Nanoscale surface properties of a Ni–Mn–Ga 10M magnetic shape memory alloy

► A surface layer of ∼2.5nm and rich in Mn and Ga oxides is established with XPS. ► Nanoindentation is applied on electropolished and mechanically polished crystals. ► A similar type of oxide layer appears both in the electropolished and mech. polished surface. ► In the electropolished surface elastic loading is followed by a notable pop-in of the indenter. ► The pop-in is absent when indenting the mechanically polished surface. Highly mobile twin boundaries allow straining of the 10M Ni–Mn–Ga martensite single crystals with low as 0.15MPa uniaxial stresses. This is clearly lower than what is possible to magnetically generate in this structure. Therefore, this material can be used for magnetically controlled actuation. In cyclic deformation occurring by twin variant reorientation, fatigue life up to 2×109 cycles has been demonstrated without decrease of the magnetic-field-induced strain or without failure of the specimen. However, the actuation behavior can be strongly impaired by unfavorable twin variant configuration or surface constraints which decrease the twin mobility and increase the twinning stress. In the present work, the surface properties of the 10M Ni–Mn–Ga are studied in details. The existence of an ultra-thin surface layer having chemical composition different from the bulk crystal is established with the XPS analysis. The influence of this layer on the nano-mechanical properties of the surface is investigated using instrumented nanoindentation both on electropolished crystals and crystals where surface deformation has been introduced by mechanical polishing. The results show that the surface layer appears both in the electropolished and mechanically polished surface. However, the nano-mechanical properties of the surface are distinctly different in the two cases. In the case of electropolished surface essentially elastic loading occurs first followed by a notable pop-in of the indenter, while in the case of mechanically polished surface minor or no pop-in is observed. This behavior is attributed to the differences of the initiation of plastic deformation in the two types of surfaces. The contribution of the layer to the performance and actuation properties of the material is discussed.