Metallography

6.2.1. Optical microscopy

directly to alpha while between 10$ and 15$ Mn the uncomplet­

ed sequence V — >£-— is followed. The alpha formed in

this range is found sandwiched between pre-existing bands of

epsilon phase which formed initially along the planes

of the parent gamma phase. This state of affairs is shown in figure 49 for a 12.5$ Mn alloy at a magnification of X1600. The alpha phase is revealed as light grey lens shaped masses crossing light coloured epsilon bands. The similarity

between the appearance of alpha in Fe - Mn alloys and that found in stainless steel of low stacking fault energy leads

to the suggestion that the phase is likely to be in the form of laths and that a similar orientation relationship is

likely to exist i.e.

(lll)y II

(oooi)t II (101)^

C110]v || C1210J£ || [111]^

Alloys in the range 16-50$ Mn are composed of varying percentages of gamma and epsilon phase. Figure 50 shows a

typical microstructure for an 18.5$ Mn alloy at X150. The structure has a pronounced widmanstatten pattern, the epsilon phase forming on the (ill) planes of the parent structure.

Above 28$ Mn the structure at room temperature con­ sists of equiaxed gamma grains, a considerable number of

which are twinned. This suggests a low stacking fault energy (see figure 51).

Representative samples: of the specimens annealed at 950°C for ly hours followed by air cooling showed light and dark bands on stain etching in saturated sodium thiosulphate

plus potassium metabisulphite. The under-lying structure, however, appeared to be the same. A statistical hardness

survey, using a Knoop penetrator with a 100 gramme load, of the light and dark areas showed that for a 95$ confidence interval for Students t there was no significant difference in hardness. On the basis of the hardness survey it was

therefore concluded that any residual segregation was insignificant. For details see appendix A.4.0.

In order to estimate the percentages of each phase present in the alloy a series of point counts were carried

out using a grid with a 100 intersections. The magnification was adjusted to reveal the structural details present.

Figure 52 shows the structures resulting from annealing at 950°C for ly hours followed by air cooling. Each result shown is the average of five counts.

Samples annealed at 1150°C for 24 hours plus 950°C for ly hours gave a uniform appearance on etching showing no banding effect. The phases present on the basis of point

counting are shown in figure 55, which is a plot of counts (number of intersections with epsilon phase) versus the man­ ganese content of the alloy. The crosses indicate the actual values obtained while the ringed crosses indicate the middle value of a sample of five results. A similar count for the gamma phase is shown in figure 54. If the values for gamma and epsilon are added together, a number of intersections are unaccounted for, these are the bands of gamma criss-crossed by numerous laths of epsilon phase. Examination at a higher magnification revealed that these areas contained about 50$

of each phase in the case of alloys of higher manganese con­ tent (greater gamma content), while in the region of 15$ Mn almost 100$ epsilon is found (see figure 55). The dotted line indicates the revised counts based on the original count plus 50$ of unallocated counts in the case of the higher manganese alloys, while in the case of the 15.6$ Mn alloy, the original count plus 100$ unallocated count is used as an estimate of the true value. Figure 56 shows a plot of percentage phase versus manganese content based on the above

revised values.

The epsilon phase appears to reach a maximum of 85$ at about 14$ Min. The gamma to alpha curve is very steep in this region which means that slight changes in composition would lead to considerable changes in microstructure.

Figure 57 shows the effect of etching time on the percentage of apparent epsilon phase counts present in a 15.6$ Mn alloy. It can be seen that the longer the etching time the lower the apparent epsilon content. This is due to the fact that the reagent is also capable of staining the ■ epsilon phase when the etching time is prolonged. Obviously this can be a source of error, therefore the minimum etching time which gave a uniform stain etch was used.

6.2.2. X-ray diffraction

6.2.2.1. Forged bar samples air cooled from 950°C

Figure 58(a) shows the results of an X-ray diffraction study carried out on a series of alloys air cooled from 950°C. On the whole the results were similar to those obtained by

Schumann for material heated to 1000°C and air cooled with the exception that the epsilon content appears to reach a maximum at about 15.0$ Mn.

In view of the banding effect previously-noted :.on etching, it was decided to extend the period of heat treat­ ment in order to ensure homogeneity in the specimens examined. Therefore, samples were annealed at 1150°C for 24 hours plus 950°C for ly hours followed by air cooling. The sample

showed no banding on stain etching. Figure 58(b). shows the results of this study. It can be seen that the epsilon con­ tent reaches a maximum at about 15$ Mn then begins to

decrease.

6.2.2.2. Tapered tensile specimens

A diffractometer study was also made of sections cut from the tapered tensile test pieces pulled at various temp­ eratures ranging from -100°C to 300°C. The results of this investigation are shown in figure 59. The alloys containing 32.5$ Mn and 27.0$ Ivin respectively showed no phase change when pulled in tension and are therefore excluded from the figures. The 15.6$ Mn alloy transforms completely to epsilon after about .06 strain. However, the degree of deformation achievable never reaches that required to cause the appear­ ance of alpha martensite, even at the point of fracture. Nevertheless, if figure 59 is compared with figure 60(a) it can be seen that the same pattern is followed under both tensile and compressive stresses.

28.25$ Mn alloy, (see figures 60(c) and 59). It should he borne in mind, however, that no attempt has been made to

correct the percentage phase for orientation effects and to all intents and purposes the different modes of deformation achieve the same result. Deformation by rolling has, however, the advantage that large reductions can be achieved although it is difficult to control the actual deformation tempera­ ture. Large strains would, however, probably be achieved by machining the gauge length parallel when it begins to neck down. The disadvantage of this procedure is that the test has to be interrupted, which may affect the ultimate result.

6.2.2.3. Gold rolling experiment

A diffractometer study was made of the phases present at each stage of reduction (see figures 60 a-c). No attempt was made to correct the results for preferred orientation,

therefore, they may be regarded only as semi-quantitative. Nevertheless, they clearly indicate the effect of cold defor­ mation on the phases present.

In the case of the 23.0$ Mn alloy (initially gamma plus epsilon) the amount of gamma decreased with increasing

deformation until at 50$ reduction the structure consisted entirely of epsilon martensite.

The 15.6$ Mn alloy showed somewhat different behaviour,