METAL STRUCTURE

New Zealand Times, Volume LXXI, Issue 4298, 6 March 1901, Page 7

METAL STRUCTURE

SOME POPULAR IDEAS EXPLODED

It is a popular idea that metals are perfectly homogeneous and consistent throughcut, being composed of the molecules of which we read so much in the books on chemistry. The experiments which Professor Ewing l as been making, and which he described at the Mechanical Engineers on Monday evening, show (says the London "Leader" of the 16th January) that this idea is not correct.

For the purpose of examining the internal structure of metals ho polished them carefully on the surface. Alien subjected them to the action of nitric acid, so that they might become etched, and then he examined the etched surfaces through the microscope. When the surface is prepared in this way it is seen to bo composed of a, number of irregular patches, but these with nil the same' shape, which indicates that the metal is composed of crystals, all lying close together. Further, it jo noticed that there are well-defined areas quite irregular in shape, and that the boundary lines between the areas are on a microscopic scale like fissures caused by an earthquake. In each of the areas the similar shapes or crystals all lie the same way, each having its own crystals lying their own way. Professor Ewing speculated on the manner in which this was brought about. It might be assumed that metal began to cool at a great number of different points at once, and that each of these points became a centre of cooling. One crystal formed, and formed in its own way. Then other crystals formed found it, lying with their sides touching, and arranging them, selves to suit the first, so that they all lay in tho same direction as the first. The same would be proceeding from all the cooling centres until the crystals of one area began to meet the crystals of another, and then the crystals which met could not accommodate their directions, and a boundary line formed. The action was like that of childr ,r. in a nursary, all beginning to build with bricks on'the floor of the nursary, and to continue building until their systems met, when it-would be seen that each: had his bricks lying parallel to each other, but the bricks of one system were not parallel to those of another, and so there resulted lines of demarcation between one set and another set. In a metal the boundary line is made no of the metal which takes longest to solidify, and has not so strong a tendency to crystallise as the rest.

The effects of a stretching force on a metal was shown by the microscope to elongate the crystals, and make them to slightly slide over one another, forming ridges between one crystal and the next. In such a ease the'microscope shows a

"rcat family of lines going in ono direction while the next to them would he in another direction, and the next to that would be- a strip with another family of lines parallel to the firstset. Cold rolling the metal produced exactly the same effect a sal-retching. The effect of temperature is to completely change the construction of the metal. Lead which was heated to a temperature of 290 deg. and. kept at that temperature for a mouth would show a certain constitution of crystals,-formed in areas; after three .month.: the,, areas and the crystals would he quite different; and after further time there would he a further change. One crystal would bs. seen to encroach on another, and in „tin?o..completely absorb it. In another case an archipelago of crystals would come together. It was thought that this.was due to the different electrical states of separate crystals. If two separate pieces were brought together aud tin scattered over a part of the surfaces the pieces on each side would become part of tho same crystal —that is. the crystal would extend across both sides of the junction line. Annealing and tempering were thus shown to be changes in the arrangements of crystals of Ihe same metal, and the degree to which they became annealed or tempered was a question of time.