INFLUENCE OF PARAMETERS DEFORMATION ON THE
STRUCTURAL PHASE TRANSITIONS IN STEEL AND Ti-Al
ALLOYS
Sheyko S.P., Zaporozhye National Technical University, cand. sci. tech.
Belokon’ Y.A. Zaporozhye State Engineering Academy, cand. sci. tech.
The influence of plastic deformation on structural and phase transformations in low-alloy
steel and Ti-Al alloys. The optimal hot-working conditions for the investigated low-alloy are in the
temperature range 850-950 oC and at the strain rate of 100 s-1. The structure is formed 58-62 %
ferrite and 38-42 % pearlite. The optimal hot-working conditions for the investigated γ-TiAl alloy
are in the temperature range 1000-1100 oC and at the strain rate of 0.001 s-1 to 0.1 s-1. The structure
is formed γ-TiAl and α2-Ti3Al.
Keywords: deformation, structural, low-alloy stell, Ti-Al alloy.
Currently, hot plastic deformation is considered one of the most promising ways
to obtain a fine-grained structure of metals, which can provide a high the level of
mechanical properties. In this work we study the effect of deformation parameters at
constant and variable temperature on the structure and properties of steels and Ti-Al
satisfied to test complex Gleeble-3800 [1]. Operating parameters of plastometer:
- temperature t = 20…1700 °C,
- rate of movement of puncheon to 2000 mm/s,
- logarithm of deformation  сom  0,01...1,2;  ten  0,01...0,15.
At research on plastometer, standards were measuring d x h = 10 x 12 mm
chambered, into that air was pumped out and created a vacuum for the exception of
oxidization of metal. Management by a plastometer it was come by true by the
special computer programs on a temperature, speed and degree of deformation. At
stated intervals in the process of ladening tension of fluidity and logarithmic
deformation was fixed. In table. 1 the thermomechanical parameters of the deformed
standards are presented.
Table 1
Parameters of deformation
Standards
1
2
3
4
5
Temperature, °C
770
800
850
900
950
1
100
Speed of deformation, s
Degree of deformation
0,01…1,2
To develop modes of plastic deformation, ensuring the best possible structure
refinement of low-carbon low-alloy steels, requires detailed information about the
influence of the mode of deformation of austenite on the formation of structural
elements in steel. To determine the critical deformation degree, to ensure the
formation of the recrystallized austenitic grains were studied influence of the degree
of deformation and temperature on the structure of the low alloy carbon steel.
Deformation temperature was varied from 770 °C to 950 °C, samples were deformed
by 50% with a strain rate of 100 s-1. Microstructure and distribution diagrams for
grain size and phase are shown in Fig. 1.
а
b
c
d
f
g
Fig. 1 - Alloy steel microstructures 10ХФТБч 50% after deformation at a
temperature of 770 оС (а), 800 оС (b), 850 оС (c) 900 ° C (d) 950 ° (f) 1100 ° C (g)
and strain rate of 100 s-1, x5000.
50% after deformation at a temperature of 770 °C in the steel is formed of
ferrite-pearlite structure with an average grain size of 11 points, the maximum grain
size of 14 2.42% 28.344% in the structure of the structural elements comprise grains
score 10 h (Fig. 2 a). In the steel produced 81.5% and 18.5% ferrite pearlite. This is
possible due to the fact that the deformation takes place in the two-phase region to
form a large proportion of the ferrite (Fig. 2 b).
а
b
Fig. 2 - Results of studies of the structure of steel after deformation of 50% at a
temperature of 770 °C and a strain rate of 100 s -1: a - grain size distribution; b - the
distribution of the phases.
After deformation of 50% at 850 °C formed structure with an average grain size
of 12 points in the structure there is 33.358% grain size of 11 points, 31.091% - the
size of 12 points, the maximum score of 14 grains of 5.633%. The structure is formed
62.041% ferrite and 37.959% pearlite. This testifies to the grinding α-phase - the
formation of subgrain structure within the grains.
A further increase in temperature to 950 °C leads to a slight increase in the
average size of the structural member after the strain of 50% (Fig. 3 a) 24.52% in the
structure of the grain size is 11% 20.166 - 12. The maximum grain size of 14 points
of the grain size is 10.503 %. The structure is formed 58.469% ferrite and 41.531%
pearlite (Fig. 3 b).
а
b
Fig. 3 - Results of studies of the structure of steel after 50% deformation at 950 °
C and a strain rate of 100 s-1: a - grain size distribution; b - the distribution of the
phases.
When considering the distribution histograms of structural elements in size grain
size (Fig. 2-3 a) it can be seen that the distribution of approximately the same at
different deformation temperatures. When analyzing the histogram distribution
phases (Fig. 2-3 b) visible differences: the deformation temperature of 770 °C comes
to the peak of the distribution and the proportion of ferrite is 80% Bolle at higher
temperatures, the proportion of ferrite is reduced and is in the range 58-62% that
indicates a change in the ratio of the structural components of the ferrite
(predominantly of large-misorientations between grains).
On the samples was measured microhardness. The dependence of the
microhardness and medium-sized structural element of the deformation temperature.
In sample deformed with a degree of 50% at a temperature of 770 °C, the lowest
microhardness value and is 260 HV, raising the temperature to 850 °C increases the
hardness of 60 HV, a further increase in temperature does not affect the value of
microhardness. The lowest microhardness value was obtained for samples deformed
at a temperature of 770 ° C, although these samples had very fine-grained structure.
Perhaps this is due to the fact that the plastic deformation extends in the two-phase
region during deformation and a large amount of ferrite, characterized by low
strength (hardness).
The data obtained are in good agreement with modern ideas about the
mechanisms of structure formation depending on the temperature deformation of
steel. According to this concept [2], at a temperature of 770 °C strains prevalent
mechanism of grinding structure is fragmented, consisting in the partition of
austenitic and ferritic grains uniform initial orientation disoriented subgrain
(fragments) low-angle dislocation boundaries of deformation origin. At higher
temperatures, implemented two competing mechanisms - fragmentation and initial
processes of dynamic recrystallization of austenite. By forming the ferrite at a
temperature of 770 °C minimum observed hardness values, the maximum values of
hardness in combination with the small size of the structural element after
deformation observed at 850 °C deformation degree of 50%.
Thus, an appropriate choice of modes on the steel of the same composition can
be obtained higher strength and plastic properties that allows you to manage
obtaining a given set of properties, and in the future - steel uniform chemical
composition to obtain a rolled sheet of various categories of strength or sheet rental
with different plastic properties, depending on the operating conditions.
The Al-Ti system is characterized by presence of main compounds TiAl, Ti 3Al
and TiAl3. The compounds TiAl and TiAl3 are formed by peretectic reaction at
temperature 1460 and 1340 ºC agreeably and is formed by peretectoid reaction at
1600 ºC. The Ti3Al compound has the hexagonal lattice, TiAl has the tetragonal
lattice and TiAl3 has the space-centered tetragonal lattice. The formation of Ti3Al
phase takes place by reaction β + γ → α2 [3].
The strength decreasing with synchronous plasticity increasing in intermetallic
compounds is observed with aluminum content increasing. Thus the titan
monoaluminide has the considerable high-temperature strength and low plasticity at
normal temperature. It happens cause this compound has approximately 70 %
metallic and 30 % covalent bond.
The structurization mechanism is greatly depended on reactionary mix
proportion.
The initial stage of titan aliminides structurization is the melting of aluminum
which was evoked by heat impulse. Then the melted aluminum flows through the
canals in capillary-porous mediums. The further diffusion of aluminum atoms into
lattice of titan particles leads to generation of inermetallic TiAl 3 compound in
diffused zone. The internal compressive stress external tighten stress appears during
intermetallic formation. They can lead to titan aluminide destruction. In system where
aluminum is 39.6 % wt. the formed layer limits the displacement of aluminum atoms
into titan base. At the same time the layer TiAl3 is piled up. It leads to depletion of
aluminum mass and to following formation of titan monoaluminide. With process
propagation deep into titan mass the aluminum concentration decreases. It becomes
the reason of Ti3Al formation. The final structurization stage is the homogenizing of
intermetallic layers first of all owing to recrystallization of Ti 3Al into TiAl.
The hot deformation behavior of a γ-TiAl alloy has been studied using the
processing map approach (Fig. 4). Compression tests were conducted in the
temperature range of 1000-1150 and the strain rate range of 10-3 s-1 to 0,5 s-1 on a
Gleeble-3800 testing system. The flow stress was found to be strongly dependent on
the temperature and the strain rate. The optimal hot-working conditions for the
investigated TiAl alloy are in the temperature range 1000-1100 oC and at the strain
rate of 0.001 s-1 to 0.1 s-1. Thematerial exhibited dynamic recrystallization to produce
a fine-grained microstructure in these conditions. In the temperature range 1150 oC
with the strain rate 0.001 s-1, the alloy exhibited superplasticity.
a
b
c
d
o
Fig. 4 – Flow curves of γ-TiAl alloy deformed at 1000 C (a), 1050 oC (b), 1100
o
C (c), 1150 oC (d): 1 – 0,5 s-1, 2 – 0,1 s-1, 3 – 0,01 s-1, 4 – 0,001 s-1.
Conclusions
Study experienced steels 10ХФТБч plastometer using special computer
programs installed critical points of phase transitions and optimal power parameters
of hot deformation, allowing to choose the temperature range of hot deformation is
850-950 ° C.
The optimal hot-working conditions for the investigated γ-TiAl alloy are in the
temperature range 1000-1100 oC and at the strain rate of 0.001 s-1 to 0.1 s-1.
References
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