Microstructural features of sintered steels, which comprise both phases and porosity, strongly condition the mechanical behaviour of the material under service conditions. Many research activities have dealt with this relationship since better understanding of the microstructure-property correlation is the key of improvement of current powder metallurgy (PM) steels. Up to now, fractographic investigation after testing has been successfully applied for this purpose and, more recently, the in situ analysis of crack evolution through the microstructure as well as some advanced computer assisted tools. However, there is still a lack of information about local mechanical behaviour and strain distributions at the microscale in relation to the local microstructure of these steels, i.e. which phases in heterogeneous PM microstructures contribute to localisation of plastic deformation or which phases can impede crack propagation during loading. In the present work, these questions are addressed through the combination of three techniques: (i) in situ tensile testing (performed in the SEM) to monitor crack initiation and propagation; (ii) digital image correlation technique to trace the progress of local strain distributions during loading; (iii) fractographic examination of the loaded samples. Three PM steels, all obtained from commercially available powders but presenting different microstructures, are examined: a ferriticpearlitic Fe-C steel, a bainitic prealloyed Fe-Mo-C steel and a diffusion alloyed Fe-Ni-Cu-Mo-C steel, with more heterogeneous microstructure (ferrite, pearlite, upper and lower bainite, martensite and Ni rich austenite).

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The use of low melting point master alloy (MA) powders contributes beneficially to sintering by increasing the distribution rate of alloying elements, enhancing homogenisation and sometimes also promoting densification. However, working with liquid phases poses important challenges like maintaining a proper dimensional control and minimising the effect of secondary porosity on the final performance of the steel. In this work, three different MA systems are compared: a low dissolutive Cu-based MA, and two systems with a higher degree of iron dissolution but different content in oxidation-sensitive elements. The combination of wetting experiments, step sintering tests and dilatometry studies show how the evolution of the microstructure, dimensional stability and overall densification are strongly affected by the characteristics of the liquid MA and in particular by its ability to dissolve the iron base particles, and by the amount of oxidationsensitive elements present in the composition of the MA powder.

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Introducing alloying elements through Master Alloy (MA) additions provides the unique opportunity of designing their composition to enhance sintering by forming a liquid phase. However, working with liquid phases poses important challenges like maintaining a proper dimensional control and minimizing the effect of secondary porosity on the final performance of the steel.The critical parameters for designing low melting point compositions are analyzed in this work by combining the use of thermodynamic software tools, wetting angle/infiltration experiments, and advanced thermal analysis techniques. Due to their low ability to dissolve iron, Cu-based liquids present remarkable infiltration properties that provide homogeneous distribution of the alloying elements. Dissolutive liquids, on the other hand, tend to render more heterogeneous microstructures, rapidly solidifying in contact with the matrix. As a consequence of their lower infiltration capacity, dimensional changes upon liquid formation are significantly lowered. When using master alloys with high content in oxidation-sensitive alloying elements, the differences in oxygen affinity cause an oxygen transfer from the surface of the iron base particles to the surface of the master alloys. The change in the surface chemistry modifies the wetting capability of the liquid, and the dimensional stability becomes increasingly sensitive to the processing atmosphere. KEY WORDSmaster alloys, liquid phase sintering, oxygen-sensitive alloying elements, dimensional stability, wetting/infiltration IntroductionThe so called "master alloys" emerged in the very early seventies as a vehicle to introduce in low alloyed sintered steels elements with high oxygen affinity. The combination of these elements with others with lower sensitivity for oxygen -such as Fe -reduced the risk of oxidation during the early stages of sintering 1-5) .A master alloy could be defined as a powder with a high concentration of alloying elements that is designed to be mixed with a base iron powder in order to provide the desired final composition of the steel after the sintering process. Compared with the use of fully prealloyed steel grades, the master alloy route presents interesting advantages such as preserving the compressibility of the base powder, and yielding flexibility in the selection of the final composition. But probably one of the most interesting benefits of using master alloys is the fact that their composition can be specifically designed to promote the formation of a liquid phase that enhances the distribution of alloying elements and accelerates the sintering mechanisms.Several authors have considered the use of master alloys specifically as a means of activating sintering through the formation of a liquid phase with specific characteristics. For instance, Cu-based master alloys provide a liquid with the ability to distribute through the pore network and penetrate the iron grain boundaries, reducing the diffusion distances needed to homogeneously distribute other alloying elements [6][...