Work hardening during rolling of Zn-Al-Mg and Zn-Cu-Mg sheet alloys containing up to 0.30 at.% Al, up to 2.2 at.% Cu and up to 0.10 at.% Mg was tested. This phenomenon non appears in pure zinc and typical zinc alloys, showing work softening as a result of recovery and recrystallization at room temperature [1]. The changes in hardness of series of cold rolled model alloys shows Table 1.
Sheets' hardness, HB (comp. in at.%) a,b) before cold rolling supersaturated: a) at 570K /3h, b) at 660K /3h
Cold deformation % 0 10 30 50 90 Zn-0.3Al 37 47 46 41 39 Zn-0.2Al-0.1Mg 51 58 63 68 75 Zn-0.2Cu-0.1Mg 63 67 70 72 73 Zn-0.7Cu-0.1Mg a 51 62 69 70 64 Zn-2.2Cu-0.1Mg a 70 84 90 89 79 Zn-2.2Cu-0.1Mg b 69 92 101 108 115Changes in residual stresses and microstructure of these alloys when rolling also have been defined. The reason of hardening of the alloys tested has been found.
Residual stresses in Zn-Cu-Mg sheet alloys increase with deformation degree increasing (Table 2). The stronger supersaturated is (Zn) solid solution the greater are the stresses. Zn-Cu-Mg alloys show also shear bands forming at higher deformations [2], non typical for hcp metals. Additional feature occurring in Zn-Cu-Mg alloys is precipitation of $\varepsilon$-CuZn phase during cold rolling [2,3], causing (Zn) solution softening. Its influence on the alloys' hardness is the stronger the greater is deformation degree and the greater is supersaturation of alloys. Sufficient amount of Mg in the solid solution inhibits precipitation process.
Residual stresses in Zn-Al and Zn-Al-Mg alloys increase up to the deformation of 10% or 15%, when the deformation by twinning predominates. Subsequently they decrease - down to zero in the case of Zn-Al alloy.
Table 2. Residual stresses $\Delta$ a/a and subgrain size $\Phi$ ({1011}
pl., comp. in at.%, def. 90%
Cold deformation Delta a/a Phi(nm)
Zn-0.3Al 0.00 166
Zn-0.2Al-0.1Mg 0.18 60
Zn-0.2Cu-0.1Mg 2.96 106
Zn-0.7Cu-0.1Mg 13.4 81
Zn-2.2Cu-0.1Mg 21.3 71
All alloys containing Mg show simultaneous work hardening
(i.e. increase of their hardness and strength) and partial
recovery, exhibiting by subgrain diameters increasing.Basing on reached results the presence of Mg in alloys tested seems to be the main reason of zinc alloys hardening. A form of Mg occurring is considerable feature determining hardening degree. The most efficacious source of defects generating during deforming and causing work hardening are Mg atoms in (Zn) solid solution. Their solubility in (Zn) increase with Cu amount increasing. Weaker is strengthening by Mg-rich GP zones appearing in supersaturated solid solution, especially in Zn-Cu-Mg alloys. The weakest is the influence of Mg$_{2}$Zn$_{11}$ phase, existing mostly in Zn-Al-Mg alloys.
1. M.Staszewski: Thesis, INM, Gliwice, Poland 1989
2. M.Staszewski et al: Proc.CAC '94, Cieszyn, Poland 1994 (in
print)
3. M.Staszewski et al: Proc.AMT '95,"Scripta" Gliwice-%
Zakopane, Poland 1995, 107