Grain size modeling of a Ni-base superalloy using cellular automata algorithm

Nickel-base superalloys are used in highly demanding applications such as energy and aerospace industries. These alloys present good corrosion resistance, weldability and mechanical stability at high temperatures. Numerical methods are commonly used to predict the mechanical and microstructural behavior of heat resistant alloys. The aim of the present work was to model recrystallized grain size evolution under isothermal conditions using the cellular automata (CA) technique. The CA model was applied to simulate hot compression of Inconel 718 nickel-base alloy at 980 °C and 1020 °C. A finite element analysis was conducted to acquire input parameters to the model such as strain and strain rate. Hardening and recovery coefficients were calculated in order to represent the competitive effects during deformation. The influence of local changes of initial grain with fully and partial recrystallized microstructures were simulated by CA and compared with isothermal hot compression results. The model was able to comprehensively predict necklace type microstructures. The average grain size was generally in good agreement with the experimental data.     

Effect of semicrystalline copolymers in solid dispersions of pioglitazone hydrochloride: in vitro-in vivo correlation

Objective: The study was aimed to improve the dissolution and bioavailability of developed stable amorphous solid dispersions (SDs) of pioglitazone hydrochloride (PGH), a poorly water-soluble drug.

Significance: Poor aqueous solubility of PGH was overcome by the design of SDs. Level A correlation demonstrated between in vitro release and bioavailability of PGH, suggest its biowaiver potential.

Methods: The effects of semicrystalline copolymers (poloxamer 407 and gelucire 50/13) and methods of preparations on dissolution behavior, in vivo performance, and stability of PGH SDs were investigated. All the SDs were characterized by FTIR, TGA, DSC, XRD, and SEM.

Results: FTIR and TGA showed the compatibility with the polymers. The significant change in melting pattern of the PGH observed in the DSC thermograms supported by XRD patterns & SEM indicated a change from a crystalline to an amorphous state. Gelucire 50/13 was observed to have greater ability to form SDs than poloxamer 407 in solvent evaporation method (SM). Prevention of recrystallization during storage suggested stability of the formulation. Gelucire 50/13 based SD, prepared by SM remarkably increased the dissolution within 15?min (87.27?±?2.25%) and was supported by dissolution parameters (Q₁₅, IDR, RDR, % DE, f₁, f₂). These SDs showed pH-dependent solubility. In vivo test showed significantly (p?<?.05) higher AUC_0–t and C_max, which were about 3.17 and 4.34 times that of the pure drug respectively.

Conclusion: Gelucire 50/13 was found to be a suitable carrier for SM for preparation of SDs of PGH as evident from increased dissolution and bioavailability.     

铌锆管表面开裂原因分析及热处理行为的研究

铌锆管表面SEM(EDS)表明:成品铌锆管表面存在大量的Si、O元素,在临界加工率下表面剧烈变形,最终导致后续加工严重开裂,通过对成品铌锆管的两因子-五变量温度-时间正交实验设计,结果表明:成品铌锆管在40%~62.5%加工率下1350×60 min退火下,晶粒组织较好,再结晶充分,力学性能稳定,铌锆管的表面开裂倾向较小。     

高速逆流色谱-重结晶法制备高纯度三尖杉宁碱和紫杉醇

通过高效液相色谱测定紫杉醇粗提物中三尖杉宁碱和紫杉醇的纯度分别为33.1%和36.5%,采用正己烷-醋酸乙酯-甲醇-乙醇-水(5∶7∶5∶1∶6.5,v/v)体系,利用高速逆流色谱仪分离得到三尖杉宁碱和紫杉醇馏出物纯度分别为88.4%和89.7%,以50%乙腈进行重结晶后,三尖杉宁碱和紫杉醇纯度分别为98.7%和97.6%。     

Grain-refining and strengthening mechanisms of bulk ultrafine grained CP-Ti processed by L-ECAP and MDF

The microstructural evolution and mechanical properties of ultrafine-grained(UFG)CP-Ti after an inno-vative large-volume equal channel angular pressing(L-ECAP)and multi-directional forging(MDF)were systematically examined using monotonic tensile tests combined with transmission electron microscope(TEM)and electron backscatter diffraction(EBSD)techniques.Substantially refined and homogeneous microstructures were achieved after L-ECAP(8-pass and 12-pass)and MDF(2-cycle and 3-cycle),respec-tively,where the grain size distribution conformed to lognormal distribution.The grain refinement of 450℃L-ECAP is dominated by dynamic recrystallization(DRX)and dynamic recovery(DRV),while that of MDF is dominated by DRX.The iron impurities promote recrystallization by pinning-induced dislocation accumulation so that DRX is prone to occur at iron segregation regions during L-ECAP.The monotonic tensile results show that the strain hardening rate of CP-Ti increases with the decrease of grain size,while the continuous strain hardening ability decreases.The relationship between the average grain size and yield strength is in accordance with Hall-Petch relationship.Meanwhile,the individual strength-ening mechanisms were quantitatively examined by the modified model.The results indicate that the strengthening contribution of dislocation accumulation to yield strength is greater than that of grain refinement.     

Microstructural Characterization of the Shear Bands in Fe-Cr-Ni Single Crystal by EBSD

An investigation has been made into the microstructural characterization of the shear bands generated under high-strain rate （≈10^4 s^-1） deformation in Fe-15%Cr-15%Ni single crystal by EBSD-SEM （electron backscatter diffraction-scanning electron microscopy）, TEM （transmission electron in microscopy） and HREM （high- resolution electron microscopy）. The results reveal that the propagation of the shear band exhibits an asymmetrical behavior arising from inhomogenous distribution in plasticity in the bands because of different resistance to the collapse in different crystallographic directions; The γ-ε-α′phase transformations may take place inside and outside the bands, and these martensitic phases currently nucleate at intersections either between the twins and deformation bands or between the twins and ε-sheet. Investigation by EBSD shows that recrystallization can occur in the bands with a grain size of an average of 0.2μm in diameter. These nano-grains are proposed to attribute to the results of either dynamic or static recrystallization, which can be described by the rotational recrystallization mechanism. Calculation and analysis indicate that the strain rate inside the shear band can reach 2.50×10^6 s^-1, which is higher, by two or three orders of magnitude, than that exerted dynamically on the specimen tested.     

Exploring suitable ZSM-5/MCM-41 zeolites for catalytic cracking of n-dodecane: Effect of initial particle size and Si/Al ratio

In this study, various ZSM-5/MCM-41 micro/mesoporous zeolite composites have been prepared by alkalidesilication and surfactant-directed recrystallization of ZSM-5. The effects of particle size and Si/Al ratio of initial ZSM-5 zeolites on the structure and catalytic performance of ZSM-5/MCM-41 composites are studied. The results of XRD, TEM N₂-adsorption-desorption, NH₃-TPD and in situ FT-IR revealed that ordered hexagonal MCM-41 mesopores with 3-4 nm pore size were formed around ZSM-5 crystals, and the specific surface area and mesopore volume of composites increased with increasing the Si/Al ratio of initial ZSM-5. Catalytic cracking of n-dodecane (550 ℃, 4 MPa) showed that the ZSM-5/MCM-41 composites obtained from the high Si/Al ratio and nano-sized initial ZSM-5 zeolites exhibited superior catalytic performance, with the improvement higher than 87% in the catalytic activities and 21% in the deactivation rate compared with untreated zeolites. This could be ascribed to their suitable pore structure, which enhanced the diffusion of reactant molecules in pores of catalysts.     

Sub-solvus Cellular Recrystallization and P Phase Formation in a Single-Crystal Superalloy Containing Re

Sub-solvus recrystallization behavior of a second-generation single-crystal superalloy has been studied by transmission electron microcopy and scanning transmission electron microcopy. Surface local stress facilitated cellular recrystallization accompanied with formation of twin structure and TCP phase of P during annealing at sub-solvus temperature of 1,100 °C. The precipitation of P phase is considered to be attributed to the coarsening of c0 phase in the recrystallized aggregates which lower the activation energy for atomic migration.     

Strain Softening and Hardening Behavior in AZ61 Magnesium Alloy

The deformation behavior of AZ61 Mg alloy during hot deformation has been investigated in wide temperature and strain rate range by a Gleeble simulator. Specimens are deformed in compression in the temperature range of 523～673 K and at strain rates of 0.001～1 s-1. It is found that the flow curves exhibit a peak and then decrease towards steady-state of classical DRX, which decrease with rising temperature and decreasing strain rate. The deformation behavior of the specimens can be attributed to the occurrence of strain hardening and softening. As stress decreases, the strain hardening rate declines at a fast rate when temperature rises or strain rate decreases. The shapes of θ-σ curves indicate some important features such as subgrain formation, the critical stress, the peak stress and steady stress. The onset of DRX can be determined by the point of inflection on θ-σ or Inθ-σ curves.