Electron Beam Melting (EBM) is a powder-bed additive manufacturing technology enabling the production of complex metallic parts with generally good mechanical properties. However, the performance of powder-bed based additively manufactured materials is governed by multiple factors that are difficult to control. Alloys that solidify in cubic crystal structures are usually affected by strong anisotropy due to the formation of columnar grains of preferred orientation. Moreover, processing induced defects and porosity detrimentally influence static and cyclic mechanical properties. The current study presents results on processing of a metastable austenitic CrMnNi steel by EBM. Due to multiple phase transformations induced by intrinsic heat-treatment in the layer-wise EBM process the material develops a fine-grained microstructure almost without a preferred crystallographic grain orientation. The deformation-induced phase transformation yields high damage tolerance and, thus, excellent mechanical properties less sensitive to process-induced inhomogeneities. Various scan strategies were applied to evaluate the width of an appropriate process window in terms of microstructure evolution, porosity and change of chemical composition.

Within the Collaborative Research Center 799 novel composite materials which consist of a highly alloyed TRansformation-Induced Plasticity/Twinning-Induced Plasticity (TRIP/TWIP) CrMnNi cast steel matrix and a partially stabilized zirconium dioxide (Mg-PSZ) ceramic, referred to as TRIP-Matrix-Composites, are developed. [1] By applying an external load larger than the yield strength, the TRIP steel matrix shows a strain-induced phase transformation from metastable austenite to a 0 -martensite which leads to a concurrent increase of strength and ductility. By contrast, stress-induced formation of a 0 -martensite occurs at stresses below the yield strength, i.e., during elastic deformation. [2] The stress-induced transformation of the partially stabilized ZrO 2 from tetragonal phase to monoclinic modification can result in a further increase of the strength. Currently the interstitial-free austenitic CrMnNi cast stainless steels with TRIP/TWIP effect are in use. Representative alloys are 16Cr-7Mn-3Ni, 16Cr-7Mn-6Ni, and 16Cr-7Mn-9Ni. [3,4] These austenitic steel grades typically show low yield strengths of the order of 180-200 MPa. As a result of the low stress levels, only small fractions of the ceramic phase can transform to the monoclinic structure. In order to assist the phase transformation of the ceramic phase from tetragonal to monoclinic, the current steel research in the CRC 799 is focused on the increase of strength, especially the yield strength of the austenitic TRIP/TWIP CrMnNi cast steels by solid solution strengthening by nitrogen. Furthermore, the reduction of delta ferrite volume fractions, the adjustment of a pronounced TRIP/TWIP effect at operating temperatures and improved resistance against the intercrystalline corrosion by the addition of nitrogen is desired. In recent years, highly alloyed TRIP and TWIP steels have received much attention in both academia and industry because of their superior mechanical properties. [5][6][7]12] The high-manganese TWIP steels can be divided in the alloying systems FeMnC and CrMnC with additions of the main alloying elements such as N, Al, Si. In these steels g ! e transformation and deformation-induced twinning can appear in dependence of the SFE of the austenite. It is commonly believed that stacking fault energies below 18-20 mJ m À2 favor the g ! e phase transformation and higher values favor the twinning of austenite. [8][9][10] The longest known FeMnC steels are characterized by manganese contents between 15 and 30% and an austeniticThe effect of the manganese content (0-11%) on the transformation temperatures, the mechanical properties and microstructure development of five highly alloyed 14Cr-XMn-6Ni cast stainless steels with 0.1% nitrogen was studied. The examinations reveal that the M s , A s , and A f temperatures decrease with increasing manganese contents. As a result of low austenite stability, room temperature austenitic-martensitic as-cast microstructure was formed at manganese contents between 0 and 3%. At manganese levels of 6% and higher a fu...

Electron beam melting (EBM) is an established powder bed-based additive manufacturing process for the fabrication of complex-shaped metallic components. For metastable austenitic Cr-Mn-Ni TRIP steel, the formation of a homogeneous fine-grained microstructure and outstanding damage tolerance have been reported. However, depending on the process parameters, a certain fraction of Mn evaporates. This can have a significant impact on deformation mechanisms as well as kinetics, as was previously shown for as-cast material. Production of chemically graded and, thus, mechanically tailored parts can allow for further advances in terms of freedom of design. The current study presents results on the characterization of the deformation and strain-hardening behavior of chemically tailored Cr-Mn-Ni TRIP steel processed by EBM. Specimens were manufactured with distinct scan strategies, resulting in varying Mn contents, and subsequently tensile tested. Microstructure evolution has been thoroughly examined. Starting from one initial powder, an appropriate scan strategy can be applied to purposefully evaporate Mn and, therefore, adjust strain hardening as well as martensite formation kinetics and ultimate tensile strength.

No abstract

In spite of the formation of a high fraction of deformation-induced a¢ martensite, the tensile elongation of a cast high-nitrogen austenitic stainless steel was found to enhance at lower temperatures, a behavior deviating from that exhibited by wrought and homogenized austenitic stainless steels. The observed behavior was explained by the presence of microstructural regions with different stabilities with respect to deformationinduced a¢ martensite formation caused by the segregation of alloying elements. Tensile elongation near room temperature of low stacking fault energy (SFE) austenitic steels including stainless, [1][2][3][4][5][6][7][8][9] high Mn, [10][11][12][13] and high Ni [14] steels varies in three temperature regimes as follows. At the highest temperature range (regime I), tensile elongation remains more or less constant or exhibits a weak temperature dependence. At intermediate temperatures (regime II), elongation increases as the temperature decreases. The enhancement of ductility at reduced temperatures in regime II is commonly attributed to such deformationinduced microstructural changes as e martensite formation, the e-TRIP effect, [15] and deformation twinning, the TWIP effect.[2] These are common microstructural features of deformed austenitic stainless steels [16,17] and high Mn [12,18,19] steels and are thought to be consequences of high glide planarity. Enhanced glide planarity caused by reduced cross slip of screw dislocations at lower temperatures has been correlated with an underlying temperature dependence of SFE.[20] The latter dependence is available for many fcc metals and alloys including austenitic steels. [20][21][22][23][24] The enhancement of tensile elongation at lower temperatures in regime II of austenitic stainless steels is interrupted at the temperature below which the deformation-induced formation of a¢ martensite is enabled, namely below the M d cfia¢ temperature. The loss of ductility caused by the deformation-induced formation of a¢ martensite initiates the regime III of elongation, characterized by reduced elongations at lower temperatures. The large number of investigations in support of the detrimental effect of a¢ martensite formation on the tensile elongation of metastable austenitic stainless steels [1][2][3][4][5][6][7][10][11][12] indicates that any possible contribution to the ductility of a TRIP effect would not be large enough to prevent the loss of ductility below the M d cfia¢ temperature. This has been also suggested by the modeling of the a¢ TRIP effect contribution to the ductility.[25] The present study demonstrates how compositional inhomogeneities in a cast high-nitrogen austenitic stainless steel can cause deviation from the behavior described above.