In Situ 3D-µ-Tomography on Particle-Reinforced Light Metal Matrix Composite Materials under Creep Conditions

In transportation light metal matrix composites (L-MMCs) are used increasingly due to their improved creep resistance even at higher application temperatures. Therefore, the creep behavior and failure mechanisms of creep loaded particle reinforced L-MMCs have been investigated intensively. Until now, creep damage analyses are usually performed ex situ by means of interrupted creep experiments. However, ex situ methods do not provide sufficient information about the evolution of creep damage. Hence, in situ synchrotron X-ray 3D-µ-tomography investigations were carried out enabling time and space resolved studies of the damage mechanisms in particle-reinforced titanium- and aluminum-based metal matrix composites (MMCs) during creep. The 3D-data were visualized and existing models were applied, specifying the phenomenology of the damage in the early and late creep stages. During the early stages of creep, the damage is determined by surface diffusion in the matrix or reinforcement fracture, both evolving proportionally to the macroscopic creep curve. In the late creep stages the damage mechanisms are quite different: In the Al-MMC, the identified mechanisms persist proportional to creep strain. In contrast, in the titanium-MMC, a changeover to the mechanism of dislocation creep evolving super-proportionally to creep strain occurs.TU Berlin, Open-Access-Mittel – 202

Synchrotron Sub-μ X-ray Tomography of Kirkendall Porosity in a Diffusion Couple of Nickel-Base Superalloy and Nickel after Annealing at 1250 \ub0C

Kirkendall porosity that forms during interdiffusion in a diffusion couple of nickel-base superalloy CMSX-10 with pure nickel is investigated. The diffusion experiments are conducted at a temperature of 1250 \ub0C, where the strengthening γ′-phase is partially dissolved. The porosity is studied by X-ray sub-μ tomography with a spatial resolution of about 0.353 μm3 at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. It is found that depending on the distance from the interface the Kirkendall pores take different shapes: octahedral, rounded pyramidal, drop shaped, dendritic, pear shaped, and joint shapes. Such a variety of pore morphologies indicates a complex multistage process of porosity nucleation and growth under vacancy supersaturation of different degrees. The experimental findings are interpreted on the basis of the results of diffusion modeling. It is shown that the kinetics of porosity growth is essentially influenced by the dissolution of the γ′-phase

In-situ investigation of the damage behavior of multiphase materials during thermal and mechanical loading

Es werden die Kriechschädigungsmechanismen in Leichtmetall-Verbundwerkstoffen (Metal Matrix Composites = MMC) untersucht. Die in-situ µ-Tomographie mit Synchrotron-Röntgenstrahlung als abbildendes Verfahren bietet den Vorteil zeitaufgelöster Messungen. Die mikrostrukturellen Mechanismen, die die Schädigung hervorrufen und zum Kriechbruch führen, werden zeitabhängig und lokal im Volumen analysiert sowie mit dem makroskopischen Kriechverhalten korreliert. Anhand bestehender Modelle zur Beschreibung der Kriechschädigung werden die Porosität in der metallischen Matrix, das Versagen der keramischen Verstärkungen sowie die Matrixablösungen von den Verstärkungen charakterisiert. Die Modelle nutzen phänomenologische Kriterien für die qualitative Analyse der Schädigung in den einzelnen Kriechstadien einerseits und Bestimmung der Entstehungs- sowie Entwicklungsmechanismen andererseits. Aus der Kombination der Modelle, die die rechnerischen Grundlagen liefern, und deren Erweiterung für den 3-dimensionalen Anwendungsfall werden Methoden für die zeitabhängige quantitative Beschreibung der Kriechschädigung entwickelt. Die Schädigungsprozesse zeigen eine Abhängigkeit von den gewählten Kriechbelastungen. Allen untersuchten MMC-Werkstoffen -Titan+15%(SiC)p, AA6061+22%(Al2O3)p sowie AE42+10%(Saffil®)f+10%(SiC)p- ist in den frühen Kriechstadien die Schädigung aufgrund von Porenbildung und –wachstum in der Matrix durch den Mechanismus der Oberflächendiffusion gemeinsam. Es zeigt sich innerhalb der späteren Kriechstadien, dass in den drei Werkstoffen letztendlich jedoch unterschiedliche Mechanismen zum Versagen führen. In den partikelverstärkten Werkstoffen Titan+15%(SiC)p und AA6061+22%(Al2O3)p verursachen bei höheren Temperaturen nach wie vor die Schädigungsprozesse in der Matrix durch Porenbildung und ˗wachstum den Kriechbruch. Bewirkt in dem MMC Titan+15%(SiC)p nach einem Mechanismuswechsel das Versetzungskriechen ein überproportionales Porenwachstum in Relation zur Kriechdehnung, so bleibt in dem Werkstoff AA6061+22%(Al2O3)p die Oberflächendiffusion weiterhin kriechschädigungsbestimmend und proportional zur Kriechdehnung. Bei einem bei niedriger Temperatur und hoher mechanischer Belastung durchgeführten Experiment des Werkstoffs AA6061+22%(Al2O3)p und dem hybridverstärkten MMC AE42+10%(Saffil®)f+10%(SiC)p sind Verstärkungsbrüche ursächlich für das Versagen. Das Wachstum der Bruchspalten der Partikel (Al-MMC) bzw. der Fasern (Mg-MMC) findet in beiden Werkstoffen proportional zur Kriechdehnung statt. Für das zeit-, spannungs und temperaturabhängige Kriechschädigungs- und Kriechbruchverhalten durch mikrostrukturelle Schädigungsmechanismen in der metallischen Matrix, Brüche der Partikel- bzw. Kurzfaserverstärkungen sowie Matrixablösung von den Verstärkungen werden modellhafte grafische Darstellungen entwickelt.The creep damage mechanisms in Lightmetal based Metal Matrix Composites (MMCs) are analysed. In-situ µ-tomography using synchrotron X-ray radiation is a 3-dimensional imaging technique and exhibits the advantage of time-resolved studies. This enables in-situ investigations of damage during creep. The mechanisms of creep damage and creep fracture are analysed time-resolved and space-resolved in the volume. Thereby, microstructural damage mechanisms such as void formation in the matrix, reinforcement fracture and delamination of the matrix from the reinforcements can be detected and correlated to the macroscopic creep curve. To characterize the creep damage in the different creep stages existing models were used, which are specifying the phenomenology of the damage and give qualitative information about damage formation and evolution. These models were combined and extended for the application to 3-dimensional data sets. Hence a method is developed which gives the equations for time-resolved quantification of the determined creep damage mechanisms. The damage processes are depending on mechanical stresses and temperature. During the early creep stages surface diffusion dominates the creep damage through void formation and evolution regardless of the investigated MMC-materials Titan+15%(SiC)p, AA6061+22%(Al2O3)p and AE42+10%(Saffil®)f+10%(SiC)p. In the later creep stages the damage mechanisms which lead to creep fracture are different for each MMC. In the particle reinforced MMCs Titan+15%(SiC)p and AA6061+22%(Al2O3)p void formation and evolution in the matrix results in creep fracture at higher temperatures. The main process in the Titan+15%(SiC)p-MMC is super-proportionally void growth by power-law creep in relation to creep strain. In contrast the surface diffusion determines void growth which remains proportional to creep strain in the AA6061+22%(Al2O3)p-MMC. However, at high mechanical stress and low temperature reinforcement fracture is observed in AA6061+22%(Al2O3)p as well as in the hybrid reinforced AE42+10%(Saffil®)f+10%(SiC)p-MMC. The growth of the particle breakage (Al-MMC) respectively short-fiber breakage (Mg-MMC) is the main mechanism of creep failure and proportional to creep strain. The analysis of the experimental data enabled the generation of illustrations visualizing the creep damage and failure as a function of time, stress and temperature. These illustrations consider the microstructural damage mechanisms in the metallic matrix as well as fracture of reinforcements (particles or short fibers) and matrix-reinforcement delamination

In Situ 3D-µ-Tomography on Particle-Reinforced Light Metal Matrix Composite Materials under Creep Conditions

In transportation light metal matrix composites (L-MMCs) are used increasingly due to their improved creep resistance even at higher application temperatures. Therefore, the creep behavior and failure mechanisms of creep loaded particle reinforced L-MMCs have been investigated intensively. Until now, creep damage analyses are usually performed ex situ by means of interrupted creep experiments. However, ex situ methods do not provide sufficient information about the evolution of creep damage. Hence, in situ synchrotron X-ray 3D-µ-tomography investigations were carried out enabling time and space resolved studies of the damage mechanisms in particle-reinforced titanium- and aluminum-based metal matrix composites (MMCs) during creep. The 3D-data were visualized and existing models were applied, specifying the phenomenology of the damage in the early and late creep stages. During the early stages of creep, the damage is determined by surface diffusion in the matrix or reinforcement fracture, both evolving proportionally to the macroscopic creep curve. In the late creep stages the damage mechanisms are quite different: In the Al-MMC, the identified mechanisms persist proportional to creep strain. In contrast, in the titanium-MMC, a changeover to the mechanism of dislocation creep evolving super-proportionally to creep strain occurs

The Effect of Specimen Size and Test Procedure on the Creep Behavior of ME21 Magnesium Alloy

Lightweight constructions and materials offer the opportunity to reduce CO2 emissions in the transport sector. As components in vehicles are often exposed to higher temperatures above 40% of the melting temperature, there is a risk of creep. The creep behavior usually is investigated based on standard procedures. However, lightweight constructions frequently have dimensions not adequately represented by standardized specimen geometries. Therefore, comparative creep experiments on non-standardized miniature and standardized specimens are performed. Due to a modified test procedure specified by a miniature creep device, only the very first primary creep stage shows a minor influence, but subsequently, no effect on the creep process is detected. The creep behavior of hot extruded and heat treated ME21 magnesium alloy is investigated. It is observed that the creep parameters determined by the miniature and standard creep tests are different. As the deviations are systematic, qualitatively, evidence of the creep behavior is achieved. The creep parameters obtained, and particularly the creep strain and the strain rate, show a higher creep resistance of the miniature specimen. An initial higher number of twinned grains and possible multiaxiality in the gauge volume of the miniature specimen can be responsible

Interdiffusion in cmsx-4 related ni-base alloy system at a supersolvus temperature

Interdiffusion in Ni-base superalloy CMSX-4 and alloys related to CMSX-4 was investigated at the temperature 1288 \ub0C, which is 8 \ub0C above the y’-solvus temperature of this superalloy, 1280 \ub0C. This temperature is of a special interest because it is a temperature of hot isostatic pressing applied to CMSX-4 and modeling of this process needs verified diffusion data for this specific temperature. Various diffusion couples were assembled from the investigated alloys, annealed at 1288 \ub0C and studied by electron probe microanalysis. So far as the annealing temperature was higher than the y’-solvus temperature of CMSX-4 and other investigated alloys have no strengthening y’-phase, interdiffusion occurred in the fcc solid solutions of nickel. It was found that in the case when the y’-forming and y-stabilizing elements diffuse in the same direction (towards nickel) the diffusion rate accelerates, but when they diffuse in the opposite directions (counter diffusion) it slows down. Such an interdiffusion behavior is in agreement with the results predicted with diffusion simulation software Dictra