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Microstructure And Martensitic Transformation And Mechanical Properties Of Ni-Mn-Ga-RE Magnetic Shape Memory Alloys

Posted on:2008-05-31Degree:DoctorType:Dissertation
Country:ChinaCandidate:L GaoFull Text:PDF
GTID:1101360245997369Subject:Materials Physics and Chemistry
Abstract/Summary:PDF Full Text Request
The microstructures, martensitic transformations, mechanical and magnetic properties, magnetic-field-induced strain (MFIS) and their influences on Ni-Mn-Ga-RE magnetic shape memory alloys have been investigated by means of SEM, TEM, XRD, DSC, compressive tests, ac susceptibility and MFIS measurements as well. The effects of RE (RE=Gd, Dy, Y) addition on martensitic transformation temperature and the mechanism for that have been revealed. The mechanism of the improved mechanical properties by adding RE was also discussed.The experimental results show that the crystal grains of Ni-Mn-Ga-RE alloys can be apparently refined by increasing RE content. According to the microstructure observations, Ni-Mn-Ga-RE alloys consists of the Ni-Mn-Ga matrix and the hexagonal RE(Ni,Mn)4Ga phase. With the increase of RE content, the RE(Ni,Mn)4Ga phase grows and its volume fraction increases gradually. When the content of RE is less than 2 at.%, the RE (Ni,Mn)4Ga phase distributes mainly along the grain boundaries, while an eutectic-like structure composed of the matrix and the RE (Ni,Mn)4Ga phase can be observed in alloys with 5 at.% RE addition.Ni-Mn-Ga-RE alloys undergo one-step thermoelastic martensitic transformation during the process of cooling and heating. Martensitic transformation temperatures of Ni-Mn-Ga-RE alloys notably increase with the increase of RE content, which is mainly due to the presence of the RE(Ni,Mn)4Ga precipitate, resulting in the increase of Mn content in the matrix. In addition, the thermal hysteresis and thermal enthalpies also increase with the increase of RE content. At the meanwhile, Ni-Mn-Ga-RE alloys indicate excellent thermal stability based on the DSC results. The martensitic transformation and reverse transformation temperatures almost keep constant even after five thermal cycles.It is shown that the type of martensites in Ni-Mn-Ga-RE alloys can be modified from 5M (bct) to 7M (orthorhombic), and then to non-modulated T martensite (bct) with increasing RE content. TEM observations show that the martensite variants of Ni-Mn-Ga-RE alloys exhibit well self-accommodation configuration, where the intervariant boundaries are clear and straight. The ( 20 2) and ( 2(2|-)2) type twin relationship have been detected between 7M and T martensite variants, respectively. Stacking faults-like microstructure has been observed in Ni50Mn29Ga21.9Gd0.1 alloy.Compression tests show that an appropriate amount of Gd, Y or Dy addition has significantly improved the mechanical properties of Ni-Mn-Ga alloy. Both the compressive strength and strain have a strong dependence upon the RE content. When the RE content is less than 1 at.%, the compressive strength enhances rapidly as RE addition increases and then becomes stable when the addition is more than 1 at.%. However, the compressive strain increases dramatically and reaches the maximum value with 1at.% RE addition. Further increase of RE content makes the compressive strain of the alloys decrease slightly. The fracture type of Ni-Mn-Ga-RE alloys changes from intergranular fracture of Ni-Mn-Ga alloys to transgranular cleavage fracture with increasing RE content. For the alloys with excessive RE addition, the interphase fracture is observed, which leads to the increase of the brittleness.A minor RE addition, i.e. no more than 1 at.%, has little influence on the Curie temperature. However, when the content of RE is up to 2at.%, the Curie temperature decreases slightly. The saturation magnetization and magnetocrystalline anisotropy constant of Ni-Mn-Ga-RE alloys at 300 K decrease gradually with the increase of RE content. The saturation magnetic-field-induced strain increases at first and then decreases gradually as the operation temperature increases, the maximum saturation MFIS can be obtained around the martentisic transformation start temperature. The minor addition of RE makes the MFIS increase, while the MFIS decreases in the alloys with higher RE addition.
Keywords/Search Tags:Ni-Mn-Ga alloys, magnetic shape memory alloys, rare earth element, martensitic transformation, mechanical properties
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