Abstract
The reversion of deformation-induced α′-martensite in tension-deformed 1.4301 (EN) stainless steel was investigated using positron annihilation spectroscopy. The Doppler broadening of the annihilation line and positron lifetime spectroscopy were applied to study defect structure and its annealing behavior in samples with a similar deformation level but varying in α′-martensite amount. The difference in α′-martensite was obtained by applying different deformation temperatures, i.e., liquid nitrogen temperature, room temperature, and 200 °C. The cumulative annealing of the tension-deformed samples and measurement of the positron annihilation characteristics show the gradual annealing of defects in the temperature range between 200 and 400 °C due to the recovery and recrystallization. However, in the temperature range between 450 and 650 °C, the generation of vacancy clusters which trap positrons is revealed. This temperature range coincides with the temperature range of α′-martensite reversion which is confirmed by microhardness and magnetization measurements. The detected large vacancy clusters consisting of 6–9 vacancies can occur at the interface between austenite and α′-martensite phases due to the volume contraction accompanying bcc/fcc change.
Highlights
Martensitic transformation induced by plastic deformation of metastable austenitic stainless steel (SS) is a well-known phenomenon [1, 2] depending on steel composition and deformation conditions, e.g., temperature, strain rate, and state
In the temperature range between 450 and 650 °C, the generation of vacancy clusters which trap positrons is revealed. This temperature range coincides with the temperature range of a0-martensite reversion which is confirmed by microhardness and magnetization measurements
For the sample A (e = 5 %) deformed at LN2 temperature a0-martensite phase dominates. This confirms the results obtained by Herrera et al who found that amount of the a0martensite in 304L SS increases both with deformation level and the temperature lowering from 200 °C to RT and Figure 2 shows the room temperature magnetization curves, and Table 2 presents values of the specific saturation magnetization per unit mass rs and volume fraction of a0-martensite fa0 for the as-deformed samples
Summary
Martensitic transformation induced by plastic deformation of metastable austenitic stainless steel (SS) is a well-known phenomenon [1, 2] depending on steel composition and deformation conditions, e.g., temperature, strain rate, and state. Deformation-induced martensite (DIM) changes important properties of steel, e.g., the strain-hardening coefficient and formability. During plastic deformation (briefly, deformation) of metastable austenitic SS, e-martensite with hexagonal close-packed crystal structure and a0-martensite with body-centered cubic crystal structure can be formed. A0 have been reported [7, 8] The former is typical for steels with low stacking fault energy (SFE), the latter for steels for somewhat higher SFE [9]. In addition to the plastic strain, strain rate and the SFE of material related to the material chemistry and temperature, the DIM amount depends on grain size and orientation [10,11,12,13]
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