Mechanism of two Way Shape Memory Effect Obtained by Stabilised Stress Induced Martensite

Two Way Shape Memory Effect (TWSME) involves thermomechanical routines which produce some plastic deformation or complex dislocation arrays during the training procedure. The results of TWSME obtained by these thermomechanical training routines are not as good as would be expected because new dislocations are generated during thermal cycling. These dislocations may interact with those that provide TWSME to the alloy. The present authors have developed a completely new training method which involves the stabilisation of the stress induced martensite (SSIM). The present work has been carried out to improve knowledge of the mechanism of TWSME obtained by the SSIM training method. The results show that the new training method not only improves the values of TWSME but also gives improved stability to this TWSME

In Situ X-Ray Studies on New Cu-Al-Ag SMA with Martensitic Transformations Well Above 200 °C

Cu-Al-Ag and Cu-Al-Ag-Co alloy s exhibit a martensitic transformation and the shape memory effect well above 200°C. This greatly extends the field of industrial applications for the shape memory alloys (SMA). The present work was carried out to study the martensitic transformation that takes place during heating. Samples were obtained by melting pure metals in an induction furnace under Ar controlled atmosphere. The samples were then cooled in a graphite crucible to room temperature. All the samples were homogeneized at 900°C for 12 hours and quenched in water at 100°C and held for 1 minute. They were, subsequently, cooled in water to room temperature. X-Ray diffraction patterns were recorded during heating for the different alloy compositions which allowed us to follow the in-situ the structural changes during heating

Acoustic emission study on WC-Co thermal sprayed coatings

Thermally sprayed coatings contain residual stresses that are produced in the spraying process. These may reduce the coating lifetime. In order to determine the optimum spraying conditions with respect to the residual stress level present, the acoustic emission (AE) during four-point bend tests on tungsten carbide–cobalt coatings sprayed onto mild steel substrates was investigated. Samples tested at different levels of deformation were studied by means of scanning electron microscopy and AE in order to understand the cracking mechanisms. Relationships between the number and amplitude of AE events detected and the type of cracking processes occurring were established. It has been possible to compare the residual stresses caused by the effect of different spraying parameters, such as coating thickness, spraying distance and high velocity oxy-fuel gun

TWO WAY SHAPE MEMORY EFFECT OBTAINED BY STABILISED STRESS INDUCED MARTENSITE IN Cu-Zn-Al-Co AND Cu-Al-Mn ALLOYS

Two way shape memory has been achieved in Cu-Zn-Al-Co and Co-Al-Mn alloys by a training process based on the stabilisation of stress induced martensite. The effect of training temperature on the amount of two way shape memory obtained has been studied by training at various temperatures of between 50 °C and 170 °C above the Af. After training the samples were thermally cycled various times (up to 100) through their martensitic transformation temperatures in the absence of applied stress to evaluate the degree of recovery of both the hot and cold shape and hence assess the two way shape memory effect. The results obtained showed that there existed an optimum training temperature at which maximum two way shape memory was observed

Origin of two way shape memory effect in Cu-based shape memory alloys

Two-way shape memory effect (TWSME) should be trained by thermomechanical cycling or constrained aging. After constrained aging, oriented stress fields around precipitates and stabilized stress-induced martensite were found in Cu-Zn-A1 alloys. The effect of these phases on TWSME has been studied by in-situ observations in TEM. For thermomechanical cycling, paired dislocation bands and oriented microstrain in re-oriented martensite were studied by molecular dynamics simulation. It was found tliat the oriented stress fields of the precipitates, pair dislocation bands and re-oriented martensite also have two-way memory effect during forward and reverse transformation. These stress fields can be termed two-way memory stress fields (TWMSF). The TWMSF contribue their stress fields to the formation of the preferential martensite variants that give raise the TWSME