Cryo-rolling is effectively deformation of materials at liquid nitrogen temperature (LNT) that assists in producing ultra-fine grained microstructures.
In specific specimens studied it was found that ultra fine structure was eventually formed by a huge level of deformation twinning followed by extensive shear banding.
Large deformation at cryogenic temperatures is sometimes considered as a promising cost-effective method for producing bulk ultra-fine grain materials. In the paper of T. Konkova, S. Mironov, A. Korznikov, G. Korznikova, M.M. Myshlyaev, S.L. Semiatin, the high-resolution electron backscatter diffraction (EBSD) was used to study grain structure development during cryogenic rolling of Cu–29.5Zn brass. Microstructure evolution was found to be broadly similar to that occurring during rolling at room temperature.
Specifically, favorably-oriented grains (Copper {112}<111> and S {123}<634>) experienced profuse deformation twinning followed by extensive shear banding. This eventually produced an ultrafine structure with a mean grain size of 0.2 μm. On the other hand, grains with crystallographic orientations close to Brass {110}<112> and Goss {110}<100> were found to be stable against twinning/shear banding and thus showed no significant grain refinement. As a result, the final structure developed in heavily-rolled material was distinctly inhomogeneous consisting of mm-scale remnants of original grains with poorly developed substructure and ultra-fine grain domains.
The main conclusions from this work are as follows.
Figure 1: Microstructure after 30-pct. thickness reduction (true strain of 0.4): (a) low-resolution, composite EBSD grain-boundary map (grain-boundary color code is given in the upper right corner), (b) high-resolution Kikuchi-band-contrast EBSD map taken from the twinned area, and (c) grain-boundary EBSD map from the same location as (b). The broken lines in (b) show the {111} plane traces closest to the twin habit planes.
Figure 2: Microstructure after 50-pct. thickness reduction (true strain of 0.7): (a) low-resolution, composite EBSD grain-boundary map (grain-boundary color code is given in the upper right corner), (b) high-resolution Kikuchi-band-contrast EBSD map illustrating shear banding, and (c) grain-boundary EBSD map from the same location as (b). The dark bands in (b) are shear bands; the circled area in (c) illustrates substructure within the bands.
The effect of annealing treatment on the mechanical properties and microstructure of cold-rolled Cu- 20% Zn alloys was investigated in the work of X. Wu, Y. Gong, S. Ren, J. Tao, Y. Long, L. Cheng and X. Zhu. Mechanical properties changed dramatically with the increase of temperature. According to the microhardness test, it can roughly concluded that 150 ℃ is the optimal annealing temperature for deformation, at which a uniform elongation increased from 1.4658% before annealing to about 6.89%, and the elongation to failure increased from 7.426% to 16.81% with the same strength almost retained. The changes of microstructure during the annealing process are mainly distributed to the improvement of mechanical properties.
Cu-20% Zn was produced by induction vacuum melting. Hot-rolling was conducted to these as-cast after they were made into plates with a thickness of 7.9 mm. To minimize the effect of mechanical processing, the plates were homogenized at 600°C,750°C and 800°C for 4 hours in argon atmosphere respectively. During cold rolling process, the samples were soaked in liquid nitrogen for about 5 minutes before each rolling pass. These plates were cold rolled from a thickness of 7.9 mm to 0.5 mm producing a total strain of 93.67%.
The variations of microhardness at different annealing temperatures for RT and LN Cu-20%Zn are shown in Figure 3. As indicated in Figure 3, the microhardness of LN Cu-20% Zn was greatly enhanced at the temperature of 150°C,where the microhardness can be achieved as high as 189 HV. The microhardness changes for RT Cu-20% Zn showed different trend. As can be seen in Figure 1, no hardening is observed during the annealing process.
Figure 3: The variations of microhardness in different annealing temperatures
References
1. T. Konkova , S. Mironov, A. Korznikova, G. Korznikova, M.M. Myshlyaev, S.L. Semiatin: Grain structure evolution during cryogenic rolling of alpha brass, 2015, Accessed NOV 2018;
2. X. Wu, Y. Gong, S. Ren, J. Tao, Y. Long, L. Cheng, X. Zhu: Effect of Annealing on Mechanical Properties of Copper Alloys Deformed at Cryogenic Temperature, Materials Science Forum Vols. 745-746, 2013, p. 363-370, doi:10.4028/www.scientific.net/MSF.745-746.363;