Effect of Applied Strain on the Martensite Transformati on

The effect of applied strain on the martensite reaction in Pe - Ni alloys has been studied in some detail

(71)

by Scheil^ '. Data are also available for lithium and lithium-magnesium a l l o y s c o p p e r - z i n c ^ 83^ and copper-

aluminium a l l o y s a u s t e n i t i c stainless steels(74r-*79) an(3_

(80—82 ^

manganese steelsv '. It was found that under certain

conditions martensite formation can be induced isothermally even at temperatures above Ms by the mechanical deformation of the parent phase. When stress is applied to an alloy, such as during a cold working process, the mechanical energy interacts with the thermodynamics of the martensite reaction as shown by Patel and Cohen^83^. In figure 21 P ^ and

poc* (free energy of alpha martensite) represent the relative

conditions in the unstressed system and P * and P are the

conditions in the strained system where P*^ and Ms are dis-

the applied stress.

A F V - “4 Hs = + A F V " “ii;B . • .(30)

It can he seen that the Ms in the unstrained system can he moved to a higher temperature Ms because at this temperature the algebraic sum of the mechanical and thermo-

>✓ ^ |

dynamic energies is equal to A F tL™ , the critical cool-_l»ig

ing energy at Ms . Thus cold working at temperatures above Ms may result in the formation of martensite and the response to such treatment will depend upon the chemical analysis and also on the temperature of cold work. (Figures 22 and 23).

In the study of the mechanical properties of cold

worked materials both the cold working process and the tensile testing itself must be regarded as cold working. When cold rolled materials are strained during tensile testing the

amount of strain that can take place is controlled by the type of deformation that has taken place previously. In alloy com­ positions of the Y austenite type in which extensive slip has already occurred further slip to produce tensile elonga­ tion at room temperature will be limited. If the temperature of testing is lowered slip becomes more difficult and marten­ site formation is favoured; consequently, elongation values increase as the test temperature decreases.

Watson and Christian^®^ showed this effect in cold worked stainless steels. They demonstrated that the ductil­

ity of standard grades in the cold worked condition increased as the temperature of testing decreased, although at very low temperatures the ductility reached a maximum then decreased.

This increase in ductility was shown to have "been produced hy the formation of martensite during tensile testing.

The influence of plastic strain is quite pronounced with respect to hoth the Ms point and the reaction rate.

Schiel’s results shown in figure 24 were obtained hy com­ pressing a series of samples at different temperatures. For

a given temperature, the amount of transformation increases with the amount of cold work. When slip planes are produced in the parent phase, the Ms point on subsequent cooling is lowered apparently because of the introduction of extra barriers. Thus a parent phase with slips bands acts as if

it were a fine grained material, with a lower Ms temperature.

Schiel also studied the effect of tension after pre­ vious compression. A specimen of Pe - 30% Ni alloy with an original Ms of 5°C was plastically deformed at 100°G. Its new Ms was -5°G, the decrease being presumably due to the introduction of slip planes at the high temperature of deformation. Elastic stress changed the Ms as follows:

M °G

Tensile Stress p.s.i. * s

1,760 -5

17,600 +5

35,200 *14

The phenomenon corresponds to a ’Bauschinger effect’ in that the tensile stress tries to undo the effect of

previous compression.

Considerable data are available on the effect of cold

work on the 1$ - <*.' transformation in stainless steels^74^.

temperature and can form ferrite "by a martensite reaction as discussed above.

Llewellyn and M u r r a y s t u d i e d the effect of alloy­ ing additions on the work hardening rate of stable austenitic structures. Alloying additions of 10% Co, 10% Mn, 10% Ni, and 4% Cu were made to a basic 18% Cr - 13% Ni alloy.

Results of tests on the alloys in the solution treated condi­ tion are shown in figure 25. It can be seen that 10% Mn

causes a very slight increase in work hardening whereas a similar addition of cobalt causes a considerable increase. They also show that C and Cr additions also increase the rate of work hardening. These effects are consistent with expected changes in the stacking fault energy of the austenitic struc­ ture. Ai alloying element such as cobalt readily undergoes transformation from cubic to an hexagonal structure and such a condition is recognised as being indicative of low-stacking fault energy. The association of the hexagonal structure and

stacking faults arises out of the fact that a. hexagonal

structure would be formed by having stacking faults on alter­ nate planes in a f.c.c. lattice. From the data it was con­ cluded that nickel and copper raise the stacking fault energy and decrease the rate of work hardening of 18% Ni 8% Cr type steels, whereas the stacking fault energy and the rate of work hardening are respectively decreased said increased progress­ ively by Mn, Cr and Co. Carbon is generally thought to lower the stacking fault energy but the marked effect on the rate of work hardening could be explained by dislocation atmos­ phere effects.

Dulieu and Nutting^133^ studied the influence of solute additions on the stacking fault energies of austen- ites hy measuring the radii of dissociated nodes, and hy measuring the twin interface frequency per grain after a recrystallising anneal. On the assumption that there is a monotonic variation of stacking fault energy with atom per cent of solute over the range of the solute concentration used, the change in stacking fault energy per atom per cent

of solute was calculated. A corresponding adjustment to the measured stacking fault energy was then made and the varia­ tion in stacking fault energy per atom per cent recalculated. See tahle 1 and figure 26. Further experiments showed that

the addition of 5% Lin lowered the stacking fault energy hy

—2

3 ergs cm . The 1Q% Cr 10% Ni hase material had a stacking

-2