Sabine Decker

Improved mechanical properties by high-energy milling and Spark Plasma Sintering of a TRIP-matrix composite

A composite reinforced with 5 vol% MgO partially stabilized zirconia and a high-alloyed TRIP-steel matrix (16% Cr, 6% Ni, 6% Mn) was processed. The objective was to trigger a phase transformation under mechanical stresses within the steel and the ceramic. Therefore, the composite was processed by both mixing (110 r/min) and dispersing by intense milling (250 r/min) of the initial powders, and subsequent Spark Plasma Sintering. During Spark Plasma Sintering, the dwell time was varied (5 min, 10 min). The steel/MgO partially stabilized zirconia interfaces were improved through the intense milling and the 10 min dwell time. Furthermore, 50% of the MgO partially stabilized zirconia transformed to the monoclinic phase under quasi-static compressive loading (10−3 s−1) and the composite had a 1% offset compressive yield strength of 665 MPa. Independent from the processing route, 65% of the steel transformed to α′-martensite.

Spark Plasma Sintering and Strength Behavior under Compressive Loading of Mg-PSZ/Al2O3-TRIP-Steel Composites

Composite materials, which consist of a metastable austenitic TRIP-steel matrix (CrMnNi TRIPsteel; TRansformation Induced Plasticity) reinforced by alumina particles (25 vol.% ceramic, designated as AT 25/75) and reinforced by alumina and MgO partially stabilized zirconia particles (Mg-PSZ) (35 vol.% ceramic, designated as AT 25/75 + MgPSZ) were synthesized through spark plasma sintering (SPS). In the AT 25/75 + MgPSZ, the steel particles were mainly surrounded by alumina. Hence, mostly steel/alumina and alumina/MgPSZ interfaces existed. The mechanical behavior of the as-sintered samples was characterized by compression tests at room temperature and 40 °C and in a range of strain rates between 103 s-1 and 103 s1. The influence of the ceramic content, strain rate and temperature on TRIP-effect of the steel matrix was investigated. Due to the increasing ceramic volume fraction, AT 25/75 + MgPSZ exhibits the highest compressive yield strength under all loading conditions and no strain rate sensitivity. This composite showed no measurable TRIP-effect, due to the low fracture strain. The deformation-induced α’martensite within the steel particles in pure steel and AT 25/75 primary depends on the testing temperature and the strain rate. This is attributed to an increase of stacking fault energy with rising temperature. High strain rates cause adiabatic heating, counteracting the martensitic transformation.