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Journal ArticleDOI

Characteristics of superplasticity domain in the processing map for hot working of an Al alloy 2014---20vol.%Al2O3 metal matrix composite


Abstract: The processing map for hot working of Al alloy 2014-20vol.%Al2O3 particulate-reinforced cast-plus-extruded composite material has been generated covering the temperature range 300-500 degrees C and the strain rate range 0.001-10 s(-1) based on the dynamic materials model. The efficiency eta of power dissipation given by 2m/(m + 1), where m is the strain rate sensitivity, is plotted as a function of temperature and strain rate to obtain a processing map. A domain of superplasticity has been identified, with a peak efficiency of 62% occurring at 500 degrees C and 0.001 s(-1). The characteristics of this domain have been studied with the help of microstructural evaluation and hot-ductility measurements. Microstructural instability is predicted at higher strain rates above (ls(-1)) and lower temperatures (less than 350 degrees C).
Topics: Strain rate (58%), Superplasticity (56%), Hot working (54%), Metal matrix composite (51%)
Citations
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Journal ArticleDOI
J.C. Shao1, B.L. Xiao1, Quanchao Wang1, Z.Y. Ma1  +2 moreInstitutions (1)
Abstract: Constitutive flow behavior and hot workability of the powder metallurgy processed 20 vol.%SiC(P)/2024Al composite were investigated using hot compression tests. The modified Arrhenius-type constitutive equations were presented with the values of material constants in consideration as a function of strain. Dynamic material model (DMM) and modified DMM were used to construct the power dissipation efficiency maps, and Ziegler's instability criterion and Gegel's stability criterion were used to build instability maps. The presence of finer SiC(P) and more boundaries resulting from smaller 2024Al powders shifted the dynamic recrystallization domain of the 2024Al matrix to higher strain rate and lower temperature ranges and decreased the peak value of power dissipation efficiency. Large instable regions were found in the form of flow localization and cavitations located at the matrix/SiC(P) interfaces and within the SiC(P) clusters. By comparison, the Gegel's stability criterion was more sensitive to the instability zones than the Ziegler's instability criterion for this material. (c) 2010 Elsevier B.V. All rights reserved.

60 citations


Journal ArticleDOI
Abstract: A simple instability condition based on the Ziegler's continuum principles as applied to large plastic flow, is extended for delineating the regions of unstable metal flow during hot deformation of 2014 Al–20 vol% Al2O3 metal matrix composite. The optimum hot working conditions for this material are suggested.

43 citations


Journal ArticleDOI
Abstract: The hot working characteristics of 2124 Al alloy matrix composites reinforced with 0, 5, 10, 15, and 20 vol pct of SiC particulate, produced by the powder metallurgy route, were studied using processing maps. The maps based on the dynamic materials model were generated from the flow stress data obtained from hot compression tests, carried out at strain rates ranging from 0.001 to 10 s−1 and temperatures ranging from 300°C to 525°C. All the compositions studied exhibited domains of dynamic recrystallization (DRX) and superplasticity. Flow instabilities were found at higher strain rates and lower temperatures. The composite with 10 vol pct SiC showed a tendency for abnormal grain growth at lower strains, which manifested itself as a shift in the DRX domain to lower strain rates and the disappearance of the superplasticity domain.

32 citations


Journal ArticleDOI
Abstract: The constitutive relationships between stress, strain rate, and temperature were analyzed to obtain a unified description of creep and plasticity of aluminum alloys produced by powder metallurgy and of aluminum-based metal-matrix composites. As both classes of materials are characterized by the existence of a threshold stress (σ 0), a unified description of creep (low strain-rate regime) and plasticity (high strain-rate regime) was obtained by substituting the conventional power-law equation with the sinh relationship, where the applied stress is replaced by the difference between the applied stress and a threshold stress. The stress exponent was n = 3 or 5, and the activation energy was equivalent to the activation energy for self-diffusion or to the activation energy for diffusion of solute elements in the matrix. The model was applied to an unreinforced alloy (2014PM) and a composite (6061 + 20 pct Al2O3) tested in tension (under constant load) or torsion (at constant strain rate) in the temperature range between 300 °C and 500 °C. The results were compared with data available in the literature.

28 citations


Journal ArticleDOI
Zhiye Huang1, Zhiye Huang2, Xingxing Zhang1, Bolyu Xiao1  +1 moreInstitutions (2)
Abstract: Hot deformation behavior of a stir cast and hot extruded 14 vol% SiCp/2014Al composite was studied at temperatures from 355 to 495 °C and strain rates from 0.001 to 1 s−1, including microstructure evolution and damage formation. Stress-strain rate fitting was optimized to construct accurate processing maps based on modified dynamic materials model (MDMM). In addition, the strain rate sensitivity maps were plotted, indicating more significant effect of temperature on deformation mechanism than strain rate. The dissipation efficiency versus temperature curves indicated: (i) a transition from dynamic recovery (DRV) to dynamic recrystallization (DRX) at 400 °C; (ii) occurrence of dynamic grain growth (DGG) at 400–440 °C; (iii) existence of equicohesive point (Teq) of 450 °C (∼0.8 Tm) above which grain boundaries weakened and contributed to plastic deformation. The particular fluctuation of temperature sensitivity at 440 °C was caused by an abnormal grain growth.

20 citations


References
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Journal ArticleDOI
Abstract: A new method of modeling material behavior which accounts for the dynamic metallurgical processes occurring during hot deformation is presented. The approach in this method is to consider the workpiece as a dissipator of power in the total processing system and to evaluate the dissipated power co-contentJ = ∫o σ e ⋅dσ from the constitutive equation relating the strain rate (e) to the flow stress (σ). The optimum processing conditions of temperature and strain rate are those corresponding to the maximum or peak inJ. It is shown thatJ is related to the strain-rate sensitivity (m) of the material and reaches a maximum value(J max) whenm = 1. The efficiency of the power dissipation(J/J max) through metallurgical processes is shown to be an index of the dynamic behavior of the material and is useful in obtaining a unique combination of temperature and strain rate for processing and also in delineating the regions of internal fracture. In this method of modeling, noa priori knowledge or evaluation of the atomistic mechanisms is required, and the method is effective even when more than one dissipation process occurs, which is particularly advantageous in the hot processing of commercial alloys having complex microstructures. This method has been applied to modeling of the behavior of Ti-6242 during hot forging. The behavior of α+ β andβ preform microstructures has been exam-ined, and the results show that the optimum condition for hot forging of these preforms is obtained at 927 °C (1200 K) and a strain rate of 1CT•3 s•1. Variations in the efficiency of dissipation with temperature and strain rate are correlated with the dynamic microstructural changes occurring in the material.

945 citations


Journal ArticleDOI
Rishi Raj1Institutions (1)
Abstract: A fracture initiation map is developed which should be useful in fast forming operations at strain rates greater than about 10-3 s-1 at elevated temperatures. Two types of cavitation mechanisms, one pertaining to cavity formation at second phase particles, as in ductile fracture, and the other pertaining to wedge type microcracking at grain boundaries, are considered. In addition, dynamic recrystallization and adiabatic heating effects are considered. When these concepts are applied to aluminum, it is shown that there may be an intermediate region in the strain rate and temperature field in which neither of these mechanisms should operate and within which the material would, therefore, be safe from fracture.

290 citations


Journal ArticleDOI
Abstract: Mise en evidence, dans un alliage commercial d'aluminium renforce par des whiskers de SiC (SiC/2124 Al) soumis a des traitements thermomecaniques adequats, d'un comportement de type superplastique lors d'une deformation isotherme a des taux de deformation relativement elevees

205 citations


Journal ArticleDOI
Mu Yeh Wu1, Oleg D. Sherby1Institutions (1)
Abstract: Metal matrix composites can be made to flow superplasticity under thermal cycling conditions of uniaxial testing. This mechanical response is attributable to the generaration of internal stresses during thermal cycling arising from the difference in thermal expansion coefficient of the constituents making up the composite. The net effect of the internal stresses is to assist plastic flow of the matrix material in the direction of the applied stress resulting in an increase in the strain rate sensitiviy exponent as the magnitude of the applied stress decreases. At low applied stresses the strain-rate sensitivity exponent is unity and the matrix behaves as a Newtonian fluid. A silicon carbide whisker-reinforced aluminum alloy (20% SiC fibers in 2024 aluminum) is shown to exhibit tensile ductilities in the order of 300% when deformed under thermal cycling conditions ( 100 ⇇ 450° C ) and at low stresses ( σ ⋍ 20 MPa ).

113 citations


Journal ArticleDOI
Murray W. Mahoney1, Amit K. Ghosh1Institutions (1)
Abstract: The superplastic properties of a rapidly solidified, high strength P/M Al alloy and the same alloy reinforced with SiC particulates (SiCp) have been studied. To prepare superplastic test materials, a matrix alloy powder of composition 7.2Zn-2.4Mg-2Cu-0.2Zr-0.12Cr-0.2Co (Kaiser PM-64) and the powder mixed with 10 to 20 vol pct SiCp (~5 μm diameter) were thermomechanically processed to very fine equiaxed grain structures of ~6 μm and ~8 μm, respectively. Superplasticity in these materials was evaluated by characterizing (1) high temperature stability, (2) dynamic grain growth, (3) strain rate sensitivity, (4) flow stress behavior, (5) cavitation and cavitation control, and (6) total superplastic strain. It was observed that the PM-64 alloy could achieve a total elongation of over 800 pct, while the SiCp reinforced alloy could attain an elongation greater than 500 pct before failure. Also, it was shown that with the use of hydrostatic pressure during superplastic flow, cavitation could be controlled. Observations were made of the effect SiCp reinforcement particles had on the superplastic flow stress behavior. Interpretations are proposed to explain the role of particulates during superplastic straining.

89 citations


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