Influence of equal-channel angular extrusion on impact toughness of aluminum and brass at room and low temperatures
© Wang et al.; licensee Springer 2014
Received: 28 August 2014
Accepted: 18 September 2014
Published: 15 October 2014
Equal-channel angular extrusion is a severe plastic deformation process that can be used for grain refinement to improve material properties of bulk metals. In this paper, the effect of equal-channel angular extrusion on the impact toughness of aluminum 1100 and brass C26000 was investigated.
Brass and aluminum materials were extruded in two and four passes using two equal-channel angular extrusion processing routes. Specimens were tested for hardness and Charpy impact toughness. Microstructure and fractography were examined.
The results showed the hardness remained almost constant after two passes for both brass and aluminum. Impact energy of brass after two passes decreased due to increase in dislocation density whereas for aluminum it remained almost constant after four passes due to the formation of ultrafine grains in addition to deformation/dislocation structures. Impact energies of specimens tested at room temperature and low temperature (−70°C) were almost the same due to their face-centered crystal structure. It was also found that the impact toughness of a specimen with a non-distorted notch surface is higher than that of a specimen with a distorted notch surface.
It is evident from the present study that the number of passes determines the extent of ultrafine grain structure that is known to increase impact toughness of equal-channel angular extrusion processed materials. All of these observed characteristics can influence material and process selections in practical design applications.
Equal-channel angular extrusion (ECAE), twist extrusion, high pressure torsion, and friction stir processing are all severe plastic deformation (SPD) processes that can result in ultrafine grain (UFG) microstructures in metals. As UFG materials are known to exhibit enhanced mechanical properties, SPD processes have been extensively studied (Valiev and Langdon 2006). In ECAE, the workpiece/billet is pressed through a die with intersecting channels having the same cross section. The material is subjected to simple shear at the intersecting plane of the two channels. As the billet can be extruded repeatedly in successive passes, ECAE has the advantage of producing large cumulative strain without changing the cross section of the workpiece (Rebhi et al. 2009). The workpiece possesses fine grains with enhanced properties that can have various design applications.
For multiple-pass ECAE, there are different routes that can be used to introduce different slip systems. A commonly used designation of the processing routes was presented in Rebhi et al. (2009) and Iwahashi et al. (1997a). In route A, the workpiece is pressed without rotation between passes. In route BA, the workpiece is rotated 90° in alternate directions, clockwise then counterclockwise or vice versa, between consecutive passes. In route BC, the workpiece is rotated 90° in the same direction, either clockwise or counterclockwise, between consecutive passes. In route C, the workpiece is rotated 180° between passes. With different processing routes, different microstructures can be produced from the same initial workpiece. Research has shown that the large equiaxed grains in a billet can be significantly distorted during ECAE. The influence of various ECAE routes on the texture and plastic anisotropy of the material has been investigated (Iwahashi et al. 1997a; Xiao et al. 2012; Li et al. 2004; Stolyarov et al. 2001; Xu et al. 2008).
It is known that the strength of a polycrystalline material is related to the grain size. As the yield strength increases with a decreased grain size, ultrafine grain materials produced from ECAE have gained significant attention. While high strength materials resulting from straining generally exhibit low ductility, ECAE processing typically leads to a reduction in the ductility which is less than that resulting from conventional bulk forming processes such as rolling, drawing, and extrusion. It is also reported that most of the materials processed by ECAE have a relatively low ductility, but they usually demonstrate significantly higher strength than their coarser-grained counterparts (Valiev and Langdon 2006). Physical and mechanical properties, such as hardness, yield strength, and the strain hardening exponent, have been investigated (Reihanian et al. 2008; Firstov et al. 2003). Other attractive characteristics such as superelasticity, wear resistance, and enhanced fatigue behavior of UFG materials have also been observed (Li and Cheng 2010; Crone et al. 2001; Höppel et al. 2006).
Impact toughness, while not a direct measure of the fracture toughness, is commonly used to evaluate the relative fracture behavior of engineering materials. Experimental results regarding the effect of ECAE on the impact toughness are very limited. Although it was reported that the impact toughness of two-phase Zn-40Al alloy can be improved through ECAE (Purcek et al. 2008), it is noticed that the tested material also exhibited strain softening that is not common in cold forming of metals. It has also been reported that when the grain size is below a critical value, the dominating deformation and fracture mechanisms can change (Li and Ebrahimi 2005). The impact toughness of nanostructured Ti processed by SPD was enhanced with decreasing testing temperature. The phenomenon is attributed to the small fracture dimples at lower temperatures (Stolyarov et al. 2006). However, it is not known if any other UFG materials have a similar behavior. The present study assesses the hardness and the impact toughness of aluminum 1100 and brass C26000 before and after ECAE processing. Charpy impact tests of specimens prepared from two ECAE routes were conducted at two different testing temperatures. The fractographs were examined, and the fracture morphology of the specimen was discussed.
z plane (top)
C, two passes
z plane (top)
C, two passes
y plane (side)
BC, four passes
z plane (top)
z plane (top)
C, two passes
z plane (top)
C, two passes
y plane (side)
Microstructural evaluation, hardness measurement, and impact toughness test
Aluminum and brass samples were cut from the middle sections of the billets; specimens for metallographic observation were prepared using standard polishing techniques and etched with Keller's reagent (2.5 mL HNO3, 1.5 mL HCl, 1.0 mL HF, and 95 ml water) for aluminum, and ammonium persulfate solution (10% in water) for brass. An optical microscope Leica DM750P (Leica Microsystems, Wetzlar, Germany) was used to examine the microstructures of the materials before and after ECAE.
Hardness tests were performed on the samples, also from the center of the billets, using a Buehler Macromet 3 hardness tester (Buehler, Lake Bluff, USA). The hardness test was repeated three times for each material parameter shown in Table 1. The hardness tests were performed on both the top (z plane) and side (y plane) of the billets. For aluminum, HR15T was used, while HRA was used for brass.
Charpy impact tests were conducted on the notched specimen using a Tinius Olsen Model 104 Impact Tester (Tinius Olsen, Inc., Horsham, USA). The specimens were tested at room temperature (24.7°C) and low temperature (−70°C). For low temperature testing, the specimens were submerged in methanol in a Kinetics Multi Cool MC480A1 chiller for a minimum of 5 min. In each test, the time to extract and place the specimen in position and strike the specimen was less than 5 s. SEM examination of the fractured impact specimens was carried out using HITACHI-4500 (Hitachi High Technologies America, Inc., Pleasanton, USA) and LEO 1430 VP scanning electron microscopes.
Results and discussion
Microstructure evolution and its influence on impact toughness
It is clear from the micrographs that microstructural evolution leading to fully ultrafine grains (UFG, grain size less than 1 μm) typical of the ECAE process was not achieved after a maximum of two passes in brass. After one and two passes, dislocation cell structure with low angle grain boundaries was formed leading to increase in microstrain (dislocation density) and is evident from impact energy values (presented in Figure 7). Microstructure after ECAE processing with two passes has a predominant deformed structure, due to increase in dislocation density, as demonstrated in Figure 6 (Georgy et al. 2004; Iwahashi et al. 1997b).
ECAE-processed aluminum with severe plastic deformation would demonstrate mostly equiaxed grains and well-defined, sharp grain boundaries that are absent in our present investigation. It has been reported that large grains in UFG materials contain dislocations while grains smaller than a certain size are dislocation free (Valiev et al. 2000; Zhu et al. 2003). Based on the impact energy results (presented in Figure 8), aluminum material used in the present investigation requires more than four passes (to have severe enough plastic deformation) to achieve completely dislocation-free UFG structures.
To summarize, the hardness almost remained constant after two and four passes during ECAE processing as demonstrated for samples A2, A3, and A4 (for aluminum) and after two passes (B2, B3) for brass in Figure 9a,b. This is attributed mainly to the combination of deformation and UFG structures in aluminum and only deformation structure in the case of brass.
From Figures 7 and 8, it can be observed that the impact energies of specimens tested at room temperatures were almost identical to the specimens tested at low temperatures. This is due to the well-known non-ductile-brittle transition temperature (DBTT) behavior of face-centered cubic (FCC) metals like aluminum and their alloys and the FCC crystal structure of alpha phase brass.
Effect of notch surface on impact toughness
ECAE-processed aluminum demonstrated deformation/dislocation structures after two or three passes but showed a combination of dislocation and UFG structures as evident from the impact energy values.
In the case of brass, ECAE processing with two passes resulted in decrease in impact energy due to dominant deformation/dislocation structures as a result of increase in dislocation density/microstrain.
There is no appreciable difference in impact energy for materials tested in room temperature and in low temperature. The non-ductile brittle transition temperature behavior of FCC metals/alloys (both aluminum and brass in the present study) was observed.
The impact toughness can be affected by the surface where the notch was machined. It was found that the specimens with notch machined on an undistorted surface have higher impact energy than the specimens with notch on a distorted and restored surface.
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