The Mechanism of Plastic Deformation of Crystals. Part I.—Theoretical. By G. I. Taylor, F.R.S., Royal Society Yarrow Professor. (Received February 7, 1934.) Experiments on the plastic deformation of single crystals, of metals and of rock salt have given results which differ in detail but possess certain common characteristics. In general the deformation of a single crystal in tension or compression consists of a shear strain in which sheets of the crystal parallel to a crystal plane slip over one another, the direction of motion being some simple crystallographic axis. The measure of this strain, which will be represented by s, is the ratio of the relative lateral movement of two parallel planes of slip to the distance between them. Thus it is defined in the same way as the shear strain considered in the theory of elasticity. The resistance to shear, which will be denoted by S, is defined as the component of shear stress in the direction of slip which must act parallel to the slip plane in order that plastic deformation may occur. It has been found that when the results of tests on single crystals of a metal are analysed the stress-strain curve which represents S as a function of s is independent of the stress normal to the slip plane and of the components of shear stress perpendicular to the direction of slip. Thus the (S, s) curve is a unique curve which defines the strength of the single crystal at any stage of distortion. When the (S, s) curves for single crystals of different metals are compared they are found to differ considerably in detail, but they all possess one general characteristic, a very small stress will produce a small plastic deformation, but as the deformation increases the stress necessary to increase it also increases. With some crystals it has been found difficult to assign a definite stress at which plastic distortion begins, with others, such as rock salt and zinc, experimenters have found such a limit, but it is very small compared with the strength ultimately attained by the material. In some such crystals it has been found that the observed lower limit of strength depends very much on the degree of purity of the material, thus Schmid, working with single crystals of zinc, found that a decrease in the total amount of metallic impurity from 0.03% to 0.002% causes the stress at which the plastic deformation begins to decrease from 94 to 49 gm. per sq. mm. Except for these differences in the early stages of distortion the (S, s) curves for many metallic single crystals are very similar. Some of them are shown in figs. 1, 2, and 3. Fig. 1 refers to aluminium. The data from which this curve has been constructed were given in a previous paper. In fig. 1, however, the unit of stress has been changed from lbs. per square inch to dynes per square centimetre. FigThe Mechanism of Plastic Deformation of Crystals. Part I.—Theoretical. By G. I. Taylor, F.R.S., Royal Society Yarrow Professor. (Received February 7, 1934.) Experiments on the plastic deformation of single crystals, of metals and of rock salt have given results which differ in detail but possess certain common characteristics. In general the deformation of a single crystal in tension or compression consists of a shear strain in which sheets of the crystal parallel to a crystal plane slip over one another, the direction of motion being some simple crystallographic axis. The measure of this strain, which will be represented by s, is the ratio of the relative lateral movement of two parallel planes of slip to the distance between them. Thus it is defined in the same way as the shear strain considered in the theory of elasticity. The resistance to shear, which will be denoted by S, is defined as the component of shear stress in the direction of slip which must act parallel to the slip plane in order that plastic deformation may occur. It has been found that when the results of tests on single crystals of a metal are analysed the stress-strain curve which represents S as a function of s is independent of the stress normal to the slip plane and of the components of shear stress perpendicular to the direction of slip. Thus the (S, s) curve is a unique curve which defines the strength of the single crystal at any stage of distortion. When the (S, s) curves for single crystals of different metals are compared they are found to differ considerably in detail, but they all possess one general characteristic, a very small stress will produce a small plastic deformation, but as the deformation increases the stress necessary to increase it also increases. With some crystals it has been found difficult to assign a definite stress at which plastic distortion begins, with others, such as rock salt and zinc, experimenters have found such a limit, but it is very small compared with the strength ultimately attained by the material. In some such crystals it has been found that the observed lower limit of strength depends very much on the degree of purity of the material, thus Schmid, working with single crystals of zinc, found that a decrease in the total amount of metallic impurity from 0.03% to 0.002% causes the stress at which the plastic deformation begins to decrease from 94 to 49 gm. per sq. mm. Except for these differences in the early stages of distortion the (S, s) curves for many metallic single crystals are very similar. Some of them are shown in figs. 1, 2, and 3. Fig. 1 refers to aluminium. The data from which this curve has been constructed were given in a previous paper. In fig. 1, however, the unit of stress has been changed from lbs. per square inch to dynes per square centimetre. Fig