Martensitic Transformation And Microstructure Of ZrCu-based High Temperature Shape Memory Alloy

ZrCu-based alloys have advantages of low price and controllable martensitic transformation temperature from 25 to 1050? compared with other conventional high temperature shape memory alloys(HTSMAs),which make it considered as a promising HTSMA candidate.In addition,ZrCu-based alloys could form shape memory bulk metallic composites and the crystal phase could show the stress-induced martensitic phase transformation,enhancing both plasticity and toughness.Martensitic transformation behavior and microstructural evolution of deformed martensite of ZrCu-based alloys are closely related with the properties of shape memory alloys and shape-memory and amorphous composite materials,which are the keys to increase the function and mechanical properties.Optimized composition is designed via doping to increase martensitic transformation temperature,decrease temperature hysteresis as well as improve thermostability and shape memory effect.The martensitic transformation,microstructure and interface structure,mechanical behavior and shape memory effect of ZrCu-based alloys are systematically investigated by X-ray diffraction(XRD),transmission electron microscopy(TEM),differential scanning calorimetry(DSC)and tensile test at room and high temperature.The law of microstructural evolution of martensite during deformation is illuminated and the inherent mechanism of microstructural evolution of ZrCu-based alloys is revealed.Experimental investigation shows that martensitic transformation temperature is remarkably increased while temperature hysteresis is effectively decreased and thermostability is also improved by Ni and Co doping.With the increased Co content of Zr50Cu25Ni25-x Cox alloys,both martensitic transformation temperature and hysteresis are decreased,but thermostability is first increased and then decreased.For optimized composition Zr50Cu25Ni7.5Co17.5,the martensitic transformation start temperature is 564 K and the hysteresis is 59 K that decrease 124 K compared with Zr50Cu50 alloy.Simultaneously,the transformation temperature becomes stable and the change is only 3 K after 3 cycles.The morphology of martensite is spear-like,parallel-like and mosaic-like for solution treated Zr50Cu50 and Zr50Cu25Ni25-x Cox alloys.The intervariant martensites show(021)type I and(111)type I twins relationship.The substructure of Zr50Cu50 alloy are lots of(001)compound twins and small amount of(001)stacking faults,while the substructure of Zr50Cu25Ni25-xCox alloys are small amount of(001)compound twins and lots of(001)and(11 0)stacking faults.The phenomenological theory indicates that the lattice invariant strain during martensitic transformation process is(021)I type twin for ZrCu-based alloys,consistent with the TEM observations.TEM observations shows that dislocation glide and twin-microstructural evolution concurrently happen during the deformation process of martensite of Zr50Cu50 alloy.At initial deformation stage,(001)compound twins first detwin.With increasing deformation strain,(001)compound twins further detwin and new nanoscale(021)and(201 )twins form,leading to the intersected variant,bad interface mobility and decreased strain recovery characteristics.For Zr50Cu25Ni7.5Co17.5 alloy,(001)compound twins detwin and then thin(111)and(021)type I twins newly form.The intersected morphology does not happen and the interface mobility is good,thus resulting in the large strain recovery characteristics.The 1/10[012] Shockley partial dislocations homogeneously shear on(021)glide planes that results in forming(021)type I deformation twins for ZrCu-based alloys.Better strain recovery characteristics can be obtained for Zr50Cu25Ni25-xCox alloys,compared with Zr50Cu50 alloy.With the increased Co content,shape memory effect is first increased and then decreased.The maximum recoverable strain is 5.92% and 6.87% for Zr50Cu50 alloy and Zr50Cu25Ni7.5Co17.5 alloy under the pre-strain of 8%,respectively.Zr50Cu25Ni25-x Cox alloys exhibit good and stable linear superelasticity.The maximum linear elastic strain is up to 6%,which does not show degradation during the latter cyclic process.