Papers by Keyword: Microstructural Evolution

Abstract: Using equal channel angular extrusion (ECAE) process, which combine the advantages of grain refinement and induced strain, for preparing semisolid billets is a relatively new Strain Induced Melt Activation(SIMA)method. This paper investigates the remelting and semisolid isothermal holding behavior of Mg-9Al alloy after ECAE processing at 350°C. It has been found that 2 passes ECAE-ed billets are qualified for obtaining spherical grains after remelting and isothermal holding treatment. Increasing the ECAE process from 2 passes to 8 passes has no further significant effects on the size and roundness of the solid particles at the semisolid state. The effects of temperature and isothermal holding time on microstructural evolution have also been investigated. In addition, the solid solution treatment before the ECAE processing affected greatly on microstructural evolution of the alloy during ECAE processing and thus the following remelting and isothermal holding behavior. The Mg17Al12 precipitates were uniformly decomposed from the saturated solid solution in the solutionized samples during ECAE processing. The coalescence of grains together with the self-blocking effect generated more entrapped liquid in the solutionized sample at semisolid state. Moreover, the solid particles of solutionized samples have bigger size and grow faster, which are detrimental to SSM processability.

Abstract: The microstructural evolution of SiCp/AZ61 composites during partial remelting was investigated. The results indicated that the suitable technological parameters of SiCp/AZ61 composites were the reheating temperature of 595°C ~ 600°C and isothermal holding time of 30min ~ 60min. The separation of microstructure of AZ61 matrix was faster than that of SiCp/AZ61 composites during the initial stage. In the meantime, SiCp/AZ61composites hold high stability during remelting process and a fine semi-solid thixoforming microstructure can be obtained with increases in temperature and holding time. In addition, the samples of SiCp/AZ61composites were susceptible to serious deformation and running out above 610°C.

Abstract: The microstructure simulation of spinodal decomposition was carried out in the isothermally-aged Cu-Ni-Fe alloys using the phase field method. The numerical simulation was based on a solution of the Cahn-Hilliard partial differential equation by the finite difference method. The calculated results were compared to those determined by atom-probe field ion microscope analyses of the solution treated and aged alloys. Both the numerically simulated and experimental results showed a good agreement for the concentration profiles and microstructure evolution in the aged Cu-Ni-Fe alloys. The aging process caused the change of initial morphology, iregular and interconected, to an equiaxial shape of the decomposed Ni-rich phase aligned in the elasticallysoftest crystallographic direction <100> of Cu-rich matrix.

Abstract: Tensile creep tests were conducted at 980°C under a constant stress on a single crystal nickel base superalloy. Some tests were interrupted at different stages during the creep process. The strain-time curves indicated that this alloy exhibited conventional primary, steady-state and tertiary stages at this temperature. The transmission electron microscope (TEM) observations of foils taken from the gauge sections of specimens were made to interpret the microstructural evolution that occurred during the creep process. It was found that the γ′ particles were rafted in the direction perpendicular to the applied stress. The acceleration of the creep rate was related to the change of the dislocation density.

Abstract: Thixoextrusion, one of the thixoforming processes, has advantages of high productivity, reduction of the extrusion pressure and cost saving because of the low energy consumption compared with the conventional extrusion processes. For the thixoextrusion, the low liquid fraction (fL<0.3) should be achieved and also the liquid fraction and average grain size should be uniform according to the reheating profile at the desired low liquid fraction. The main emphasis of this study is to investigate the feasibility of thixoextrusion for 2024 Al wrought alloy without additional pretreatment. The results show that the liquid fraction and average grain size were almost uniform with respect to isothermal holding temperature and time. It was very useful for thixoextrusion in terms of process control such as billet temperature control and actual extrusion time.

Abstract: The evolution of microstructure during production and elevated temperature service of type 347 austenitic stainless steel in the temperature range 700-800°C was modelled using commercial software packages such as Thermo-Calc and DICTRA and characterized using various microscopical techniques. The growth and coarsening of niobium carbonitrides and σ- phase were modelled as well as nitrogen uptake. Good agreement between predictions and microstructural observations was found.

Abstract: Fine-grained fully lamellar (FGFL) structures of XD TiAl alloys (Ti-45 and 47Al-2Nb-2Mn+0.8vol.%TiB2) (at.%) were stabilized to varying degrees by different aging treatments. Specimens with and without aging were creep tested at 760°C and 207 MPa. It was found that during creep deformation, degradation of the lamellar structure involving coarsening within the colonies and spheroidization at colony boundaries occurred, forming fine globular structures at the colony boundaries and increasing the creep rate. Aging treatments stabilized the lamellar structure and retarded the coarsening and spheroidization processes during creep deformation. As a result, the aged specimens exhibited lower minimum creep rates and longer creep lives than the unaged specimens. A multiple step aging stabilized the lamellar structure to the greatest extent and suppressed other degradation processes during aging, resulting in the best creep resistance. These results demonstrate that the multiple step aging is the optimal aging condition for stabilizing FGFL XD TiAl alloys.

Abstract: The present paper outlines the effect of small addition of Sc,Ag and Mg on the ageing characteristics and microstructural evolution in some microalloyed Al alloys,such as Al-Cu-Mg, Al-Ag and Al-Li alloys.Small quantities of these microalloying elements can modify the ageing characteristics of these alloys,and significant changes have been found in their microstructural evolution after small microalloying element additions. The observed effects are explained in terms of the atomic behaviour during the early ageing stage,and one of the key factors governing this effect is Sc/Vacancy mechanism.

Abstract: An experimental apparatus using cooling slope and thin metal container has been developed to study controlled rheocasting process. The microstructural characteristics of Al-8%Si industrial alloy processed by this route were determined statistically. The statistics and quantitative analysis of the dispersions of particle’s diameters were used for process optimization. Emphasis is placed on factors which determine the final grain size in a billet. It is suggested that much of the refinement of the microstructure in the process may be caused by temperature fluctuations in solid shell formed at cooling slope. The nuclei which could be formed by melting of eutectic phase in solid shell and transferring by overheated liquid metal to the container mostly survived and contributed to final globular microstructure of a billet and ring type feedstocks.

Abstract: The feasibility study for thixoextrusion of 7075 Al wrought alloys was carried out with respect to reheating profile, isothermal holding temperature and time during the partial remelting through simple partial remelting. The reheating rates were 40
/min and 60
/min. The isothermal holding temperatures were controlled at 609
, 622
, 628
and 632
for 0, 2, 5, 10 and 30 min. The interesting point of this study was that the thixotropic structures of 7075 Al wrought alloy without additional pretreatment could be obtained through simple partial remelting. The average grain size was significantly smaller in the quickly heated specimen. The liquid fraction was increased with increasing isothermal holding temperature and time while the average grain size was inversely proportional to isothermal holding temperature and time. However, there was no big change of liquid fraction and average grain size with respect to isothermal holding time. The important fact that the liquid fraction and average grain size were almost uniform after 5 min is considered very useful for thixoextrusion in terms of actual extrusion time.