminage à la température ambiante, de la phase métastable β dans des alliages de zirconium contenant du molybdène, du vanadium, du niobium, de l'étain et de l'aluminium. La stabilitè des alliages a été determinée en termes d'un diagramme de stabilité proposé antérieurement pour des alliages isomorphes de titane. Le molybdène et la vanadium stabilisent fortement la phase β alors que le niobium n'avait qu'un effet peu marqué. L'aluminium et l'étain stabilisent plutôt la phase α l'aluminium ayant un effet nettement plus fort que l'étain. La transformation, induite par la déformation, de la phase métastable β en phase matensitique αD ne semblaient pas avoir lieu dans la plupart des alliages. L'alliage Zr-0.8 % Mo-2 % V-2% Nb- 2% Sn (poids) s'est avéré comme étant le plus prometteur et nous nous proposons d'étudier ses propriétés mécaniques.     

Structural heat-resistant β-NiAl + γ′-Ni<Subscript>3</Subscript>Al alloys of the Ni–Al–Co system: I. Solidification and structure

When analyzing the ternary Ni–Al–M phase diagrams, where M is a group VI–VIII transition metal, we chose the Ni–Al–Co system, where the γ′ and γ phases are in equilibrium with the β phase, as a base for designing alloys with the following physicochemical properties: a moderate density (≤7.2 g/cm³) and satisfactory heat resistance at temperatures up to 1300°C. The structure formation in heterophase β + γ′ alloys during directional solidification is studied. It is found that, in contrast to cobalt-free β + γ′ alloys (where the γ′-Ni₃Al aluminide forms according to the peritectic reaction L + β ? γ′), the alloys with 8–10 at % Co studied in this work during directional solidification at 1370°C contain the degenerate eutectic L ? β + γ. The transition from the β + γ field to the β + γ′ + γ field occurs in the temperature range 1323–1334°C, and the γ′ phase then forms according to the reaction β + γ ? γ′.     

Effect of additions of zirconium,chromium, and nickel on the structure and characteristics of superplasticity of alloys of the Al-Zn-Mg-Cu system

By optical and transmission microscopy as well as by X-ray structural analysis, the structure and properties of alloys of the Al-Zn-Mg-Cu system with Ni, Zr, and Cr additions during superplastic deformation are investigated. It is shown that introduction of 0.26% Zr into the alloys leads to the formation of a nonrecrystallized or partially recrystallized structure in the billets. Such materials manifest the effect of superplasticity at t = 515°C in a range of deformation rates of (0.15–1.0) × 10^?2 s^?1, and the relative elongation reaches 450–680%.     

On the creep behavior of the Ti3Al titanium aluminide Ti25Al10Nb3V1Mo

The temperature and stress dependence of the steady-state creep behavior of the Ti₃Al alloy Ti25Al10Nb3V1Mo (at.%) has been evaluated. Two microstructural conditions were evaluated as follows: As processed (rolled) consisting of the fine grained (approx. 6–10 μm) β plus ordered α₂ phase and beta heat treated consisting of coarse grained (approx. 150 μm) retained ordered B2 phase with a fine Widmanstatten structure within the grain interiors. The steady-state creep behavior of both microstructural conditions was studied over the temperature range of 650–815°C. The apparent creep activation energies and stress exponents were measured for both microstructural conditions. The temperature and stress dependence of the steady-state creep rate of both microstructures can be described well by the power law creep equation suggesting dislocation motion as the operative deformation mechanism. Over the temperature-stress regime of the present study, the creep deformation of the fine grained microstructure possibly breaks down into a low temperature (dislocation core diffusion controlled) regime and a high temperature (bulk diffusion controlled) regime within the power law creep region as indicated by the apparent creep activation energies measured. Upon β heat treatment, creep deformation is found to be governed by a single rate limiting process. At temperatures and stress levels where a direct comparison can be made, the steady-state creep rates of the β heat treated Ti-25-10-3-1 exhibit an order of magnitude decrease over those of the processed material. This suggests the possibility of some mechanism other than power law creep controlling within the regime corresponding to the low apparent activation energy of the fine grained microstructure.     

循环热处理及形变对TC17钛合金片层组织球化和取向的影响

将循环热处理与形变相结合,利用电子背散射衍射等手段探究该工艺对TC17钛合金片层组织球化和取向的影响.结果表明:TC17钛合金在两相区进行单纯的循环热处理其片层组织球化程度有限,而经过循环热处理+压缩变形后,其魏氏组织消失,片层α相得到明显球化,但是其取向均匀性仍没发生较大变化.此外,变形中两相的再结晶速度及其强韧性导致了两相取向的差异性.α相的再结晶速度快于β相,在变形过程中,α相的各向异性首先降低;另一方面,由于α相比β相硬度高,热变形过程中,α相的变形程度小于β相,应变主要集中在与α相邻近的较软的β相,从而导致α相的取向均匀性高于β相.      

The structure and properties of thermomechanically treated β-III titanium

Beta-III titanium (Ti-11.5Mo-5.5Zr-4.5Sn) was solutionized above the β-transus, water-quenched and deformed by rolling at room temperature. The deformation accelerated the aging kinetics at all temperatures up to the β transus. The thermomechanically treated (TMT) alloy always had higher strength than the conventionally heat treated (CHT) alloy; the effect being most marked when the aging product was normally α in a β matrix. In addition, the ductility and notched impact resistance of TMT β-III was greater than that of the CHT alloy in the over-aged condition. The TMT did not alter the morphology of the ellipsoidal α phase formed at low aging temperatures and short aging times, respectively. Here the strengthening increase is attributed to strain hardening of the initial β + ω microstructure. The deformation did substantially change the morphology of the Widmanstätten α phase that formed at higher aging temperatures. In particular, the Widmanstätten α plates were much finer and the β grain boundaries were no longer a preferred precipitation site following TMT. At the highest aging temperatures, the TMT material developed a cell structure of about 1 μ in diameter.     

Microstructure and mechanical properties of RS NiCrAl melt-spun alloys

The microstructure and mechanical properties of three melt-spun NiCrAl alloy ribbons have been studied in the as-cast condition as well as after thermal treatments. The microstructure of the alloys is dendritic-microcellular in as-cast condition and phases present for 10 at.% Al and 30 at.% Al alloys are as is predicted by the equilibrium phase diagram. In the 20 at.% Al alloy, γ' has frozen in metastable form and partial ordering takes place during cooling in the solid state. After thermal treatments the ribbons generally maintain a refined microstructure; α phase precipitates are always found in β and γ' phases in 20 and 30 at.% Al alloys. The hardness of the alloys increases with aluminum content. The tensile strength at room temperature is related to the phases present in the material for each state of treatment. The alloys are brittle, a higher ductility always being obtained in the as-cast condition.     

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

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

Fatigue deformation of TiAl base alloys

The fatigue behavior of Ti-36.3 wt pct Al and Ti-36.2 wt pct Al-4.65 wt pct Nb alloys was studied in the temperature range room temperature to 900°C. The microstructures of the alloys tested consisted predominantly of γ phase (TiAl) with a small volume fraction of γ phase (Ti₃Al) distributed in lamellar form. The alloys were tested to failure in alternate tension-compression fatigue at several constant load amplitudes with zero mean stress. Fracture modes and substructural changes resulting from fatigue deformation were studied by scanning electron microscopy and transmission electron miscroscopy respectively. The ratio of fatigue strength (at 10⁶ cycles) to ultimate tensile strength was found to be in the range 0.5 to 0.8 over the range of temperatures tested. The predominant mode of fracture changed from cleavage type at room temperature to intergranular type at temperatures above 600°C. The fatigue microstructure at low temperatures consisted of a high density of a/3 [111] faults and dislocation debris of predominantly a/2 [110] and a/2 [110] Burger's vectors with no preferential alignment of dislocations. At high temperatures, a dislocation braid structure consisting of all 〈110〉 slip vectors was observed. The changes in fracture behavior with temperature correlated well with changes in dislocation substructure developed during fatigue deformation. S. M. L. SASTRY was formerly NRC Research Associate in the Air Force Materials Laboratory, Wright-Patterson Air Force Base, OH     

Powder Metallurgy Group Meeting,Coventry, 24–26 October 1977: Report of Proceedings

Abstract

The feasibility of producing room temperature superplastic Zn–Al alloys by hot extrusion of gas atomised powders has been investigated. Commercially pure zinc, and Zn–8wt-%Al and Zn–28wt-%Al binary alloys were gas atomised; the resulting powders were cold compacted into cylindrical billets and extruded to form consolidated rod. Two extrusion temperatures (200 and 300°C) were used, chosen to lie on either side of the invariant (eutectoid) temperature of 275°C. It has long been established that in conventional cast alloys rapid quenching from above this temperature is required to produce a microstructure having superplastic properties. (It was anticipated that the 300°C extrusions would contain quantities of near equilibrium eutectoid and thus be unlikely to deform superplastically. The 200°C extrusions were expected to exhibit a non-equilibrium structure that might have potential in terms of superplastic deformation.) The microstructures of the extrudates were investigated by transmission electron microscopy and the mechanical properties established by room temperature tensile testing and Charpy impact testing. PM/0502