Laser nitriding investigated with Mössbauer spectroscopy

The effect of the nitrogen take‐up upon irradiation of iron or steel with excimer laser pulses in air or in nitrogen atmosphere is well established. The resulting phase compositions and nitrogen depth profiles were measured by a combination of simultaneous Conversion Electron Mössbauer Spectroscopy (CEMS), Conversion X‐ray Mössbauer Spectroscopy (CXMS), and Resonant Nuclear Reaction Analysis (RNRA) as a function of the nitrogen gas pressure during irradiation. A maximum nitrogen content and a maximum fraction of the ?-nitride was found at 0.1 MPa. This result is in accordance with hardness measurements performed by the nanoindentation technique.     

CEMS investigation of laser melted Fe−Zr alloys

Conversion Electron Mössbauer Spectroscopy (CEMS) studies are reported for as-cut and laser melted surfaces of Fe?Zr ingots in the 25–80 at.% Zr composition range. Disorder and amorphization was observed even on the as-cut surfaces due to the mechanical processing. Besides a significant enhancement of the non-crystalline fraction, surface melting by as laser pulses also results in the appearance of new metastable phases. Solidification via an extremely high cooling rate thus produces amorphous phase in composition ranges where its formation was previously assumed to be restricted to non melt-quenching methods only.     

CXMS study of mild steel laser alloyed with CrB2

Laser alloying of surfaces has attracted a great deal of attention for technical applications. By laser alloying of materials it is possible to improve hardness as well as wear and corrosion resistance of the surface without affecting the bulk material. The surface of a mild steel (C45) substrate was laser-alloyed with chromium-boride CrB₂. The chromium-boride was added to the substrate surface by powder injection during laser surface melting with a high power continuous-wave CO₂-laser. The resulting surface layers were studied by surface Mössbauer measurements. The backscattering geometry of Conversion X-ray Mössbauer Spectroscopy (CXMS) was used to study the phase formation in the laser alloyed surface. The results for the treated surfaces are discussed for different samples.     

Laser Nitriding of Iron,Stainless Steel,and Plain Carbon Steel Investigated by Mössbauer Spectroscopy

Nitriding is a common method for improving the hardness, mechanical properties, wear and corrosion resistance of metals. Laser nitriding of metals is an efficient process, where the irradiation of surfaces in air or nitrogen atmospheres with short laser pulses leads to a fast take-up of nitrogen into the irradiated surfaces. This process has been extensively investigated for pure iron, but usually, no tools or functional parts are made of pure iron. Mainly steel or cast iron is used as a base material. Therefore, when looking for technical applicability, also the influence of alloying elements on the laser nitriding process is of great interest. Besides the pure iron various carbon steels and an austenitic stainless steel were studied in laser nitriding experiments in order to investigate the influence of the material itself. Here, the process is investigated via Conversion Electron and X-ray Mössbauer Spectroscopy (CEMS and CXMS), Resonant Nuclear Reaction Analysis (RNRA), and X-Ray Diffraction (XRD). It appears that carbon steels are even better suited for the laser nitriding process than pure iron, and the laser nitriding also works efficiently for the stainless steel which is normally difficult to be nitrided.     

Some Mechanical Properties of Austempered Ductile Iron

In the present investigation the influence of the microstructure, obtained after an austempering treatment in a "process window", on the mechanical properties of austempered ductile iron has been investigated. These properties include tensile strength, elongation and hardness. Conversion electron Mössbauer spectra (CEMS) were measured, after heat treatment.     

Photon and Ion-Beam Induced Processes in Materials Studied by Hyperfine Interactions

Laser nitriding and laser cementation are investigated by Mössbauer spectroscopy and complementary methods. It is demonstrated how the backscattering versions of Conversion Electron and Conversion X-ray Mössbauer spectroscopy can contribute to the investigation of surface processes, like the laser-induced formation of nitrides and carbides. Additionally, the formation of semiconducting iron disilicide can be achieved by ion-beam mixing and pulsed laser irradiation of Fe/Si bilayers. The results of both processes are compared.

     

Laser-produced iron nitrides seen by Mössbauer spectroscopy

Mössbauer spectroscopy is a very powerful tool to investigate technological processes performed mainly at the surface of materials. Nitriding of metals and steel is well established in surface engineering, and gas nitriding is used most frequently. Laser nitriding, i.e. the nitrogen take-up from the ambient gas upon irradiation of a steel surface with short laser pulses, is presented in its application to iron, stainless steel and plain carbon steels. It will be demonstrated how Mössbauer spectroscopy in combination with complementary methods (Rutherford backscattering spectroscopy, Resonant nuclear reaction analysis, Nanoindentation) can help to reveal basic mechanisms in these processes.     

Laser nitriding of iron: Nitrogen profiles and phases

Armco iron samples were surface nitrided by irradiating them with pulses of an excimer laser in a nitrogen atmosphere. The resulting nitrogen depth profiles measured by Resonant Nuclear Reaction Analysis (RNRA) and the phase formation determined by Conversion Electron Mössbauer Spectroscopy (CEMS) were investigated as functions of energy density and the number of pulses. The nitrogen content of the samples was found to be independent of the number of pulses in a layer of 50 nm from the surface and to increase in depths exceeding 150 nm. The phase composition did not change with the number of pulses. The nitrogen content can be related to an enhanced nitrogen solubility based on high temperatures and high pressures due to the laser-induced plasma above the sample. With increasing pulse energy density, the phase composition changes towards phases with higher nitrogen contents. Nitrogen diffusion seems to be the limiting factor for the nitriding process.     

Study of Phase Formations in ε-FeSi Layers by Pulsed Laser Annealing

Pulsed laser annealing has been applied on ε-FeSi layers in order to study the formation of (meta)stable iron silicides. Laser shots of different energy density were applied and/or the number of laser shots was also varied. Characterisation was performed using Rutherford Backscattering Spectroscopy (RBS)/Channelling and Conversion Electron Mössbauer Spectroscopy (CEMS). The reaction mechanisms of the Fe/Si system as well as the stability of the phases are discussed.     

CEMS study of strain induced phase transformation in manganese Hadfield steel

A Conversion Electron Mössbauer Spectroscopy, (CEMS), study of phase transformations in a Hadfield steel induced by high rate strains is reported. Hadfield steel samples were impact deformed and the ensuing changes in the magnetic properties at the deformed zone and its surroundings have been studied by CEMS. The CEMS results are compared with wear tests and optical microscopy and show a formation of martensite by impact deformation only at the surface. Martensite is not produced by compression or tensile stresses but appears after wear tests in proportions that depend on the load and velocity conditions of test. The understanding of martensite phase formation and its evolution during deformation processes is also addressed.     

CEMS investigation of near surface structure

Conversion Electron Mössbauer Spectroscopy (CEMS) studies are reported for as-cut and laser melted surfaces of single phase crystalline Fe₂Y, Fe₂₃Y₆, Fe₂Zr, Fe₂B and FeB ingots. Disorder and the appearance of a new phase with a low value of the room temperature hyperfine field was observed for the Fe?Y and Fe₂Zr ingots even on the as-cut surfaces due to the mechanical processing. In case of these ingots surface melting by ns laser pulses resulted in the formation of amorphous alloys. In case of the Fe?B ingots the formation of amorphous phase by laser melting was observed for Fe₂B only, while in case of FeB the low temperature α-FeB modification appeared both, for mechanical processing and laser melting.     

The dynamics of piezoelectric polymer materials at high frequencies measured with conversion electron Mössbauer spectroscopy (CEMS)

Conversion Electron Mössbauer Sideband Spectroscopy is presented as a new and the most accurate method to study the dynamics of piezoelectric polymer materials at high frequencies. With raising amplitude of driven vibrations, complete oscillating behaviour of the Mössbauer sideband-intensities is found. The amplitudes of vibrations can be measured with an accuracy of 0.005 Å. Variations of the piezoelectric constant with temperature and frequency can be determined to better than 1%. Beside piezoelectricity the thermally induced dynamic behaviour of polymer surfaces can also be studied by CEMS.     

Perspectives in physical metallurgy

Physical metallurgy experienced a tremendous, impact with the advent of the Mössbauer effect. Some of these developments will be reviewed, in particular the role of⁵⁷Fe as the star performer in Mössbauer spectroscopy. Also, one must realize that in most casen metals are involved, either in the source, the absorber, or both. Mössbauer spectroscopy in its different variations (conversion electron-, conversion x-ray-, γ-transmission Mössbauer spectroacopy) is well suited for the analysis of iron based alloys. An instrument has been developed which allows simultancous triple radiation Mössbauer spectroscopy (STRMS). The backscalttering geometry of CEMS and CXMS in conjunction with the transmission mode permits non-destructive testing of surfaces and depth analysis.     

Studies of PVD Interfaces Generated by Ti Ions in a Steered Arc Discharge

A critical stage of the combined steered arc and unbalanced magnetron process is the titanium ion etch which improves the adhesion of the TiN coating. This study uses Conversion Electrom Mössbauer spectroscopy (CEMS) to investigate the phases that are formed when steel substrates are given a metal ion etch as a pretreatment prior to the deposition of ceramic coating of the type TiC/N. To detect iron containing phases of a thickness in the order of nanometers it has been necessary to enrich the substrates with the Mössbauer isotope ⁵⁷Fe. The enriched substrates have been treated by a range of Ti ion etch processes and then characterised by CEMS. This technique has identified a number of phases, including the FeTi phase.     

Studying surfaces and thin films using Mössbauer spectroscopy

Using a series of bi-layer samples, we show how Conversion Electron Mössbauer Spectroscopy (CEMS) and X-ray Backscatter Mössbauer Spectroscopy (XBS) can be done with the same experimental set up. The penetration depths of the K and L conversion electrons are measured as 51(6) and 330(240) nm, respectively, with relative contributions of 88(9) and 12(9)%. The penetration depth of the Fe-K _α X-ray signal is determined to be 3.6(2) μm. As a demonstration we show data on surface damage effects in electropolished TRIP steels, and by comparing CEMS and XBS Mössbauer patterns we estimate the thickness of a damaged layer (created by sanding) to be 550(50) nm.     

CEMS study of a nitrogen implanted CrNi 18.9 steel

Conversion electron Mössbauer spectroscopy (CEMS) has been used to study a commercial X10CrNiTi 18.9 steel implanted with varying nitrogen doses and at varying implantation temperatures. At low nitrogen doses the fcc γ-phase transformed to the bcc structure. With increasing implantation dosages we have observed the precipitation of Fe-nitrides and a reverse (α→γ) transformation. High implantation temperatures lowered the nitrogen content of the Fe-nitrides and favoured the formation of Cr-nitrides.     

Mössbauer surface studies on Tife hydrogen storage material

By means of Conversion Electron Mössbauer Spectroscopy (CEMS) Fe clusters have been found on the surface of activated TiFe specimens. The magnetic moments of the Fe precipitates order ferromagnetically with a field of 328 KOe and are aligned parallel to the surface. The Fe precipitates are produced by the thermal treatment under hydrogen atmosphere and can be regarded as catalysts for the dissociation of the H₂ molecules.     

Oxidation state and short range order at the surfaces of amorphous Fe40Ni40P14B6 ribbons

Conversion electron Mössbauer spectroscopy (CEMS) was used to study the oxidation state at the surfaces of amorphous Fe₄₀Ni₄₀P₁₄B₆ ribbons obtained by melt-spinning in air and vacuum heated at 583 K. Different concentrations of ferric and ferrous ions depend on the different behaviour of phosphorous diffusion towards the two surfaces during low annealing temperature.     

Pulsed laser treatment at Fe/C6H6 interface: A Mössbauer effect study

The pulsed ruby laser induced reactive-quenching process at Fe/C₆H₆ Ibenzenel has been investigated using conversion electron Mössbauer spectroscopy [CEMS]. It is shown that iron carbide phases can be synthesized when an iron foil immersed in benzene is treated with ruby Laser pulses [λ=694 nm, pulse width ～30 ns, energy density =15 J/cm²]. The results indicate the formation of ε-carbide and Fe₅C₂ phases in the as-treated sample and its transformation to Fe₃C upon thermal treatment. The result of the CEMS measurements are supported by small angle X-ray diffractometry.