Abstract
Structural changes (martensitic transformation, rearrangements of martensitic variants) in shape memory alloys have an intermittent character that is accompanied by the emission of different (thermal, acoustic, and magnetic) noises, which are fingerprints of the driven criticality, resulting in a damped power-law behaviour. We will illustrate what kinds of important information can be obtained on the structural changes in shape memory alloys. It was established that the power exponents of distributions of acoustic emission (AE) parameters (energy, amplitude, etc.), belonging to martensitic transformations, show quite a universal character and depend only on the symmetry of the martensite. However, we have shown that the asymmetry of the transformation (the exponents are different for the forward and reverse transformations) results in as large differences as those due to the martensite symmetry. We will also demonstrate how the recently introduced AE clustering method can help to identify the different contributions responsible for the asymmetry. The usefulness of the investigations of time correlations between the subsequent events and correlations between acoustic and magnetic noise events in ferromagnetic shape memory alloys will be demonstrated too. Finally, examples of acoustic and magnetic emissions during variant rearrangements (superplastic or superelastic behaviour) in the martensitic state will be described.
Highlights
Structural changes in metallic shape memory alloys can be accompanied by the emission of different noises
The interpretation of this phenomenon has recently evoked a renewed interest [7,25,34] in carrying out a statistical analysis of a large number of acoustic emission (AE) signals and determining the exponents of the energy, amplitude, area, or duration distributions. This will be discussed in more detail below. It was proposed in [25] that the asymmetry is a consequence of the fact that, while nucleation is required for the forward transformation, during the reverse transformation only the fast shrinkage of the martensite domains occurs, i.e., the dynamic mechanisms are different for cooling than heating
We have demonstrated that AE, accompanied by structural changes (martensitic transformation, We have demonstrated that AE, accompanied by structural changes in shape memory alloys, has an intermittent character and transformation, rearrangements of martensitic variants) in shape memory alloys, has an intermittent the nose parameters follow the damped power-law behaviour (Equation (1))
Summary
Structural changes in metallic shape memory alloys can be accompanied by the emission of different noises. It has been well-known for thousands of years that the mechanical properties of steels can be considerably improved by fast cooling Another important example is the family of shape memory alloys: these alloys, when deformed in the martensitic state, can turn back to their original shape upon heating to the temperature at which the martensite to austenite, backward, transformation takes place. In our recent paper, we made an attempt to interpret the existence of the observed two types of asymmetries (the relative changes of the exponents during cooling and heating, γε = (εh − εc )/εc as well as γα = (αh − αc )/αc , are either positive or negative). We will perform a trial to demonstrate how the recently introduced clustering method for the evaluation of AE data [10,11] can help to identify the different contributions responsible for the asymmetry
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