The correlation between flow behavior and microstructural evolution of 7050 aluminum alloy

Materials Science and Engineering A 530 (2011) 559–564

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The correlation between flow behavior and microstructural evolution of 7050 aluminum alloy J. Luo ∗ , M.Q. Li, B. Wu School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, PR China

a r t i c l e

i n f o

Article history: Received 13 June 2011 Received in revised form 24 September 2011 Accepted 10 October 2011 Available online 17 October 2011 Keywords: 7050 aluminum alloy Isothermal compression Flow stress Microstructure Processing maps

a b s t r a c t The flow behavior and the microstructural evolution of 7050 aluminum alloy are investigated at the deformation temperatures ranging from 593 K to 743 K, strain rates ranging from 0.01 s−1 to 20.0 s−1 , and height reductions ranging from 30% to 70%. The processing maps at the strains of 0.4 and 0.7 are developed on the basis of dynamic materials model (DMM). The results show that the deformation temperature and the strain rate have obvious effect on the flow stress and the microstructure of 7050 aluminum alloy. The steady flow stress–strain curves at the strain rates below 10.0 s−1 and the deformation temperatures above 703 K show continuous dynamic recrystallization character. However, a continuous flow softening behavior at high strain rates (≥10.0 s−1 ) implies the occurrence of flow instability or cracking in isothermal compression. The samples isothermally compressed at 10.0 s−1 or 20.0 s−1 and 723 K exhibit cracking. The processing maps at a strain of 0.7 exhibit two regions with high efficiency of power dissipation in isothermal compression of 7050 aluminum alloy. One is in the deformation temperature range from 614 K to 673 K and the strain rates below 0.022 s−1 , and another is in the deformation temperature range from 718 K to 743 K and the strain rates below 0.018 s−1 . On the basis of the processing maps and microstructural examination, the optimal processing parameter of 7050 aluminum alloy at a strain of 0.7 corresponds to a deformation temperature of 723 K and strain rate of 0.01 s−1 . © 2011 Elsevier B.V. All rights reserved.

1. Introduction The 7050 aluminum alloy belongs to 7XXX series (Al–Zn–Mg–Cu) aluminum alloys, which has high strength to weight ratio, high fracture toughness, good resistance to exfoliation corrosion and stress corrosion cracking. Its excellent combination of physical and mechanical properties makes it an ideal material in the automotive and aerospace industries. In the past several decades, a number of research groups have paid attention to the deformation behavior and the microstructural evolution of 7050 aluminum alloy [1–7]. For instance, Hu et al. [8,9] reported that the main soften mechanism of 7050 aluminum alloy was dynamic recovery at high Z value. Continuous dynamic recrystallization was dominant soften mechanism when the alloy deformed at low Z value. Nam et al. [10] pointed out that the large plate-type MgZn2 precipitates in a 7050 aluminum alloy sheet were fragmented into fine spherical particles with the increasing of the number of equal channel angular rolling (ECAR). Deng et al. [11] investigated the effect of deformation conditions on the flow behavior and microstructural evolution of

∗ Corresponding author. Tel.: +86 29 88460465. E-mail addresses: [email protected], [email protected] (J. Luo). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.10.019

homogenized 7050 aluminum alloy, and proposed that the subgrain size of 7050 aluminum alloy increased with the decreasing of Z value. Dixit et al. [12] made an effort to create the databases of 7050 aluminum alloy in order to consolidate the information about microstructure and mechanical properties, and applied the existing models to predict strength and fracture toughness of 7050 aluminum alloy. Moreover, Zhao et al. [13–15] studied the mechanical properties and microstructures of ultrafine grained 7075 aluminum alloy. Although many researchers had in depth studied the effect of processing parameters on the deformation behavior and the microstructural evolution, nevertheless no detail interpretations about the correlation between flow behavior and microstructural evolution in isothermal compression of 7050 aluminum alloy are reported in the open literature. And, the deformation mechanism of 7050 aluminum alloy is not yet clarified. Therefore, further more investigations are needed so as to illustrate the deformation mechanism and optimize the processing parameters in isothermal compression of 7050 aluminum alloy. The processing maps were developed on the basis of dynamic materials model (DMM), and were used to design hot working schedules for making near-net shapes in a wide variety of materials. The maps explicitly represented local peak efficiency of power dissipation which was the response of specific microstructure mechanism and regions of flow instability. With the help of the

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J. Luo et al. / Materials Science and Engineering A 530 (2011) 559–564 Table 1 EDX results of the as-extruded 7050 aluminum alloy (wt.%). Element

Al

Point A Point B Point C Point D Point E

2.14 9.09 20.62 93.64 91.34

Zn

4.15 3.16

Mg

Cu

0.80 1.05 1.18

97.86 90.91 78.58 1.16 4.32

2.2. Experimental procedures

Fig. 1. SEM micrograph of the as-extruded 7050 aluminum alloy.

processing maps, the deformation temperature and the strain rate corresponding to local peak efficiency of power dissipation in this domain were chose as the optimum processing parameter for hot working of materials. The hot working should not be performed in the regions of flow instability so as to prevent the occurrence of microstructure-based defects [16]. Recently, the processing maps had been widely used to optimize the processing parameters in hot working of aluminum alloys. Rajamuthamilselvan and Ramanathan [17] developed the processing maps of 7075 aluminum alloy and obtained that the optimal processing parameter was 623 K and 0.1 s−1 having efficiency of 28%. Li et al. [18] established the processing map of 2519A aluminum alloy at a strain of 0.5. The microstructure of 2519A aluminum alloy at a temperature of 723 K and strain rate of 10 s−1 exhibited most dynamic recrystallization (DRX) grains, which was optimal hot-working condition for 2519A aluminum alloy. In this paper, the effect of deformation temperature, strain rate and strain on the flow stress is analyzed to represent the flow behavior in isothermal compression of 7050 aluminum alloy. The deformation mechanism is clarified through the flow stress curves and the processing maps in isothermal compression of 7050 aluminum alloy. And, the microstructural examination at different deformation conditions is used to verify the deformation mechanism. Finally, the optimal processing parameter is determined in order to design hot working schedules for 7050 aluminum alloy.

2. Experimental 2.1. Experimental material As-extruded bar stock of 7050 aluminum alloy with T6 temper is 100.0 mm in diameter. The chemical composition (wt.%) of the alloy used in this investigation is as follows: 6.31 Zn, 2.30 Mg, 2.10 Cu, 0.03 Si, 0.09 Fe, 0.01 Mn, 0.01 Cr, 0.02 Ti, 0.11 Zr and the bal. Al. The morphology of as-extruded 7050 aluminum alloy is examined using scanning electron microscopy (SEM). The second phase particles are identified by energy dispersive X-ray spectrometry (EDX). Fig. 1 shows a SEM micrograph of as-extruded 7050 aluminum alloy. From Fig. 1, it can be observed that a typical microstructure of 7050 aluminum alloy at room temperature consists of ␣-Al matrix with elongated grains and second phase particles aligned in the extrusion direction. The EDX results of as-extruded 7050 aluminum alloy are shown in Table 1. The EDX analysis reveals that point A, point B and point C are Cu-rich phase.

Isothermal compression was carried out on a Gleeble-1500 simulator at the deformation temperatures ranging from 593 K to 743 K, the strain rates of 0.01, 0.1, 1.0 10.0 and 20.0 s−1 , and the height reductions of 30, 50 and 70%. Cylindrical compression specimens have 10.0 mm in diameter and 15.0 mm in height. The specimens prior to isothermal compression were heated and held for 3.0 min at the deformation temperature so as to obtain a uniform deformation temperature. The specimens and the anvils were enclosed within a vacuum tube during high temperature compression in order to prevent oxidation of the specimens. The flow stress–strain curves were recorded automatically in isothermal compression. After compression, the specimens were cooled in air to room temperature. For microstructural examination, the specimens were axially sectioned, mechanically polished, and chemically etched in a mixed solution of 2 ml HF, 3 ml HCl, 5 ml HNO3 and 190 ml H2 O. OLYMPUS PMG3 microscope was used for microstructural examination.

3. Results and discussion 3.1. Flow stress The selected flow stress–strain curves in isothermal compression of 7050 aluminum alloy are shown in Fig. 2. In general, the shapes of flow curves indicate some features that help in identifying the deformation mechanism. It is seen from Fig. 2 that the overall shapes of flow curves in isothermal compression of 7050 aluminum alloy are dependent on the strain rate. At the strain rates below 10.0 s−1 and at all deformation temperatures, the flow curves appears the steady flow behavior as the dynamic softening is sufficient to counteract the work-hardening in isothermal compression of 7050 aluminum alloy. Seshacharyulu et al. [19] pointed out that such curves were suggestive of mechanisms like dynamic recrystallization (DRX), superplasticity or dynamic recovery occurring at very high rates. In this case, the processing maps and the microstructural examination are required to determine the exact deformation mechanism in isothermal compression of 7050 aluminum alloy. At high strain rates (≥10.0 s−1 ), the flow stress exhibits a continuous flow softening behavior. The steady flow is not observed while the strain is up to 1.0. And, the degree of softening effect is more significant at high deformation temperatures (≥703 K). Such a feature implies the occurrence of flow instability or cracking in isothermal compression [19]. Therefore, more evidences are required to confirm the actual deformation mechanism of 7050 aluminum alloy. Macro pictures of the isothermally compressed samples are shown in Fig. 3. As seen in Fig. 3, the isothermally compressed samples at the strain rates of 10.0 s−1 and 20.0 s−1 exhibit cracking. This type of cracking may be triggered by the onset of intense flow instabilities [20]. According to the observation of isothermally compressed samples, it is confirmed that a continuous flow softening behavior at 10.0 s−1 or 20.0 s−1 and 723 K shows the occurrence of cracking in isothermal compression of 7050 aluminum alloy. And subsequently, this can be supported by

J. Luo et al. / Materials Science and Engineering A 530 (2011) 559–564

200

561

200

Deformation temperature: 593 K

(a)

Deformation temperature: 653 K

(b)

-1

20.0 s -1 10.0 s -1 1.0 s

160

Flow stress /MPa

Flow stress /MPa

160

120 -1

0.1 s

-1

0.01 s

80

-1

20.0 s -1 10.0 s

120

1.0 s

-1

-1

0.1 s

80

0.01 s 40

0 0.0

0.2

0.4

0.6

0.8

0 0.0

1.0

0.2

0.4

Strain 160

Deformation temperature: 703 K

(c)

-1

1.0 s

80

-1

-1

0.1 s 40

0.01 s

0.4

0.6

(d)

1.0

Deformation temperature: 743 K

-1

20.0 s

-1

10.0 s 80

1.0 s

-1

-1

0.1 s

40

-1

0.2

0.8

120

20.0 s -1 10.0 s

Flow stress /MPa

Flow stress /MPa

120

0.6

Strain

160

0 0.0

-1

40

-1

0.01 s

0.8

1.0

0 0.0

0.2

Strain

0.4

0.6

0.8

1.0

Strain

Fig. 2. Selected flow stress-strain curves in isothermal compression of 7050 aluminum alloy.

the processing maps in isothermal compression of 7050 aluminum alloy. On the other hand, it is also seen from Fig. 2 that the flow stress in isothermal compression of 7050 aluminum alloy increases noticeably with the increasing of strain rate at certain deformation temperature. Main reason is that the rate of dislocation generation increases with the increasing of strain rate. The tangled dislocation structures hinder the dislocation movement, leading to the increasing of flow stress in isothermal compression of 7050 aluminum alloy. Moreover, the flow stress decreases with the increasing of deformation temperature at certain strain rate. The phenomenon

may be attributed to the dissolution of second phase particles into Al matrix and the occurrence of dynamic recrystallization at high deformation temperatures. 3.2. Processing maps The processing maps consist of a superimposition of power dissipation map and instability map developed in a frame of deformation temperature and strain rate. The power dissipation map represents the pattern in which the power is dissipated by the alloys through microstructural evolution. The power dissipation rate is

Fig. 3. Macro pictures of the isothermally compressed samples with cracking: (1) T = 723 K, ε˙ = 10.0 s−1 ; (2) T = 723 K, ε˙ = 20.0 s−1 .

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given by a dimensionless parameter called the efficiency of power dissipation [21]: =

J 2m = Jmax 1+m

(1)

where m is the strain rate sensitivity of flow stress. A continuum instability criterion based on the extremism principles of irreversible thermodynamic as applied to large plastic flow [22] is used to identify the regimes of flow instabilities. In terms of the maximum rate of entropy production in material system, an instability criterion could be derived by [23]: ˙ = (ε)

∂log(m/1 + m) +m<0 ∂logε˙

(2)

where ε˙ is the strain rate (s−1 ). ˙ with deformation The variation of dimensionless parameter (ε) temperature and strain rate constitutes an instability map. Log  vs. log ε˙ data are fitted using a cubic spline function at a strain. The relationship between log  and log ε˙ is given as follows: ˙ 2 + d(log ε) ˙ 3 log  = a + b log ε˙ + c(log ε)

(3)

where  is the flow stress (MPa); a, b, c and d are material constants. The strain rate sensitivity is evaluated as a function of strain rate and is written as follows: m=

d(log ) ˙ 2 = b + 2c log ε˙ + 3d(log ε) ˙ d(log ε)

(4)

This is repeated at different deformation temperatures. The efficiency of power dissipation and the dimensionless instability parameter are calculated from a series of m-values to obtain the power dissipation map and the instability map. Finally, the processing maps are obtained by superimposing the instability map on the power dissipation map. Fig. 4(a) and (b) show the processing maps in isothermal compression of 7050 aluminum alloy constructed at the strains of 0.4 and 0.7. The contour numbers represent the efficiency of power dissipation, and shaded areas represent the instability regions. It can be observed from Fig. 4 that the processing maps at different strains exhibit similar feature indicating that the effect of strain on the processing maps is not significant. The processing maps at a strain of 0.7 exhibit two domains with high efficiency of power dissipation as follows: one is in the deformation temperature range from 614 K to 673 K and the strain rates below 0.022 s−1 with a peak efficiency of 0.41 at 623 K and 0.01 s−1 , and another is in the deformation temperature range from 718 K to 743 K and the strain rates below 0.018 s−1 with a peak efficiency of 0.42 at 723 K and 0.01 s−1 . The two domains appear to extend to lower strain rates and may reach even higher peak efficiency of power dissipation. The microstructure corresponding to the peak efficiency of 0.42 is shown in Fig. 5(7). From Fig. 5(7), it can be seen that the microstructure at a deformation temperature of 723 K and strain rate of 0.01 s−1 is composed of new equiaxed recrystallised grain, and the volume fraction of DRX is higher than that at other processing parameters. It is well known that DRX is beneficial to grain refinement and improvement of the intrinsic workability of material. Therefore, the optimal processing parameter of 7050 aluminum alloy at a strain of 0.7 corresponds to a deformation temperature of 723 K and strain rate of 0.01 s−1 . It can be also observed from Fig. 4 that a large region of flow instability at the strain rates above 0.03 in all deformation temperature range is predicted by a continuum instability criterion. When the deformation temperature is below 602 K, the instability region of 7050 aluminum alloy is at medium strain rates. The region appears to expand to lower strain rates in the deformation temperature range from 602 K to 660 K. With further increasing

Fig. 4. Processing maps of 7050 aluminum alloy at the strains of 0.4 (a) and 0.7 (b). Numbers represent the efficiency of power dissipation. Shaded areas represent the instability regions.

of deformation temperature, the instability region of 7050 aluminum alloy only exhibits at higher strain rates, which is consistent with above-mentioned phenomenon that the isothermally compressed samples at the strain rates of 10.0 s−1 or 20.0 s−1 and deformation temperature of 723 K exhibit cracking. The processing parameters should not be chosen in the instability region of 7050 aluminum alloy so as to prevent the occurrence of microstructurebased defects. 3.3. Microstructural examination The microstructure in isothermal compression of 7050 aluminum alloy is given in Fig. 5. It is seen from Fig. 5(1)–(3) that the microstructure in the instability region of 7050 aluminum alloy mainly consists of ␣-Al matrix with elongated grains and Cu-rich phase aligned in the elongated grains. The microstructure is a typical dynamic recovery structure and is unstable for the hot working process of 7050 aluminum alloy due to strongly oriented characteristics. By the analysis of the processing maps and microstructural evolution, it is confirmed that the dynamic recovery is dominant softening mechanism at the deformation temperatures below 703 K in all strain rate range or at the deformation temperatures above 703 K and high strain rates (≥10.0 s−1 ). At a deformation temperature of 703 K, the effect of strain rate on the microstructure in isothermal compression of 7050 aluminum alloy is shown in Fig. 5(3)–(5). It is observed from

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Fig. 5. The microstructure in isothermal compression of 7050 aluminum alloy: (1) ε˙ = 0.4, T = 683 K, ε˙ = 1.0 s−1 ; (2) ε = 0.7, T = 683 K, ε˙ = 1.0 s−1 ; (3) ε = 0.7, T = 703 K, ε˙ = 10.0 s−1 ; (4) ε = 0.7, T = 703 K, ε˙ = 1.0 s−1 ; (5) ε = 0.7, T = 703 K, ε˙ = 0.1 s−1 ; (6) ε = 0.7, T = 723 K, ε˙ = 0.1 s−1 ; (7) ε = 0.7, T = 723 K, ε˙ = 0.1 s−1 . The compression axis is horizontal in the extrusion direction.

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Fig. 5(3)–(5) that the microstructure of 7050 aluminum alloy is sensitive to the strain rate. At a strain rate of 10.0 s−1 , the microstructure mainly consists of ␣-Al matrix with elongated grains and Cu-rich phase aligned in the elongated grains. At a strain rate of 1.0 s−1 , many fine and equiaxed grains are observed along the elongated grain boundaries and also the grain interior. And, the volume fraction of recrystallised grain increases with the decreasing of strain rate. The microstructural observation illustrates that the softening mechanism is continuous dynamic recrystallization at a deformation temperature of 703 K and the strain rates below 10.0 s−1 . The steady flow stress–strain curves at the strain rates below 10.0 s−1 are also in support of this deformation mechanism, as illustrated in Fig. 2. Compared the micrograph in Fig. 5(2) with that Fig. 5(4), it is also seen that the deformation temperature has some influence on the microstructure in isothermal compression of 7050 aluminum alloy. When the deformation temperature is up to 703 K, many fine and equiaxed grains appear at grain boundaries and grain interior. Fig. 5(6) and (7) illustrate the microstructure at a deformation temperature of 723 K. The size and the volume fraction of recrystallised grains at a strain rate of 0.01 s−1 increase significantly in comparison with that at a strain rate of 0.1. This phenomenon is possibly related to the fact that the lower strain rates enhance nucleation in the predisposed regions with the result that there is an avalanche of recrystallization. In addition, the decrease of strain rate allows the existing nuclei to grow more easily. However, the creation of new nuclei from predisposed regions is made increasingly more difficult by the greatly increased strain rate [24]. Therefore, the size and the volume fraction of recrystallised grains in isothermal compression of 7050 aluminum alloy increase with the decreasing of strain rate. It is well known that the mechanical properties of 7050 aluminum alloy are dependent on the microstructural evolution, so the deformation temperature and the strain rate should be controlled in the forging process of 7050 aluminum alloy to improve the mechanical properties of component. 4. Conclusions (1) The flow stress exhibits a continuous flow softening behavior at high strain rates (≥10.0 s−1 ), which implies the occurrence of flow instability or cracking in isothermal compression. The samples isothermally compressed at 10.0 s−1 or 20.0 s−1 and 723 K exhibit cracking. (2) The steady flow stress–strain curves at the strain rates below 10.0 s−1 and deformation temperatures above 703 K show continuous dynamic recrystallization character. (3) The dynamic recovery is dominant softening mechanism at the deformation temperatures below 703 K in all strain rate range or at the deformation temperatures above 703 K and high strain rates (≥10.0 s−1 ). (4) The processing maps at a strain of 0.7 exhibit two regions with high power efficiency as follows: one is in the deformation

temperature range from 614 K to 673 K and the strain rates below 0.022 s−1 , and another is in the deformation temperature range from 718 K to 743 K and the strain rates below 0.018 s−1 . (5) The optimal processing parameter of 7050 aluminum alloy at a strain of 0.7 corresponds to a deformation temperature of 723 K and strain rate of 0.01 s−1 . (6) The instability region of 7050 aluminum alloy occurs at the strain rates above 0.03 in all deformation temperature range. The processing parameters should not be chosen in the instability region so as to prevent the occurrence of microstructure-based defects. Acknowledgement The authors thank the financial supports from the National Natural Science Foundation of China with Grant No. 50975234. References [1] N.M. Han, X.M. Zhang, S.D. Liu, D.G. He, R. Zhang, J. Alloys Compd. 509 (10) (2011) 4138–4145. [2] J. Buha, R.N. Lumley, A.G. Crosky, Mater. Sci. Eng. A 492 (1/2) (2008) 1–10. [3] X.Y. Wang, H.E. Hu, J.C. Xia, Mater. Sci. Eng. A 515 (1/3) (2009) 1–9. [4] L. Zhen, H.I. Hu, X.Y. Wang, B.Y. Zhang, W.Z. Shao, J. Mater. Process. Technol. 209 (2) (2009) 754–761. [5] L.J. Zheng, H.X. Li, M.F. Hashmi, C.Q. Chen, Y. Zhang, M.G. Zeng, J. Mater. Process. Technol. 171 (1) (2006) 100–107. [6] J.D. Robson, Mater. Sci. Eng. A 382 (1/2) (2004) 112–121. [7] S.D. Liu, J.H. You, X.M. Zhang, Y.L. Deng, Y.B. Yuan, Mater. Sci. Eng. A 527 (4/5) (2010) 1200–1205. [8] H.E. Hu, L. Zhen, B.Y. Zhang, L. Yang, J.Z. Chen, Mater. Charact. 59 (9) (2008) 1185–1189. [9] H.E. Hu, L. Zhen, L. Yang, W.Z. Shao, B.Y. Zhang, Mater. Sci. Eng. A 488 (1/2) (2008) 64–71. [10] C.Y. Nam, J.H. Han, Y.H. Chung, M.C. Shin, Mater. Sci. Eng. A 347 (1/2) (2003) 253–257. [11] Y. Deng, Z.M. Yin, J.W. Huang, Mater. Sci. Eng. A 528 (3) (2011) 1780–1786. [12] M. Dixit, R.S. Mishra, K.K. Sankaran, Mater. Sci. Eng. A 478 (1/2) (2008) 163–172. [13] Y.H. Zhao, X.Z. Liao, Z. Jin, R.Z. Valiev, Y.T. Zhu, Acta Mater. 52 (15) (2004) 4589–4599. [14] Y.H. Zhao, X.Z. Liao, S. Cheng, E. Ma, Y.T. Zhu, Adv. Mater. 18 (2006) 2280–2283. [15] Y.H. Zhao, X.Z. Liao, Y.T. Zhu, R.Z. Valiev, J. Mater. Res. 20 (2) (2005) 288–291. [16] P.S. Robi, U.S. Dixit, J. Mater. Process. Technol. 142 (1) (2003) 289–294. [17] M. Rajamuthamilselvan, S. Ramanathan, J. Alloys Compd. 509 (3) (2011) 948–952. [18] H.Z. Li, H.J. Wang, X.P. Liang, H.T. Liu, Y. Liu, X.M. Zhang, Mater. Sci. Eng. A 528 (3) (2011) 1548–1552. [19] T. Seshacharyulu, S.C. Medeiros, W.G. Frazier, Y.V.R.K. Prasad, Mater. Sci. Eng. A 284 (1/2) (2000) 184–194. [20] T. Seshacharyulu, S.C. Medeiros, W.G. Frazier, Y.V.R.K. Prasad, Mater. Sci. Eng. A 325 (1/2) (2002) 112–125. [21] Y.V.R.K. Prasad, H.L. Gegel, S.M. Doraivelu, J.C. Malas, J.T. Morgan, K.A. Lark, D.R. Barker, Metall. Trans. A15 (10) (1984) 1883–1892. [22] H. Ziegler, in: I.N. Sneedon, R. Hill (Eds.), Progress in Solid Mechanics, vol. 4, Wiley, New York, 1963, pp. 63–193. [23] Y.V.R.K. Prasad, S. Sasidhara, Hot Working Guide: A Compendium of Processing Maps, ASM International, Materials Park, OH, 1997, pp. 25–157. [24] H.J. McQueen, in: G. Krauss (Ed.), Deformation, Processing, and Structure, American Society for Metals, Metals Park, OH, 1984, pp. 231–239.