Microstructural Evolution in Niobium-Based Alloys

Nb-Ti-Cr-Si-Al-X alloys have been considered as candidate materials for high-temperature structural applications in aero engines. Using a multicomponent approach, attempts are being made to obtain good strength with adequate oxidation resistance at the expected operating temperatures (∼1073 through 1273 K) in these alloys. In the microstructure of these alloys, which essentially consist of the silicide phase distributed in the bcc β matrix, the presence or absence of the Laves phase can bring about considerable changes in the mechanical properties. The present article reports on the instability observed in the β phase and also on the formation of the Laves phase in the β matrix. The extent of the chemical separation and elements responsible for this separation has been determined. Different morphologies of the Laves phase have been obtained by aging at different temperatures for various durations and the structure, chemistry, and volume fraction of these phases have been determined. This article is based on a presentation given in the symposium entitled “Materials Behavior: Far from Equilibrium” as part of the Golden Jubilee Celebration of Bhabha Atomic Research Centre, which occurred December 15–16, 2006 in Mumbai, India.

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High Temperature Phase Stability and Microstructural Characterization of D9 Stainless Steel-Zirconium Metal Waste Form Alloys

Zirconium present in stainless steel-zirconium metal waste form (MWF) alloys form Ni?CZr and Fe?CZr intermetallic phases which act as a sink for radionuclide and improve resistance to localized corrosion as well as selective radionuclide leaching. The present study looks into the behavior of Zr intermetallics in MWF alloys with the variation of Zr content after heat treatments. Two MWF alloys of D9 SS (Ti modified 15Cr?C15Ni?C2.5Mo stainless steel) with 8.5 and 17?wt% Zr were heat treated at 1,323?K for 2 and 5?h and characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The stability of the Zr intermetallic compounds was examined by high temperature XRD. The results from XRD study showed the presence of NiZr, Ni₅Zr, Ni₇Zr₂, FeZr₂, and Fe₃Zr peaks along with fcc Fe based solid solution. The MWF alloy with 17?wt% Zr exhibited ??-ferrite peak in as-cast condition which was not observed after heat treatment. From the SEM micrograph the agglomeration of intermetallic phases was observed after heat treatment and the grain size of the intermetallic phases increased with duration of heat treatment. The high temperature XRD study revealed that all the intermetallic phases were stable up to 1,173?K and above this temperature Ni?CZr intermetallics started disappearing. However Fe?CZr intermetallics were stable till 1,373?K. The paper presents the high temperature phase stability of D9 SS-Zr MWF alloys.     

Advances in Microstructural Understanding of Wrought Aluminum Alloys

Metallurgical and Materials Transactions A - Wrought aluminum alloys are an attractive option in the quest for lightweight, recyclable, structural materials. Modern wrought aluminum alloys depend...     

Microstructural Model for the Deformation of Shape Memory Alloys

A model is proposed to describe the phase-structural deformation of shape memory alloys with allowance for the nonuniform strain hardening of the martensite part of representative volume. A scheme is developed to determine the volume fraction of martensite undergoing structural transformation during proportional nonreversible loading. The problem of reactive-stress generation in experiments on orientational transformation with constrained deformation after unloading is resolved.     

Al-Bi过偏晶合金显微组织演变的数值模拟

基于Al-Bi过偏晶合金凝固通过难混溶区阶段产生的液-液相分解及分离的运动行为,采用两相体积平均法,对质量、动量、能量、组分及液滴密度守恒方程进行数值模拟,计算中考虑了形核、扩散长大、Stokes运动及Marangoni运动等多种复杂物理现象的耦合作用,分析了两相运动速度、第二相尺寸分布、第二相体积分数分布以及液滴密度分布对过偏晶合金凝固的显微组织演变及宏观偏析的影响。结果表明,过偏晶合金凝固过程中显微组织演化在不同阶段的主要影响因素不同:凝固初始阶段主要以形核和扩散长大为主;凝固中期和后期第二相迁移运动行为将逐渐占主导作用。凝固过程中,铸件顶角位置首先获得过冷度驱动形核,并以较快的形核速率达到最大形核密度。随着凝固过程不断地进行,第二相小液滴受到的Marangoni力约为Stokes粘滞阻力的两倍,开始由铸件顶角和边缘低温区向中心高温区聚集。凝固时间为1 s时,铸件顶角和边缘第二相小液滴的生长直径和第二相体积分数大于铸件中心位置,而凝固时间为5和7 s时,第二相小液滴直径随铸件中心距离变化的曲线斜率随凝固过程的进行而逐渐变缓,长大速率逐渐变慢。     

Microstructural Evolution During Friction Surfacing of Dissimilar Aluminum Alloys

The microstructural evolution during friction surfacing of an aluminum alloy 6082-T6 rod on an aluminum alloy 2024-T351 substrate was characterized using the electron backscatter diffraction technique. Crystallographic data were obtained from several regions in the consumable material and in the deposited material. From the results, it can be deduced that the grain structure formation was a complex process governed by the geometrical effect of strain and the superposition of continuous and discontinuous dynamic recrystallizations.     

Mechanical Properties and Microstructural Evolution of Friction-Stir-Welded Thin Sheet Aluminum Alloys

Friction stir welding of thin aluminum sheets represents a potential goal for aircraft and automotive industries because of the advantages of using this new technological process. In the current work, the microstructural evolution and mechanical behavior of 6082T6-6082T6, 2024T3-2024T3, and 6082T6-2024T3 thin friction-stir-welded joints were investigated. Uniaxial tensile testing at room temperature, 443 K, 473 K, and 503 K (170 °C, 200 °C, and 230 °C) was used to determine the extent to which these ultra-thin joints can be used and deformed. The tensile stress–strain curves showed a decrease of the flow stress with increasing temperature and decreasing strain rate. The ductility of 6082T6-6082T6 joints generally improved when deformed at warm temperatures. It was almost constant for the 6082T6-2024T3 and reached the higher value in the 2024T3-2024T3 when deformed at 443 K and 473 K (170 °C and 200 °C) when compared with the room temperature value. Tensile specimens fractured in the middle of the weld zone in a ductile mode. The precipitation and growth of S’ type phases strengthens 2024T3-2024T3 joints during deformation. In the 6082T6-6082T6, β″ precipitates show some increase in size but give a lower contribution to strength. At 503 K (230 °C), recovery mechanisms (dislocation reorganization inside the deformed grains) are initiated but the temperature was not enough high to produce a homogeneous subgrain structure.     

Cr-Ge-Si Alloys for High-Temperature Structural Applications: Microstructural Evolution

The microstructural evolution of Cr-Si_1?x -Ge _x (0 < x < 15 at. pct) alloys was studied (in annealed state). The quasi-isothermal section of the ternary diagram was assessed by quantitative EPMA analysis. Morphology, phase formation, chemical distribution, and indentation hardness of the alloys were investigated as a function of Ge/Si ratio. The microstructure of all studied alloys consisted of Cr_ss solid solution and A15 Cr₃X intermetallic phases. Substitution of Si by Ge strongly altered the microstructure by transforming the morphology from a lamellar eutectic Cr-Cr₃Si system toward a peritectic one with dispersed Cr_ss phase in A15 matrix. EPMA chemical distribution maps and X-ray diffraction results prove the mutual solubility of Si and Ge in A15 phase by forming Cr₃(Si,Ge) as a complex A15 structure with Cr₃(Si_1?x Ge _x ) composition. Precipitates of the intermetallic phase within the Cr_ss phase was observed in Ge-alloyed samples. Indentation hardness results showed that upon Si-Ge substitution the hardness of both phases was reduced. However, Si substitution by Ge had a stronger influence on the hardness of the solid solution phase than on the intermetallic phase.     

Microstructural Effects in Face-Centered-Cubic Alloys after Small Charge Explosions

Effects on metal targets after an explosion include the following: fracture, plastic deformation, surface modifications, and microstructural crystallographic alterations with ensuing mechanical properties changes. In the case of small charge explosions, macroscopic effects are restricted to small charge-to-target distances, whereas crystal alterations can still be observed at moderate distances. Microstructural variations, induced on gold-alloy disk samples, as compared to previous results on AISI 304Cu steel samples, are illustrated. The samples were subjected to blast-wave overpressures in the range of 0.5 to 195 MPa. Minimum distances and peak pressures, which could still yield observable alterations, were especially investigated. Blast-related microstructural features were observed on the explosion-exposed surface and on perpendicular cross sections. Analyses using X-ray diffraction (XRD) were performed to identify modifications of phase, texture, dislocation density, and frequency of mechanical twins, before and after the explosions. Optical metallography (OM) and scanning electron microscopy (SEM) observations evidenced partial surface melting, zones with recrystallization phenomena, and crystal plastic deformation marks. The latter marks are attributed to mechanical twinning in the stainless steel and to cross-slip (prevalent) and mechanical twinning (possibly) in the gold alloy. This article is based on a presentation given in the symposium “Dynamic Behavior of Materials,” which occurred February 26–March 1, 2007, during the TMS Annual Meeting in Orlando, FL, under the auspices of the TMS Structural Materials Division and the TMS/ASM Mechanical Behavior of Materials Committee.

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Phase Transformations and Microstructural Evolution of Mo-Bearing Stainless Steels

The good corrosion resistance of superaustenitic stainless steel (SASS) alloys has been shown to be a direct consequence of high concentrations of Mo, which can have a significant effect on the microstructural development of welds in these alloys. In this research, the microstructural development of welds in the Fe-Ni-Cr-Mo system was analyzed over a wide variety of Cr/Ni ratios and Mo contents. The system was first simulated by construction of multicomponent phase diagrams using the CALPHAD technique. Data from vertical sections of these diagrams are presented over a wide compositional range to produce diagrams that can be used as a guide to understand the influence of composition on microstructural development. A large number of experimental alloys were then prepared via arc-button melting for comparison with the diagrams. Each alloy was characterized using various microscopy techniques. The expected δ-ferrite and γ-austenite phases were accompanied by martensite at low Cr/Ni ratios and by σ phase at high Mo contents. A total of 20 possible phase transformation sequences are proposed, resulting in various amounts and morphologies of the γ, δ, σ, and martensite phases. The results were used to construct a map of expected phase transformation sequence and resultant microstructure as a function of composition. The results of this work provide a working guideline for future base metal and filler metal development of this class of materials. An erratum to this article can be found at     

Schaeffler-Type Phase Diagram of Ti-Based Alloys

The α(hcp)/β(bcc) phase equilibria of Ti-based multi-component alloys can be described by a Schaeffler-type diagram, where Al and Mo equivalents (Aleq and Moeq) are used. Aleq is thermodynamically defined by the ratio of partial molar free energy changes transfer of one mole of each α forming element and Al from a dilute solution of α to β phases, while Moeq is also deduced by similar thermodynamic quantities of β forming element and Mo. Aleq and Moeq for 40 alloying elements are estimated from the thermodynamic parameters assessed by Kaufman and Murray. It is shown that three types of Ti alloys, i.e., α and near α, α+β, and β alloys, can be exactly classified using Aleq and Moeq. The Ms and β transus temperatures can also be predicted by Aleq and Moeq. The proposed Aleq and Moeq are very useful for alloy design, heat treatment, and microstructural evolution of Ti-based alloys.

     

Phase Transformations in Zirconium and Its Alloys

Abstract

A critical review is presented of all published data on phase transformation in zirconium and its alloys. Topics discussed include: The α → β transformation; factors affecting the stability of α and β zirconium; transformations in the β-phase including ω-phase formation, eutectoidal decomposition and strain induced transformations; transformations in the α-phase including the tempering of martensite and supersaturated alpha, and peritectoid formation of ordered Zr₃Al. The review concludes with examples of phase transformations in commercial zirconium alloys and the effects of these transformations on the mechanical properties of the alloys.

Résumé

Une revue critique de toutes les donnees publiees sur les transformations de phase du zirconium et de ses alliages est présentée. Les sujets traités sont: la transformation α → β, les facteurs influençant la stabilité des phases α et β, les transformations ayant lieu dans la phase β incluant la formation de la phase ω, la décomposition eutectoîdique et les transformations induites par une déformation, les transformations ayant lieu dans la phase α incluant le revenu de la martensite et de la phase α sursaturée, et la formation péritectoïdique d'une phase ordonnee Zr₃Al. Cette revue se termine sur la présentation de plusieurs exemples de transformation de phases des alliages commerciaux de zirconium et l'effet de ces transformations sur les propriétés mécaniques de ces alliages.     

细晶W-Cu合金的高温拉伸力学行为与组织演变

研究了平均晶粒度在0.5μm以下细晶W-40Cu和W-50Cu合金在200~800℃范围内的高温拉伸力学行为,并结合SEM断口形貌分析了材料在高温状态下的断裂形式及其组织变化规律。结果表明:W-Cu合金拉伸强度随温度升高而迅速降低,其延伸率在室温至400℃温度区间时变化不大;当温度大于400℃时,合金延伸率迅速上升。拉伸断口特征表明:在室温条件下,细晶W-Cu合金的断裂主要包括W晶粒的沿晶断裂与Cu相的延性撕裂;温度在400℃时,Cu相开始软化,但合金材料受铜的"中温脆性"影响而使得材料的断裂延伸率变化不大;当温度达到800℃时,材料的断裂方式主要受Cu相的影响而表现出很好的延性断裂。