observed at 650°C, when precipitates were predominantly seen
at grain boundaries and identified as M23C6 a cube-on-cube
orientation relationship with one of the contiguous grains. However, at higher ageing temperatures some non-coherent
twin-boundary precipitation was also observed having an orienta
tion relationship of (111) / <1 1 0> of // to {111$ <1 1 0>
of austenite. Such precipitation was clearly visible even
optically, Plate 6(e), but there was no evidence of any TiC
precipitation either at grain-boundaries or within the grains.
l+.U. 2.7 Low Carbon PEI6 Composition ( Alloy No. 7) ,
At lower ageing temperatures there was evidence for a small amount of carbide precipitation, Plate 6(f), but this was not detectable by optical microscopy until the highest ageing temperature of 900°C, Plate 7(a),
However, a detailed electron microscopical examination revealed grain-boundary precipitation of Mp^Cg on ageing
o /
at 500 C, but no zones were seen even at the longer ageing times at 500°c. Nevertheless, the matrix did reveal a few
super dislocations, Plate 7(b), which suggests the existence of an ordered structure* Clear signs of a mottling effect due to^zone formation were observed after ageing at 550°C,
Plate 7(c), and this was confirmed by obtaining superlattice reflections on diffraction patterns from the matrix area. With increasing ageing time and temperature the density of
zones increased, Plate 7(d), and also there was also an
increase in ^23^6 PreciPi‘ta^i°n a‘t grain-boundaries, Plate 7(e).
Not until longer ageing times at 750°C were the ^ precipitates starting to lose coherency with the matrix and appeared as semi-coherent particles uniformly distributed throughout the
matrix, Plate 7(f). A further increase in the ageing temperature
caused an overall increase in the semicoherent 'J^particle
diameter, Plate 8(a)^ which clearly revealed the spherical
shape of the particles. However, at higher ageing temperatures grain-boundary precipitation was observed to be limited to relatively few grain-boundaries, and the overall volume fraction of such precipitates was greatly reduced. At 85✓ 0°C and above,Y was observed to precipitate directly from the matrix without intermediate zone formation, even after very short ageing times.
k*U.2 .8 High Carbon PE16 Composition (Alloy 8)
The optical microstructures were very similar to those
observed for alloy 7> Plate 8(b), except for an increased
amount of carbide precipitation at grain-boundaries due to the higher carbon content.
The electron microstructures showed ^23^6 PreciPi‘ba'tion
at ageing temperatures as low as 300°C, particularly at triple points and grain boundaries, whereas f^zone formation was not observed until longer ageing times at 600°C. The delayed
zone formation could be an effect of leas titanium in solution due to increased amounts of undissolved TiC in this alloy?
The intensity of zone formation increased with increasing
ageing temperatures, Plate 8(c), and carbide precipitation
was observed to occur on non-coherent twin boundaries. Most of these carbides were uniformly distributed along grain- boundaries, Plates 8(d)and(e). After longer ageing times at
750°C the zones had grown appreciably and some showed
strained regions around them, Plate 8(f). Also, at 750°C
^23 ^6 was observec^ "k° Precipitate on the undissolved TiC/matrix
interface , plates 9(a)and(b) with a cube-on-cube orientation
relationship with the matrix, perhaps nucleating on the
interface dislocations. At longer ageing times at 750°C the*
growing ^zones were observed partially to loose coherency,
Plate 9(c), although some of the zones were still completely
coherent with the matrix. Particle growth was observed at
800°C where nearly all the precipitates were semi-coherent
after longer ageing times?Plate 9(d). This was accompanied
by growth of the grain-boundary M23C6’ ^ a'fces 9(e)and(f).
There were no signs of TiC precipitation at any ageing temperature, and the microstructural features at 850°C and
above were very similar to those described for alloy 7»
k* ho 3 Cold Worked and Aged Structures
Optical microstructures of strip samples cold rolled
by 1+0%, 60% and 80% reduction showed similar features for the
various alloys. At 1+0% reduction9parallel groups of deformation
bands, mostly aligned along the rolling direction of the elongated austenite grains were observed. At 60% reduction the number of deformation bands had increased, and they were observed in several directions within the same grain. With
80% reduction, however, no detail could be resolved in polished
and etched samples, particularly in the finer grained alloys. Electron microscopy did not reveal individual dislocations
readily even after 1+0% reduction, although occasional diffuse
cell structures were found. In the regions of deformation bands, high dislocation densities were observed. Increasing
the reduction from 1+0 to 60% resulted in an increase in the
number of deformation bands and pronounced arcing of diffraction patterns. At very heavy deformations (80%) the structure
was considerably more diffuse, and the diffraction patterns obtained from these areas approximated to ring patterns.
The following sections describe the microstructure of the deformed and aged specimens for each alloy with respect to precipitation and recrystallization effects.
h.h*3*l Medium Carbon Base Composition (Alloy 2)
Optical microstructures clearly revealed the formation of carbides on the elongated grain-boundaries, even after the
shortest ageing times at the lowest ageing temperature, Plate
10(a). The precipitates also had occurred on the deformation
bands, formed during cold working. The amount of such
precipitation was observed to increase both with increasing
cold working and increasing ageing temperature and time,Plate
10(b). There was very strong evidence that precipitation
occurred prior to recrystallization. At longer ageing times
and moderate (I4.0%) deformation, areas of recrystallization
were observed, having nucleated at prior grain-boundaries
and growing within the elongated grain, Plate 10(b). Plate
10(c) illustrates the formation of subgrains at grain-boundaries, even after shorter ageing times at lower temperatures. Nucleatio and growth of these subgrains occurred at both sides of the
boundary and such effects were not visible optically, particularl at lower ageing temperatures. Growing recrystallized nuclei
within the grains were also observed, occurring in or near to the deformation bands due to higher energy associated with such sites. These regions revealed a cell structure, with a great decrease in dislocation density within the cells, whilst the cell walls were sharp and clearly defined.
Optical microscopy of heavily (80%) deformed and aged strip showed heavy carbide precipitation and a few recrystallized grains, even after the shortest ageing times
at lower ageing temperatures, Plate 10(d). However, such
structures were seen to be almost fully recrystallized by electron
microscopy, Plate 10(e), and some cold-worked regions were
being consumed by the growing recrystallised grains. The
micrograph also showed bands of carbides precipitated
along the deformation bands. The carbides had a cube-on-cube orientation relationship with the matrix. The unrecrystallized parts of the structure indicated a general rearrangement of
the dislocations on a fine scale, when seen at higher magnifica
tions, Plate 10(f), i.e. recovery had occurred. Selected
area diffraction of these deformed regions showed that the rings and streaks in the cold worked condition had broken up
formation.
U. 1+. 3* 2 High Carbon Base Composition (Alloy No. 5)
Optical microstructures showed very similar precipitation and recrystallization structures to those in the lower carbon alloy (alloy No. 2), Plate 11(a), except that more intense precipitation v/as observed along deformation bands in the higher carbon composition. Electron microscopy confirmed the precipitation of ^23^6 on ^e^ormeci grain-boundaries prior to any recrystallization in moderately deformed material, Plate
11(b),the orientation relationship again being cube-on-cube with one of the contiguous grains. However, areas around undissolved ^23^6 Par*kicles in this alloy were observed to
show enhanced Recovery and recrystallization effects, Plate 11(c) suggesting that the carbide interfaces acted as a nucleation
site for recrystallization. Also, there were regions where recrystallized grain boundaries were observed to have been pinned by precipitated ^23^6 Pa]rticles> Plate ll(d). This
behaviour suggests that nucleation of these ^23^6 Prectpitates
occurred initially on dislocations, as such pinning particles were more evident after heavy deformation.
^•^-3*3 High Oarbon Molybdenum Alloy , ( Alloy No. h)
Together with the carbide precipitation on deformed grain-
boundaries, the recovered regions around undissolved ^23^6
particles were very clearly observed even after shortest ageing times at the lowest ageing temperatures. However, regions
around the deformed grain boundaries showed some rearrangement of
dislocation structure, Plate 11(e), suggesting an incipient
subgrain formation. However, the dislocation rearrangement was not enhanced by increasing the ageing time, particularly at
lower ageing temperatures, which indicated retardation of
dislocation movement due to the molybdenum. Such effects were not evident in the molybdenum free alloys. Increasing ageing
temperature enhanced dislocation movement, resulting in marked subgrain formation near the deformed grain-boundaries. Some
very fine precipitation v/as also observed on certain dislocations but it was extremely difficult to obtain the diffraction patterns
to identify the carbides because of their small size. Increasing
cold work resulted in enhanced recrystallization, Plate 11(f),
and the pinning effects due to ^23^6 PreciP^-'ta’tion were also evident.
!+.ho3oU High Carbon Molybdenum , Titanium Alloy (No. 3) The precipitation and recrystallization characteristics
were very similar to those of alloy No. k> showing a poorly
developed cell structure at lower ageing times and lower cold reductions. Optical microscopy revealed the precipitation of carbides along deformed grain-boundaries and deformation
bands prior to any recrystallization,and these precipitates
were identified as The temperature dependence of
recrystallization was very obvious, particularly at very hi^i
cold-reductions, Plate 12(a), which shows complete recrystalliz
ation at intermediate ageing temperatures. Some grain-boundary pinning effects due to precipitated carbides were observed.
Nevertheless, at ageing temperatures of 7 0 0°G there were regions shpwing partial recrystallization, Plate 12(b). This type of structure was not frequently observed in the molybdenum
containing alloy (No. 1+) > thereby suggesting that even slower
diffusion rates were prevalent in the titanium bearing composition However, at very high ageing temperatures 750 °C, the matrix was fully recrystallized even after intermediate ageing times,
Plate 12(c), with M23°^ carbides dispersed throughout the
structure. At this ageing temperature, TiC was observed to precipitate on dislocations within the recrystallized grains,
Plate 12(d), having an orientation of ^110j£Lll> jj to £llo} ^lll^Y- An increase in the ageing time caused the growth of
recrystallized grains, Plate 12(e), and to a limited extent
coarsening of carbides. Also, Plate 12(e) clearly reveals the precipitated ^23^6 Par^ic^es on dislocations, and some pinning effect of the growing grains.
U-k* 3«5 Low and High Carbon P^l6 Compositions (No. 7 and 8)