The pearlite-austenite growth interface in an Fe-0.8 C-12 Mn alloy

The interphase boundary structure and interface processes at the pearlite-retained austenite growth interface in Fe-0.8 wt% C-12 wt% Mn alloy have been investigated by transmission electron microscopy. Facetting, misfit correcting dislocations, and ledge defects are all observed at the previously assumed disordered boundary. Hot stage electron microscopy revealed that the ledge defects are mobile, indicating the migration of the growth interface occurs by the lateral movement of steps. It is found that the growth ledges are continuous across the growth interfaces of the pearlitic ferrite and cementite. This provides a mechanism by which the interface processes of the two pearlite phases may be coupled.     

Evolution of interaction domain microstructure during spray deposition

An interaction domain, defined in the present article as the region where semisolid, atomized droplets impinge and are collected during spray atomization and deposition, was systematically investigated on the basis of a semisolid metal-forming mechanism. Accordingly, microstructural evolution in the interaction domain was rationalized by quantitative analyses of (1) the solid fraction of semisolid metal, (2) the extent of deformation and deformation strain rate, and (3) the solidification cooling rate. The results demonstrate that the fraction of solid in the interaction domain ranges from 0.5 to 0.8 for the materials studied here: Ni₃Al, Al-6 wt pct Si, and Al-6 wt pct Fe. Moreover, the results show that the semisolid material in the interaction domain experiences a severe deformation during deposition with an associated strain rate of up to 10⁶ s^-1. As a result of this deformation; the solidification structure is modified, and, in particular, any dendritic structure that is present will undergo extensive fragmentation. The severe deformation that is experienced by the interaction domain and the presence of a solidification cooling rate that is on the order of 10 to 10₅ Ks^-1 were proposed to be critical factors that promote the formation of a spheroidal grain morphology during spray atomization and deposition. Experimental support to this suggestion was provided by microstructural observations on Ni₃Al, Al-6Si, and Al-6Fe. In particular, the morphological modification of the primary Si phase that is observed in spray-atomized and spray-deposited Al-6Si was rationalized on the basis of these factors.     

Fe-1.5C-1.5Cr-1.5Al超高碳钢碳化物的球化和力学性能

对Fe-1.5C-1.5Cr-1.5Al超高碳钢碳化物的球化工艺及力学性能进行了研究.扫描电镜观察表明,加入铝,可抑制锻后空冷条件下先共析网状碳化物的析出;利用铝合金化作用和成分不均匀化奥氏体加热控制,提出了2种无形变球化处理工艺:①离异共析等温球化;②预冷淬火 高温回火.2种球化处理工艺均能获得良好的球化组织和良好的综合力学性能:Rm≥1 000 MPa,Re≥700 MPa,A10=10%～14%.拉伸断口具有明显的缩颈,断口形貌呈具有典型的韧窝特征.     

Partition of Mn during the growth of proeutectoid ferrite allotriomorphs in an Fe-1.6 at. pct C-2.8 at. pct Mn alloy

A STEM analysis is made of the Mn distribution around grain boundary allotriomorphs of proeutectoid ferrite in an Fe-1.6 at. Pct C-2.8 at. Pct Mn alloy. Whereas the Mn enriched region is readily observed to extend along the austenite grain boundary, no substantial build-up or depletion of Mn near the ferrite : austenite interface is detected, consistent with the electron probe microanalysis previously reported. In the temperature range where the partition-local equilibrium (P-LE) mode has been proposed to prevail, measured parabolic growth rate constantsfall 1 to 2 orders of magnitude above that predicted from this model, but also below that calculated from the paraequilibrium (PE) model by roughly the same amount. A modification of the theory of grain boundary diffusion-aided growth of precipitates,i.e., the collector/rejector plate mechanism, on the other hand, accounts fairly well for the observed growth kinetics of ferrite allotriomorphs. However, only a slightly better accounting than the P-LE model is provided by this mechanism for the temperature dependence of Mn partition. Data on Ni partition, obtained in an Fe-0.5 at. Pct C-3.1 at. Pct Ni alloy, are also analyzed with the rejector plate model.     

Characterization of spray atomization of 3003 aluminum alloy during linear spray atomization and deposition

Linear spray atomization and deposition is an attractive technique to produce near-net-shape deposits, such as aluminum sheet and strip. In the present study, phase Doppler interferometry (PDI) was used in a backscatter mode to characterize, in situ, the droplet size and velocity distributions during linear spray atomization and deposition of a 3003 aluminum alloy. The PDI measurements were obtained along axes corresponding to the direction parallel to the nozzle slit and to the direction perpendicular to the slit. The PDI results delineate the temporal and spatial distribution of the droplet size and velocity during the metal spray. Both point and “line” measurements were obtained and are reported. The line measurements resulted from the integration of measurement made along a line scan obtained in real time (i.e., not ensemble averaged). Postrun analysis of the droplet size distribution using laser diffraction and sieving techniques is also reported. The PDI point measurements revealed that droplet size and velocity distribution were relatively invariant with time. The line measurements of droplet velocity showed that the droplet axial velocity exhibits a bimodal behavior, which becomes more apparent with increasing atomizing gas pressure, a result of droplet recirculation inside the spray chamber. In addition, the peak in the droplet axial velocity distribution increased as atomizing gas pressure increased. The line characterization also showed that the droplet size distribution became more homogeneous with increasing gas pressure, and that the distribution characteristic diameters of droplets decreased consistently with increasing gas pressure. Postrun characterization of the droplet size distribution of the entire metal spray using diffraction and sieving methods indicated that the mass (volume) median diameter D ₅₀ and the Sauter mean diameter (SMD) D ₃₂ decreased with increasing gas pressure in a manner consistent with PDI results.     

Growth Kinetics of Proeutectoid Ferrite in Fe-0.1C-1.5Mn-1Si Quaternary and Fe-0.1C-1.5Mn-1Si-0.2Al Quinary Alloys

The growth kinetics of proeutectoid ferrite in the early stages of transformation were studied in Fe-0.1C-1.5Mn-1Si (mass pct) quaternary and Fe-0.1C-1.5Mn-1Si-0.2Al quinary alloys. The observed kinetic transition temperatures from partitioned slow growth to unpartitioned fast growth of ferrite in both alloys are in good agreement with predictions using a local equilibrium model for multicomponent systems. The measured parabolic growth rate constants were smaller than those calculated assuming paraequilibrium in the unpartitioned growth region, but the difference between the measured and the calculated growth rate constants gradually diminished with a decreasing reaction temperature. The dissipation of driving force, derived from the diffusion of the substitutional solute within the transformation interface, possibly constitutes a major part of the discrepancy between the measured and the calculated growth kinetics.     

Cu precipitation in a prestrained Fe-1.5 wt pct Cu alloy during isothermal aging

The control of Cu precipitation at low temperatures, e.g., bake hardening of Cu bearing steels, has recently attracted considerable attention due to the potential of achieving good formability and high strength. An Fe-1.5 wt pct Cu alloy, solution treated and 10 pct prestrained, exhibits a two-step age-hardening behavior, i.e., a smaller, but substantial hardening around 200 °C to 300 °C and a major hardening around 500 °C, while only the latter hardening occurs in undeformed specimens. The precipitation behavior of nanoscale Cu particles or bcc Cu clusters that plays a major role in age hardening was simulated by Cahn-Hilliard nonclassical nucleation theory and the Langer-Schwartz model. Simulation results are compared with the distribution of Cu particles observed under three-dimensional atom probe field ion microscope (3-D APFIM) and transmission electron microscope (TEM), and age hardening behavior as well. The increase in hardness in prestrained specimens at low temperatures (≤400 °C) can be ascribed to Cu particles nucleated preferentially at dislocations or to Cu particles that were formed in the matrix as early as at dislocations presumably due to excess vacancies introduced by prestraining.     

粉末冶金60Al–30Fe–10Bi合金在高速冲击下的变形行为与微观组织演化

通过粉末冶金技术制备60Al–30Fe–10Bi合金药型罩材料,采用分离式霍普金森(Hopkinson)压杆测试了铝基合金在变形温度从25~450℃和应变速率从1300~7000 s~(-1)下的动态冲击压缩力学性能,研究了其动态变形前后微观组织的演变规律和断裂机制。结果表明:60Al–30Fe–10Bi合金在动态冲击压缩下的流变应力主要依赖于温度的变化,流变应力随变形温度的升高而降低,而应变速率对流变应力的影响不大;合金在25℃、1300 s~(-1)下的高速冲击载荷作用下的失效原因是发生了绝热剪切。     

The microstructural evolution of near beta alloy Ti-10V-2Fe-3Al during subtransus forging

The microstructural evolution of titanium alloys during subtransus isothermal forging (IF) has been effectively demonstrated using a testing methodology developed at Imperial College London. Double truncated cone specimen geometries were isothermally deformed at near β transus temperatures to obtain microstructural information for a range of strains within a single specimen. The methodology was applied to the near β alloy, Ti-10V-2Fe-3Al, to determine the effect of strain, strain rate, and IF subtransus temperature on microstructural evolution. An erratum to this article is available at .     

The tempering of martensite in an Fe-1.5 pct N alloy

The tempering of Fe 1.5 pct N martensite has been studied at temperatures up to 300°C using X-ray and electron microscope techniques. Stage 1 decomposition occurs below 270°C by the general precipitation, resembling spinodal morphology, of fine τa^" (Fe₁₆ N₂) lamellae on 001 habit planes in both matrix and twin crystals of the partially 112 twinned martensite plates. Yet, gaged by changes in the X-ray spectrum, the reaction is discontinuous, the tetragonal martensite doublets decaying in intensity without change in their Bragg positions. The anomaly and the failure to detect by electron microscopy regions exhibiting fractional stages of the fine scale α^’ → α + α^" reaction is attributed to its occurrence at different times in different martensite (or parts of martensite) plates. It is believed that transformation occurs in this manner because the nucleation of coherent α^" plates is controlled by the prevailing internal stress field. Thus the time exponent “n” for the reaction decays from a normal value between 1 and 0.67 to less than 0.3 as stress relief by recovery dominates the more protracted stages of the reaction. Above 200°C the more stable nitride γ^’ (Fe₄N) forms at an increasing rate as plates on 012 habit planes, accompanied by marked softening.     

Alloying Element Partition and Growth Kinetics of Proeutectoid Ferrite in Hot-Deformed Fe-0.1C-3Mn-1.5Si Austenite

Alloying element partition and growth kinetics of proeutectoid ferrite in deformed austenite were studied in an Fe-0.1C-3Mn-1.5Si alloy. Very small ferrite particles, less than several microns in size, were formed within the austenite matrix, presumably at twin boundaries as well as at austenite grain boundaries. Scanning transmission electron microscopy–energy-dispersive X-ray (STEM-EDX) analysis revealed that Mn was depleted and Si was enriched in the particles formed at temperatures higher than 943 K (670 °C). These were compared with the calculation of local equilibrium in quaternary alloys, in which the difference in diffusivity between two substitutional alloying elements was assumed to be small compared to the difference from the carbon diffusivity in austenite. Although the growth kinetics were considerably faster than calculated under volume diffusion control, a fine dispersion of ferrite particles was readily obtained in the partition regime due to sluggish growth engendered by diffusion of Mn and Si.     

The improvement of cryogenic mechanical properties of Fe-12 Mn and Fe-8 Mn alloy steels through thermal/mechanical treatments

An investigation has been made to improve the low temperature mechanical properties of Fe-8Mn and Fe-12Mn-0.2 Ti alloy steels. A reversion annealing heat treatment in the two-phase (α+ γ) region following cold working has been identified as an effective treatment. In an Fe-12Mn-0.2Ti alloy a promising combination of low temperature (-196°C) fracture toughness and yield strength was obtained by this method. The improvement of properties was attributed to the refinement of grain size and to the introduction of a uniform distribution of retained austenite (γ). It was also shown that an Fe-8Mn steel could be grain-refined by a purely thermal treatment because of its dislocated α martensitic structure and absence of ε martensite. As a result, a significant reduction of ductile to brittle transition temperature was obtained. formerly with the Lawrence Berkeley Laboratory, University of California.     

Plastic-flow and microstructure evolution during hot deformation of a gamma titanium aluminide alloy

The hot workability of a near gamma titanium aluminide alloy, Ti-49.5Al-2.5Nb-1.1Mn, was assessed in both the cast and the wrought conditions through a series of tension tests conducted over a wide range of strain rates (10⁻⁴ to 10⁰ s⁻¹) and temperatures (850 °C to 1377 °C). Tensile flow curves for both materials exhibited sharp peaks at low strain levels followed by pronounced necking and flow localization at high strain levels. A phenomenological analysis of the strain rate and temperature dependence of the peak stress data yielded an average value of the strain rate sensitivity equal to 0.21 and an apparent activation energy of ∼411 kJ/mol. At low strain rates, the tensile ductility displayed a maximum at ∼ 1050 °C to 1150 °C, whereas at high strain rates, a sharp transition from a brittle behavior at low temperatures to a ductile behavior at high temperatures was noticed. Dynamic recrystallization of the gamma phase was the major softening mechanism controlling the growth and coalescence of cavities and wedge cracks in specimens deformed at strain rates of 10⁻⁴ to 10⁻² s⁻¹ and temperatures varying from 950 °C to 1250 °C. The dynamically recrystallized grain size followed a power-law relationship with the Zener-Hollomon parameter. Deformation at temperatures higher than 1270 °C led to the formation of randomly oriented alpha laths within the gamma grains at low strain levels followed by their reorientation and evolution into fibrous structures containing γ + α phases, resulting in excellent ductility even at high strain rates.     

Growth Kinetics of Proeutectoid Ferrite in an Fe-0.09C-1.5Mn-0.2Si Steel

Growth kinetics of proeutectoid ferrite, including grain boundary face nucleated ferrite, grain boundary edge nucleated ferrite allotriomorph and intragranular ferrite idiomorph, were experimentally measured in an Fe- 0.09C-1.5Mn-0.2Si steel and compared with theoretical calculation in local equilibrium and paraequilibrium modes. Grain boundary edge nucleated ferrite exhibited larger growth rate than grain boundary face nucleated ferrite and in- tragranular ferrite idiomorph. Experimental kinetics of proeutectoid ferrite was within the window defined by the lo- cal equilibrium and paraequilibrium limits. A transition of growth kinetics from paraequilibrium to local equilibrium was observed in the temperature range of 650--750 ℃, which can be explained in terms of solute drag.     

Effect of intermediate heat treatment on microstructure and texture evolution of continuous cast Al-Mn-Mg alloy sheet

The microstructure and crystallographic texture evolution of continuous-cast, hot-rolled Al-Mn-Mg alloy sheet during cold rolling and subsequent annealing was investigated. All specimens cut from the as-received sheet were cold rolled and subsequently annealed, with some of these specimens receiving an intermediate heat treatment (IHT) prior to cold rolling. It was found that the degree of deformation and temperature of the annealing had a significant effect on the final grain size and texture of the sheet specimens, respectively. Furthermore, the IHT altered the development of the microstructure and texture of the final sheet specimens when compared to similarly produced specimens without it. For the sheet specimens without the IHT, a severely elongated grain structure was found in which the texture was dominated by a strong P orientation {011}<566>, despite the fact that the specimen was completely recrystallized. In contrast, specimens receiving the same cold rolling and annealing conditions but with the IHT had an equiaxed grain structure with a sharp Cube orientation {001}<100>. Counterbalancing the deformation textures from rolling with a sharp Cube orientation from annealing may lead to reduced earing behavior of CC Al-Mn-Mg alloy sheet products during deep drawing applications.     

Relationship between microstructure and mechanical properties of a spinodally decomposing Cu-15Ni-8Sn alloy prepared by spray deposition

A spinodally decomposing alloy of composition Cu-15 wt pct Ni-8 wt pct Sn processed by Osprey spray deposition was examined and the principal factors determining strength and ductility studied. Strengthening contributions are shown to arise from grain-size reduction by controlled recrystallization, and from the internal stress fields created by spinodal decomposition. Discontinuous precipitation takes place after the completion of spinodal decomposition and is partly responsible for the decrease of yield stress after such prolonged aging. At the same time, there is some loss of ductility, but the discontinuous precipitation is not believed to be the main cause. Instead, it is the appearance of additional phases, such as Ni₃Sn, in the matrix that is considered to be the main cause of the brittleness.     

微波烧结Fe-2Cu-0．6C的性能与组织研究

研究了Fe-2Cu-0.6C粉末冶金材料在1150℃时的微波烧结的性能,并与常规烧结试样进行了对比,分析了其显微组织.结果表明:与常规烧结相比,微波烧结时间短,能量利用率高,微波烧结试样的性能有明显的提高,试样密度达到7.20g/cm3,硬度75HRB,抗拉强度为413.90MPa,伸长率6.0%.金相显微和扫描电镜断口分析的结果表明:微波烧结试样的组织主要是由铁素体、片状和粒状珠光体以及极少量的孔隙组成的.大量珠光体的存在能显著改善其力学性能.由断口分析可知,常规烧结试样属于脆性穿晶断裂,而微波烧结试样为脆性穿晶断裂和韧窝型的穿晶韧性断裂的混合型断裂.