Peter Uggowitzer

Corrosion resistance of super duplex stainless steels in chloride ion containing environments: Investigations by means of a new microelectrochemical method. I. Precipitation-free states

Abstract A new mircoelectrochemical method was applied to perform potentiodynamic polarisation experiments on areas in the range of 10 μm. For the first time, the individual corrosion behaviour of both single phases in super duplex stainless steels was determined. The results show a good correlation with the empirical pitting resistance equivalent number (PREN) of the corresponding single phase. The microelectrochemical experiments have revealed two different kinds of interactions between the ferrite and the austenite phase at the phase boundary, namely a superposition or a separation of the two polarisation curves of the single phases. Potentiodynamic polarisation of large areas with representative amounts of both phases are performed in hydrochloric acid electrolyte in order to compare the corrosion behaviour of the single phases with the corrosion behaviour of the entire alloy. Both, the pitting potentials, evaluated by means of macroelectrochemical experiments in pH-neutral lithium chloride electrolyte, and the critical crevice corrosion temperatures show a good correlation with the PREN of the weaker phase. Part I of this paper deals primarily with solution annealed materials. Solution annealing of the samples was performed at temperatures where minimal amounts of precipitates were formed and where the element partitioning determines the corrosion resistance of the single phases. Part II deals with the influence of precipitations on the corrosion behaviour.

Microstructure and mechanical properties of squeeze cast and semi-solid cast Mg–Al alloys

Magnesium–aluminium castings produced by means of squeeze casting, new rheocasting and thixocasting have been investigated. These casting processes provide very different microstructures consisting of α-Mg and β-Mg17Al12. The shape and distribution of the brittle β-Mg17Al12 has a large influence on mechanical properties. Isolated particles of β-phase in squeeze cast components are less detrimental to ductility than the continuous β-phase network found in semi-solid processed parts. A heat treatment results in complete dissolution of β-Mg17Al12 and accounts for significant improvements of ductility and fracture toughness. Crack propagation in solution heat treated Mg–Al castings is associated with extensive twinning.

Wear–corrosion behavior of biocompatible austenitic stainless steels

Abstract The present work compares 316L and Rex734 stainless steels — currently used for biomedical applications — with the recently developed P558 alloy in terms of mechanical properties, corrosion and dry wear resistance as well as the combined wear–corrosion behavior. Pin-on-drum tests were conducted to determine the dry wear resistance of test materials. Wear–corrosion behavior was studied in slurry tests and pin-on-disk experiments in distilled water, Hanks artificial physiological solution, and 22% NaCl aqueous solution. Corrosion resistance was determined by potentiodynamic tests and critical crevice corrosion temperature measurements. Due to its combination of outstanding mechanical properties and excellent corrosion resistance, P558 performed much better than 316L in all tests. P558 also proved to be more resistant against dry wear, wear–corrosion, and crevice corrosion than Rex734. The resistance of P558 against pitting corrosion was found to be equal to the resistance of Rex734.

Partitioning of chromium and molybdenum in super duplex stainless steels with respect to nitrogen and nickel content

Abstract Experimental data of elemental partitioning of Cr, Mo, and Ni in a 5% Mn containing SAF 2507 super duplex stainless steel (SDSS) with varying nitrogen and nickel mass fractions are presented. Experimental results on phase equilibria and alloying element partitioning are compared to values calculated using Thermo-Calc software revealing good agreement. A trilinear model is proposed to describe the partitioning ratio of Cr, Mo and Ni with nitrogen and nickel mass percent in the austenite and the annealing temperature as variables. While nitrogen is found to reduce the partitioning of chromium and molybdenum, nickel enhances the partitioning ratio of these to alloying elements. Based on this semi-empirical model, a guideline for the future development of improved SDSS is formulated. With respect to corrosion resistance, a higher nitrogen level is not beneficial by itself but needs to be paralleled by increasing molybdenum and decreasing chromium contents.

Fundamentals of the New Rheocasting process for magnesium alloys

Casting of high strength, ductile and pressure tight components at low cost is the prerequisite for the introduction of magnesium alloys into hydraulic and structural applications. This paper introduces the New Rheocasting process (NRC) as a novel approach for semi-solid casting of light metals, in which the slurry is prepared from normal casting alloys directly at the foundry machine. The specialties of the process and the alloy requirements are explained. Ways for increasing ductility and process stability with slight alloy modifications and proper heat treatment are shown. The resulting mechanical properties are compared with data received from classical high pressure die casting parts. New Rheocasting of the alloy AZ71proves to be superior in strength and ductility, and shows excellent KJC values. 100µm AZ71

High toughness and high strength spray-deposited AlCuMgAg-base alloys for use at moderately elevated temperatures

Abstract Chemical composition and heat treatments of spray-deposited AlCuMgAg-base alloys for use at ambient and moderately elevated temperatures have been optimized for high strength and high fracture toughness. This alloy development has led in particular to two new age hardenable aluminium alloys (N213 and N232) with excellent property combinations. Alloy N213 (AlCu5Mg0.4Ag0.4Ti0.4Zr0.15Mn0.2) clearly outperforms commercial 2XXX series alloys such as 2618 in terms of fracture toughness and yield strength at temperatures up to 180 °C. Alloy N232 (AlCu4Mg0.8Ag0.4Zr0.5Mn0.4) shows the highest product of yield strength and fracture toughness ever achieved for aluminium alloys; at 23 °C yield strength and fracture toughness values are R P0.2 = 378 MPa, K IC = 100 MPa m 1 2 (T4 temper), and R P 0.2 = 452 MPa , K IC = 77 MPa m 1 2 (T6 temper). Fatigue strength of this experimentak akkits (T6 temper) is also obviously superior to that of commercial aluminium alloys such as 7475-T7351 and 2219-T8. Alloys N213 and N232 show stress corrosion cracking behaviour comparable with that of commercial 2XXX alloys; plateau crack growth rates around 10 −8 m s −1 have been measured. As revealed by transmission electron microscopy studies, the excellent property combinations of the new alloys can be related to microstructural characteristics, such as type, size, density and distribution of the precipitates. Moreover, these studies explain the nature of the microstructure tending towards overaging during extended exposure at temperatures above 150 °C; in particular the harmful impact of higher Mg contents on the microstructural stability at elevated temperatures is evidenced.

ICP-MS, SKPFM, XPS, and Microcapillary Investigation of the Local Corrosion Mechanisms of WC – Co Hardmetal

WC-Co hardmetal exhibits high corrosion susceptibility in aqueous solutions, related to complex microscale reaction mechanisms. This paper presents developed methods to characterize the local distribution of surface reactions difficult to assess by conventional electrochemical methods. Laterally resolved electrochemical potential distributions measured using scanning Kelvin probe force microscopy (SKPFM) under controlled humidity identified the more noble nature of WC and the microscale galvanic coupling with the Co areas acting as anodes. Inductively coupled plasma mass spectrometry (ICP-MS) element analysis, carried out using an online flow cell, provided simultaneous, time-resolved detection and quantitative concentration measurement of the dissolved elements. W ions in solution at the open-circuit potential indicated chemical dissolution due to the pH increase on the WC cathodes in addition to electrochemical anodic Co dissolution. Various mechanisms attributed to homogeneous dissolution of microscale phases or dissolution transients related to localized corrosion attack are identified. X-ray photoelectron spectroscopy (XPS) revealed a carbon-rich surface layer on the WC grains supporting a mechanism of selective W dissolution. These different techniques provided information on the microscale reactions on WC-Co surfaces in aqueous solution and allowed construction of a comprehensive model.

Alloy compositions and mechanical properties of 9–12% chromium steels with martensitic–austenitic microstructure

Abstract Nitrogen alloyed 9–12% chromium steels with high amounts of manganese and nickel have been investigated. The steels are of a lamellar duplex microstructure, consisting of tempered martensite and about 30 vol.% austenite. The martensite is precipitation hardened with fine and homogeneously distributed vanadium nitrides. A good combination of strength and toughness has been achieved, with a room temperature yield strength level of up to 1000 MPa. As will be shown even at temperatures up to 550°C the new steels exhibit outstanding strength and ductility properties. Two different steels have been investigated. One steel was solely nitrogen alloyed, while in the other one parts of the nitrogen have been replaced by carbon. The steel which was alloyed with nitrogen only showed marked embrittlement after long-term ageing. Small carbon additions prevented ageing embrittlement. This is explained by the different precipitation behaviours.

Effect of recrystallisation and grain size on the mechanical properties of spray formed AlCuMgAg-alloys

Investigations of spray formed AlCuMgAg-alloys containing minor additions of Ti, Zr and Mn have revealed that these alloys are prone to coarse recrystallisation during solution heat treatment. A primary recrystallisation has been observed, occurring in alloys with a zirconium (Zr) content below 0.15 wt%. Great differences between unrecrystallised and coarse recrystallised alloys have been found in the mechanical properties. Best strength and toughness values have been reached in the fine grained, unrecrystallised conditions. It is proposed that the strengthening effect in unrecrystallised alloys is based on the grain size as well as the higher density of dislocations and subgrain boundaries in the unrecrystallised material. The recrystallisation behaviour can be explained by a model which is based on the effect of particles with a bimodal size distribution. Coarse particles are necessary for the nucleation of recrystallisation and fine dispersoids control the grain growth by pinning of boundaries. Recrystallisation can be completely suppressed by a dense distribution of Zr containing dispersoids.

Magnetic properties of Cr-Mn austenitic stainless steels

Abstract The magnetic susceptibility x of three Cr-Mn austenitic stainless steels was measured as a function of temperature in the range 5–400 K. All specimens showed a characteristic susceptibility maximum. The temperature of the maximum and especially the curve shape depend strongly on specimen composition and metallurgical conditions (as-quenched, deformed). Because no significant field dependence appeared, the susceptibility maximum was identified as the antiferromagnetic Neel temperature. x;( T ) measurements above T N were fitted to a modified Curie-Weiss equation. Comparison between measurements and generalized-molecular-field-theory predictions allowed us to identify the magnetic structure as that of a first-type antiferromagnet with fcc crystal structure. The atomic magnetic moment and the molecular-field coefficients depend strongly not only on composition, but also on metallurgical prehistory, that is, on the degree of the applied mechanical deformation and heat treatment. Mainly, manganese affected the antiferromagnetic interactions, while chromium affected the ferromagnetic. Mn and Fe contributed the most to the effective atomic moment. Measurements on mechanically deformed specimens show a structure sensitivity of the molecular-field constants. This could be interpreted consistently in terms of lattice-parameter changes. The apparent structure sensitivity of the effective atomic moment can be attributed to changes in matrix composition caused by precipitation.