Papers by Keyword: Martensitic Structure

Abstract: The effect of composition on martensitic phase transformation and structure in polycrystalline NiMnGa was investigated through Ni substitution for Mn, Ga and Mn substitution for Ga. The martensitic transformation temperatures almost linerly increase with increasing Ni content in Ni_50+xMn_50-xGa₂₅, Ni_50+yMn₂₅Ga_25-y and Mn content in Ni₅₀Mn_25+zGa_25-z. The increases in rate of the martensitic transformation temperatures are different for the three conditions. It is large for Ga substituted by Ni, slow for both Mn substituted by Ni and Ga substituted by Mn. Analysis shows that electronic concentration is an important factor to influence the martensitic transformation temperature in the Ni-Mn-Ga alloys. The results show that Ni excess stabilizes the NM martensitic structure and Mn excess stabilizes the 7M martensitic strcuture in broad composition range.

Abstract: The present article is dealing with the conditioning of martensitic steel plates for armoring troops carriers. The steel alloy was processed into ingots. The ingots were hot flat rolled to 80 mm, followed by rolling to 6 mm. Specimens for detection of the critical transformation temperatures were taken. The Ac₁, Ac₃, M_s and M_f temperatures were detected as 716, 835, 356 and 218 °C respectively. The 80mm microstructure contains plate martensite colonies, while the 6mm plates reveal complete plate martensitic structure. Both cases contain embedded carbides. Heat treatment cycles were proposed for enhancing the ballistic resistance. A treatment was consisting of austentizing at 910 °C for 20 min., followed by water quenching. This process revealed fully fine lath martensitic packets. Tempering was done on the quenched as well as on rolled plates. Tempering at 350°C, causes dissociation of some martensite, and encourages migration of the entrapped carbon atoms forming carbide aggregates embedded between martensite plates packets. Tempering at 250°C for 20 min., relieves quenching induced stresses and reveals a lath martensitic structure with a small fraction of rosette like carbides. The process enhances elongation from 3% to 11.35%. The microstructure of as-rolled tempered plates at 300 ^oC for 20-30 minutes contains a thin ferrite layer on the boundaries surrounding the martensite packets, and few fractions of cotton wool shape carbides between the martensite packets. The treatment cycle improves the mechanical properties. Both treatments, at 250 and 300 ^oC, show successful ballistic resistance plates against shooting by 3 bullets.

Abstract: The kinetics of phase transformations of economically-alloy steels under continuous cooling from the austenitic region, the behavior of carbide phases at different stages of thermal bonding, and the mechanism of precipitation hardening during tempering were tested. A set of mechanical tests to measure the performance characteristics of new steels that allow optimizing thermal processing modes and improving the reliability and durability of the finished instrument were conducted.

Abstract: Ferromagnetic shape memory alloys (FSMA) are relatively new smart materials group. Recently, new FSMA from NiMnX (X=Sb, Sn, In, Co+In) systems are considered as alternative to the well known NiMnGa alloys. Four alloys of the following compositions: Ni₄₃Mn₃₅Co₄In₁₈, Ni₄₁Mn₃₅Co₄In₂₀, Ni₄₂Mn₃₅Co₅In₁₈, Ni₄₀Mn₃₅Co₅In₂₀ were studied in order to determine microstructure, phase composition and martensitic transformation temperatures versus their chemical composition. Structure of the alloys was studied by optical and transmission electron microscopy (TEM). All of the studied alloys showed macrostructure consisting of radially oriented columnar grains in the direction perpendicular to the casting axis. The structure of the phases occurred in the studied alloys depended on the cobalt and indium content. For the alloys containing 20 at. % of In at room temperature only L2₁ parent phase was observed whereas for those containing 18 at. % of In either single phase 14M modulated martensite or mixture of 14M martensite and L2₁ parent phase were seen. DSC measurements showed in studied alloys single-state martensitic transformation. Decrease In content of 2 at.% caused about 80°C fall of martensitic transformation temperatures. Curie temperature T_c increases of 20°C with 1 at% rise of the cobalt content.

Abstract: Multifunctional ferromagnetic shape memory Heusler alloys are frequently characterized by structural modulation in martensitic phases. In particular, modulated martensitic phases, showing the higher magnetic field induced strain (MFIS) performance, are the most promising candidates for technological applications. Depending on the composition, as well as pressure and temperature conditions, this periodic structural distortion, consisting of shuffling of atomic layers along defined crystallographic directions, accompanies the martensitic transformation. Over the years, different Ni-Mn-Ga modulated martensitic structures have been observed and classified depending upon the periodicity of corresponding ideal nM superstructure (where n indicates the number of basic unit cells constituting the superlattices). On the other hand, it has been demonstrated that in most cases such structural modulation is incommensurate and the crystal structure has been fully determined by applying superspace formalism. The results, obtained by structure refinements on powder diffraction data, suggest a unified crystallographic description of the modulated martensitic structures, here presented, where every different “nM” periodicity can be straightforwardly represented.

Abstract: A Mn-enriched Ni46.4Mn32.8Sn20.8 Heusler alloy has been prepared by the induction melting and casting method. The samples used were annealed at 900°C during 72 h with a subsequent water quench. The magnetostructural transformation (MST) in the vicinity of Tm=230K and Ta=250K has been found, and characterized by DSC, magnetization and resistivity measurements. Changes in the crystal structure due to the MST were observed using neutron diffraction at different temperatures. The crystal structure of martensitic phase shows a good fits to the 4-layered orthorhombic martensite with a Pmma space group. The linear shift of transformation temperatures was found in the entire high magnetic field range up 14T to be dTm/dB0 = -1.50 K/T and dTa/dB0 = -1.26 K/T, whereas in the low field range we found an increase of both characteristic temperatures. The entropy changes at the MST were evaluated using the Clausius-Clapeyron relationship. The transformation-induced magnetoresistance has been measured.

Abstract: Magnetic and structural properties in multifunctional FSMA (Ferromagnetic Shape Memory Alloys) belonging to Heusler family are frequently related to the occurrence of structural modulation in martensitic phases. The highest MFIS (Magnetic Field Induced Strain) effect has been observed in Ni-Mn-Ga alloys showing martensitic modulated structures. Depending on the composition, pressure and temperature conditions, this periodic structural distortion, consisting of shuffling of atomic layers along specific crystallographic directions, accompanies the martensitic transformation. Over the years, different modulated martensitic structures have been observed and classified depending upon the periodicity of corresponding superstructure (nM with n=3, 5, 6, 7, 12 etc). On the other hand, it has been demonstrated that in most cases such structural modulation is incommensurate and the crystal structure can be solved by applying superspace approach. The crystallographic representation of different modulated structures, obtained by structure refinement on powder diffraction data, suggests a unified description where every different “nM” periodicity can be straightforwardly represented. It will be presented an overview illustrating structural features of several displacive modulated martensitic lattices. For a specific Ni-Mn-Ga composition, the evolution of structural modulation upon temperature change will be illustrated.