Structural evolution during mechanical alloying of stainless steels under nitrogen

Materials Characterization

et al. 2013

Mechano-synthesis of Fe-32Mn-6Si alloy by mechanical alloying of the elemental powder mixtures was evaluated by running the ball milling process under an inert argon gas atmosphere. In order to characterize the as-milled powders, powder sampling was performed at predetermined intervals from 0.5 to 192 h. X-ray florescence analyzer, X-ray diffraction, scanning electron microscope, and high resolution transmission electron microscope were utilized to investigate the chemical composition, structural evolution, morphological changes, and microstructure of the as-milled powders, respectively.According to the results, the nanocrystalline Fe-Mn-Si alloys were completely synthesized after 48 h of milling. Moreover, the formation of a considerable amount of amorphous phase during the milling process was indicated by quantitative X-ray diffraction analysis as well as high resolution transmission electron microscopy image and its selected area diffraction pattern. It was found that the α-to-γ and subsequently the amorphous-to-crystalline (especially martensite) phase transformation occurred by milling development. © 2013 Elsevier Inc. All rights reserved. Keywords:Fe-Mn-Si shape memory alloys Mechanical alloyingPhase transformation Nanostructural/amorphous phase Microstructure IntroductionFe-Mn-Si alloys are a class of smart materials due to their suitable shape memory effect (SME), excellent machinability and formability, low cost, good weldability and corrosion resistance, and high strength having several industrial applications such as dampers, pipe couplings, big shape memory devices, hard metals or alloys joining, oxygen blowing nozzles and so on [1][2][3][4][5][6][7][8][9][10][11]. The production of stress induced martensite (ε, hcp) from parent austenite phase (γ, fcc) and the reverse transformation (γ to ε) during the heating cycle are the origins of SME in this alloying system [12][13][14][15][16][17][18]. This non-thermoelastic or semi-thermoelastic conversion occurred by the creation of stacking faults (SFs) due to the movement of the Shockley partial dislocations (a γ /6 <112>) in the parent phase. SFs are suitable sites for nucleation of the martensite phase; eventually, the ε-phase can be formed by their overlap [12,[17][18][19][20][21][22][23].Although induction melting under an argon gas atmosphere is widely utilized to produce Fe-based SMAs, solid-state routes such as mechanical alloying (MA) can be used to produce the alloys in the powder form [24]. During MA, by applying the high energy collision between ball and particles and consequently the repeated cold welding and fracture of the powders, not only is the alloying process attained but also the synthesis of the non-equilibrium structures such as supersaturated solid solutions, nanocrystalline and amorphous structures, and intermetallic compounds is possible [24][25][26][27][28].Recently, MA has been widely used to synthesize nanocrystalline and amorphous SMAs [29][30][31], although limited investigations have been done on Fe-Mn-Si alloys.

Section: Introductionmentioning

confidence: 99%

Phase transformation during mechano-synthesis of nanocrystalline/amorphous Fe–32Mn–6Si alloys

Amini

Shamsipoor

Materials Characterization

et al. 2013

“…Apart from this contribution, as noted in the experimental section, the stainless steel powders were synthesized by mechanical alloying, a process that induces nanocrystallization and amorphization. 27,28,29,30,31,32 During sintering, the shorter duration of the 30 min sample results in lower grain/crystal growth; and the smaller the grain/crystallite size the higher the corrosion rate and ion release rate. 33 albeit from the stainless steel matrix.…”

mentioning

confidence: 99%

Is cell viability always directly related to corrosion resistance of stainless steels?

Salahinejad

Materials Science and Engineering: C

Vashaee

et al. 2016

Self Cite

It has been frequently reported that cell viability on stainless steels is improved by increasing their corrosion resistance. The question that arises is whether human cell viability is always directly related to corrosion resistance in these biostable alloys. In this work, the microstructure and in vitro corrosion behavior of a new class of medical-grade stainless steels were correlated with adult human mesenchymal stem cell viability. The samples were produced by a powder metallurgy route, consisting of mechanical alloying and liquid-phase sintering with a sintering aid of a eutectic Mn-Si alloy at 1050 °C for 30 and 60 min, leading to nanostructures. In accordance with transmission electron microscopic studies, the additive particles for the sintering time of 30 min were not completely melted. Electrochemical impedance spectroscopic experiments suggested the higher corrosion resistance for the sample sintered for 60 min; however, a better cell viability on the surface of the less corrosion-resistant sample was unexpectedly found. This behavior is explained by considering the higher ion release rate of the Mn-Si additive material, as preferred sites to corrosion attack based on scanning electron microscopic observations, which is advantageous to the cells in vitro. In conclusion, cell viability is not always directly related to corrosion resistance in stainless steels. Typically, the introduction of biodegradable and biocompatible phases to biostable alloys, which are conventionally anticipated to be corrosion-resistant, can be advantageous to human cell responses similar to biodegradable metals.

“…(b) Atomic size mismatch: A large atomic size mismatch among the constituent elements induces significant strains to the lattice and increases the strain and free energy of crystalline phase. This effect can cause the crystals free energy to exceed the amorphous free energy, encouraging amorphization [11,20]. Particularly, carbon and nitrogen have considerable atomic size differences with the other constituent elements (Fe, Cr, Mn, Mo, Si, N, and C atoms are 156, 166, 161, 190, 111, 56, and 67 pm in size).…”

Section: Amorphizationmentioning

confidence: 99%

“…[10,11]. Additionally, liquid-phase sintering of mechanically-alloyed stainless steel powders, with the chemical composition of ASTM F2581, using a Mn-Si sintering aid has been detailed recently [12][13][14].…”

Section: Introductionmentioning

confidence: 99%

Microstructural characterization of medical-grade stainless steel powders prepared by mechanical alloying and subsequent annealing

Salahinejad

Hadianfard

Advanced Powder Technology

et al. 2013

Self Cite

a b s t r a c tThe harmful effect of nickel ions released from conventional stainless steel implants has provided a high level of motivation for the further development of nickel-free stainless steels. In this paper, the microstructure of medical-grade nickel-free stainless steel powders, with the chemical composition of ASTM F2581, is studied during mechanical alloying and subsequent annealing. Rietveld X-ray diffraction and transmission electron microscopy evaluations reflect nanocrystallization, austenitization and amorphization of the powders due to mechanical activation. It is also realized that annealing of the as-milled powder can develop a single austenitic structure with nanometric crystallite sizes, implying a considerable inherent resistance to grain growth. This study demonstrates the merit of mechanical alloying and subsequent annealing in the development of nanostructured medical-grade stainless steels.