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The role of matrix microstructure in the creep behaviour of discontinuous fiber-reinforced AZ 91 magnesium alloy
No full textConstant stress tensile creep tests were conducted to failure at temperatures of 423 and 473 K on an AZ 91 (Mg–9wt.%Al–1wt.%Zn) alloy reinforced with 20 vol.% Al2O3 short fibres and on an unreinforced AZ 91 matrix alloy. The creep resistance of the reinforced material was considerably improved by comparison with the matrix alloy. A microstructural investigation revealed that the most frequent morphology of the ?-phase precipitates in the composite is continuous Mg17Al12 platelets. Detailed investigations using transmission electron microscopy indicate that the matrix microstructure has no significant influence on the creep properties of these two materials. This result confirms the proposal that the creep strengthening of the composite is controlled by an effective load transfer between the matrix and the fibres
Microstructural processes in creep of an AZ 91 magnesium-based composite and its matrix alloy
No full textConstant stress tensile creep tests were conducted on an AZ 91–20 vol.% Al2O3 short fiber composite and on an unreinforced AZ 91 matrix alloy. The creep resistance of the reinforced material is shown to be considerably improved compared with the matrix alloy. The creep strengthening arises mainly from the effective load transfer between plastic flow in the matrix and the fibers. Microstructural investigations by TEM revealed good fiber–matrix interface bonding during creep exposure. The microstructures of the AZ 91 alloy and its composite were similar with regard to two types of β-phase precipitates; the enhanced precipitation of the Mg17 (Al, Zn)12 phase on the fibers is promoted by heterogeneous nucleation due to the Al enrichment of the matrix near to the alumina fibers
Flow mechanisms in creep of short fibre AZ91 alloy-based composite
No full textA comparison between the creep characteristics of an AZ91 magnesium alloy reinforced with 20 vol.% Al2O3 short fibres and an unreinforced AZ91 matrix alloy shows that the creep resistance of the reinforced material is considerably improved compared to the matrix alloy. It is suggested that the creep strengthening in the composite arises mainly from the existence of a threshold stress and the load transfer effect. The values of the threshold stress in the creep of the composite at temperatures in the range from 373 to 673 K were estimated using standard methods. It is proposed that the threshold stress arises from an attractive interaction between mobile dislocations and Mg17(Al, Zn)12 precipitates
Factors influencing creep flow and ductility in ultrafine-grained metals
No full textThe creep behaviour of high purity aluminium and copper and their Al–0.2 wt%Sc and Cu–0.2 wt%Zr alloys was examined after processing by equal-channel angular pressing (ECAP) with an emphasis on creep ductility and the ECAP microstructural homogeneity. It was found that, under the same loading conditions, the creep ductility of the ultrafine-grained materials processed by ECAP continually increases with increasing numbers of ECAP passes. A detailed quantitative microstructural study was conducted using the electron backscatter diffraction (EBSD) methods. This analysis revealed that, with increasing numbers of ECAP passes, the mutual misorientation of neighbouring subgrains grows and the subgrains continuously transform to grains having high-angle grain boundarie
Creep processes in magnesium alloys and their composites
No full textA comparison is made between the creep characteristics of two squeeze-cast magnesium alloys (AZ 91 and QE 22) reinforced with 20 vol pct Al2O3 short fibers and the unreinforced AZ 91 and QE 22 matrix alloys. The results show the creep resistance of the reinforced materials is considerably improved by comparison with the unreinforced matrix alloys. It is suggested that creep strengthening in these short-fiber composites arises primarily from the existence of a threshold stress and the effect of load transfer. By testing samples to failure, it is demonstrated that the unreinforced and reinforced materials exhibit similar times to failure at the higher stress levels. A detailed microstructural investigation by transmission electron microscopy (TEM) reveals no substantial changes in matrix microstructure due to the presence of the reinforcement. This suggests that direct composite strengthening dominates over indirect effects
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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