Influence of Boron on the Microstructural and Mechanical Properties of Ni 53.5 Mn 26.0 Ga 20.5 Shape Memory Alloy
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ORIGINAL PAPER
Influence of Boron on the Microstructural and Mechanical Properties of Ni53.5Mn26.0Ga20.5 Shape Memory Alloy M. Ramudu 1 & A. Satish Kumar 2 & V. Seshubai 3 Received: 1 October 2020 / Accepted: 18 November 2020 # Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract Boron addition to Ni53.5Mn26.0Ga20.5 alloy is found to modify the microstructure and mechanical properties substantially. Studies on (Ni53.5Mn26.0Ga20.5)Bx alloys reveal that boron addition causes grain refinement which led to an increase in compressive strength in x = 0.5 alloy which also retained multimode twinning. Maximum compression strengths of 124 MPa, 198 MPa, and 111 MPa and corresponding compression strains of 5.2%, 4.3%, and 5.5% respectively have been recorded for B-0, B-0.5, and B-1.0 alloys. Shape memory strain of 1.5% was recorded even after addition of 0.5 at.% boron which reduced to 0.4% at 1 at.% boron and showed a correlation to the extent of multimode twinning. Substantial second-phase segregation rich in Ni was seen at grain boundaries, the extent of which increased with boron content. This led to a compositional shift in the matrix phase which resulted in a reduction in the martensitic transformation temperature and which in turn caused an easy deformation at low stresses and suppression of multimode twinning in x = 1.0 alloy. Microhardness shows an increase with boron content. B-0.5 alloy with larger compression strength, substantial shape memory effect, and higher hardness has a potential for shape memory applications at room temperature. Keywords Shape memory . Mechanical properties . Microstructure . Grain boundaries . Ni-Mn-Ga-B . Phase transitions
1 Introduction Ferromagnetic shape memory alloys of Ni-Mn-Ga system exhibit multifunctional properties like magnetic field–induced strain, magnetocaloric effect, and high-temperature shape memory effect. In order to realize practical applications of these alloys, one needs to fine tune the relevant properties. Application of these alloys as actuators is constrained by their extreme brittleness and low strength. It is well known from detailed investigations on Ni-Al [1], Cu-Ni-Al [2], and Ti-PdNi [3, 4] alloys that microstructural modifications rendered to
* A. Satish Kumar [email protected] 1
Department of Physics, School of Sciences, National Institute of Technology Andhra Pradesh, Tadepalligudem, Andhra Pradesh 534101, India
2
Department of Physics, Rajiv Gandhi University of Knowledge Technologies, Nuzvid 521201, India
3
School of Physics, University of Hyderabad, Central University P. O., Hyderabad, Andhra Pradesh 500 046, India
the martensitic phase of these materials have a strong bearing on the resultant mechanical properties. This results from grain size refinement and/or segregation of a second phase at grain boundaries caused by addition of boron. In the case of Ni-Mn-Ga alloys, there has been growing interest in investigating the effect of boron, as well as other elements like Cu, Ge, Co, Fe, and Ti and rare earth elem
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