Microstructure Changes in TRIP Steel/Mg‐PSZ Composites Induced by Low Compressive Deformation

Abstract

During the last years, a new kind of composite material called TRIP (TRansformation Induced Plasticity) matrix composites has been developed that was intended to meet the requirements on high-strength structural materials with excellent formability and increased damage resistance. [1][2][3][4][5] This specific kind of metal matrix composites (MMC) consists of a high-alloy austenitic CrMnNi TRIP steel matrix, which is reinforced by zirconia partly stabilized by the addition of magnesium (Mg-PSZ). Via powder metallurgy, both kinds of material can be joined, for example as particle reinforced MMC. During mechanical loading, both components, the metallic matrix and the ceramic particles, are intended to show phase transformation, and thus improve the mechanical properties of the MMC.The phase transformation in Mg-PSZ utilizes the polymorphism of the partially stabilized zirconia ceramic. [6] After reaching a critical stress during the compression test, tetragonal PSZ shows a stress-assisted martensitic phase transformation to monoclinic zirconia (m-ZrO 2 ). [7] As m-ZrO 2 has a larger specific volume than the cubic or the tetragonal phase of zirconia, [8] this phase transformation induces compressive stresses within the microstructure of MMC.In contrast to the stress-assisted phase transformation in Mg-PSZ, the phase transformation in the ductile metallic steel matrix is triggered by plastic deformation. The highly metastable austenitic TRIP steel shows a martensitic g ! e ! a 0 transformation. [9][10][11] The alloying elements reduce the stacking fault energy (SFE) of the austenite, [10][11][12][13] which can be estimated around 14-16 mJ m À2 at room temperature by empiric equations. [14,15] Due to the low SFE plastic deformation of austenitic TRIP steels is usually accompanied by the formation of partial dislocations, stacking faults, e-martensite, and twins. [16][17][18] Since the SFE depends on temperature, it is expected that the glide behavior of dislocations varies with temperature as well. [19][20][21][22] Moreover, the rise of the SFE with increasing temperature is the reason for a transition from the pronounced intrinsic stacking fault and e-martensite formation at low temperatures to the mechanical twinning at elevated temperatures. [17,18,23] Consequently, a transition from the TRIP to the TWIP (TWinning Induced Plasticity) effect is observed at higher temperatures that results in a lower stress level but an increased uniform elongation by tensile testing. [13,24] By combining the TRIP steel matrix with the Mg-PSZ reinforcing particles, shear stresses are transmitted from the steel matrix to the ceramic particles and trigger the phase transformation in the Mg-PSZ. Due to the volume expansion of Mg-PSZ during its martensitic phase transformation, compressive stresses are induced in the TRIP-steel and a higher stress level is achieved compared to the unreinforced TRIP steel. [25] The aim of the present paper is to investigate the microstructural evolution in the MMC at different temperatures, at 20 and 1008...

Deformation Mechanisms in Austenitic TRIP/TWIP Steel as a Function of Temperature

“…20 vol pct of e-martensite after 20 pct strain. [44] With ongoing plastic deformation, the deformation bands grew in width and further new deformation bands form alongside because the motion of the (partial) dislocations in the initial deformation band was obstructed by the a¢-martensite nuclei. This process continued until the whole grains were filled with deformation bands, which transformed successively into a¢-martensite (Figure 3(g)).…”

“…20 vol pct of e-martensite can be quantified using XRD. [44] Due to the fact that e-martensite is consumed by a¢-martensite formation, a much higher cumulative Fig. 3 taken from Refs.…”

Joining of Zirconia Reinforced Metal–Matrix Composites by a Ceramics‐Derived Technology

“…[2][3][4][5][6] Composite materials based on austenitic stainless steels with transformation-induced plasticity (TRIP) and/or twinninginduced plasticity (TWIP) combined with zirconia reinforcing particles were intensively researched in the past. [7][8][9][10] These steels offer a matrix material with outstanding ductility, high strength and reasonably high energy absorption capacity for high mechanical load applications at room temperature. The addition of minor volume fractions of magnesia partially stabilized zirconia (Mg-PSZ) leads to a rise in the yield strength and the stress-strain level in a certain range of deformation, especially under compressive loading.…”

“…spark plasma sintering or hot pressing. [8,18] As these composite materials have grown in interest, the joining technique is a key section in processing and testing structural elements. Numerous processes for joining metals and MMC materials have been investigated and reported on in the past.…”

Selective Dissolution of Retained Austenite in Nanostructured Bainitic Steels

“…Attractive mechanical properties of bainitic steels including a uniquely high combination of strength (≈2.3 GPa), toughness (≈30 MPam 1/2 ), and ductility (≈30%), makes them a potential candidate for impact and wear resistance applications in the automobile, building, offshore and defence industries. Generally, grain refinement has been shown to improve the mechanical properties of metals, and for the high alloyed bainitic steels this has been achieved through a relatively low transformation temperature of 150–350 °C and high carbon (≈0.8 wt%) content . The resultant structure is an interlace of retained austenite and bainitic ferrite laths, ranging from 20 to 300 nm in thickness depending on the isothermal holding temperature .…”