J.G. Byrne

A study of work hardening in austenitic FeMnC and FeMnAlC alloys

Abstract Two austenitic FeMnAlC alloys with aluminium contents of 0 and 2.7 wt% were strained in tension between 193 and 823 K. Serrated stress-strain curves, inverse strain-rate dependence of flow stress, and high work hardening exhibited in particular temperature ranges for both alloys were characteristic of dynamic strain aging. The apparent activation energy for the onset of serration increased from 14.4 to 22.3 kcal/mol due to the addition of 2.7 wt% Al. It was found that the high work-hardening rate cannot be attributed to strain-induced deformation twinning when serrated stress-strain curves occurred. From the evidence of the present study and the known effect of aluminium on the diffusivity and activity of carbon in austenitic high-manganese steel, it is suggested that dynamic strain aging is the major cause of work hardening within the intermediate temperature range from 298 to 493 K for 0 wt% Al and 393 to 593 K for 2.7 wt% Al in the present austenitic FeMnAlC alloys.

Phase transformation in a duplex FeMnAlC alloy

Abstract Precipitates with various shapes and crystal structures produced during cooling an Fe-27.3wt.%Mn-6.8wt.%Al-0.22wt.%C alloy at various rates from 1300°C were investigated by light microscopy, scanning electron microscopy, transmission electron microscopy, and electron diffraction. Morphological, microstructural, and crystallographic information is presented for each precipitate type. Based on precipitate shape and location, the γ phase developed can be classified into the following types: (1) needle-like phase (18R(5 1 )3), (2) grain-boundary allotriomorphs (f.c.c.), (3) Widmanstatten sideplates (f.c.c.) and (4) intragranular phase (f.c.c.). Electron diffraction studies verify that the f.c.c. (γ) phase precipitates from the b.c.c. matrix in Femnal alloys. A Kurdjumov-Sachs orientation relationship holds between the b.c.c. matrix and the precipitates, i.e. the needles, Widmanstatten sideplates, and intragranular phase.

Long-period stacking-fault structure of the needle-like phase in a duplex FeMnAlC alloy

Abstract The purpose of this investigation was to study the crystal structure of the needle-like phase which appears in b.c.c. regions of water-quenched Fe-27.3 wt% Mn-6.7 wt% Al-0.22 wt% C alloy. The morphology, crystal structure, transformation mechanism and microstructure of the needle-like phase have been studied by transmission electron microscopy and electron diffraction. A structure change from the b.c.c. (matrix) to the 18R (5 l ) 3 needle-like phase is proposed to occur by shearing in the b.c.c. directions of the {110} b.c.c. planes in a five-to-one sequence repeated three times. Since we find a close relationship between the {110} b.c.c. and {111} f.c.c. , we can utilize an existing description of 18R (5 l ) 3 as a stacking sequence of ABCABC − BCABCA − CABCAB − A in terms of {111} f.c.c. planes.

The utility of positions for studies of metals and alloys

Abstract Four distinct processes have been described in which positron measurements can provide clear non-destructive indicators of changes in the state of a material. Many other examples may be found in the literature, however, the above should give the reader an indication of the utility of positions for studies of metals and alloys.

A postulate for diffusionless transformation in duplex FeMnAlC alloys

Study of a phase transformation of the kind BCC-to-18 R (51) 3 during water quenching of duplex Fe-Mn-Al-C alloys by means of the light microscopy (LM), scanning electron microscopy (SEM) and TEM

Annealing of a cold-worked duplex FeMnAlC alloy

Abstract The effects of cold working and aging on a two-phase (austenite and ferrite) Fe-27.3wt.%Mn-6.7wt.%Al-0.22wt.%C alloy were studied using differential scanning calorimetry (DSC) and isochronal annealing at different temperatures. Reactions were detected by DSC at 145 °C, 346 °C, 531 °C and 630 °C. Both the reactions at 145 °C and 346 °C are controlled by the diffusion of carbon. The products of both reactions can be described as carbon atoms segregated to interstitial sites around manganese and aluminum atoms forming very small carbon-rich clusters which are structurally coherent with the parent solid solution and are too small to be detected by transmission electron microscopy (TEM). The clusters formed in ferrite at 346 °C induce an internal stress causes an increase in the hardness of the ferrite. The reactions at 531 °C and 360 °C are controlled by the diffusion of manganese and aluminum. The reaction at 531 °C evidently was a homogeneous nucleation of precipitate with the same crystal structure as ferrite. At 620 °C, the latter homogeneously nucleated precipitate transforms into austenite.

Parametric studies of the hot isostatic pressing of rapidly solidified 304 stainless steel powder

Abstract The increasing importance of powder materials fabrication by use of hot isostatic pressing (HIP) has led to recent emphasis on analytical techniques for describing and understanding the process. Understanding of particle consolidation during the HIP process has been attempted through the modelling of densification behavior by considering the deformation of a representative particle due to forces transmitted through the particle contacts. However, the properties of HIPed material have not been thoroughly investigated in terms of their deformation maps and HIP parameters. Mechanical properties of a compact can be quite different depending on the location of various deformation map boundaries. Diffusional creep is involved not only in densification but also in bonding at particle contacts. HIP pressure increases mechanical contact and enhances density but not particle bonding per se. Discrepancies between experimental and calculated data points for shorter HIP times may have been affected by oxide film layer on the original powder.

The formation of bcc phase during the solution treatment of duplex FeMnAlC alloys

Abstract The bcc-to-fcc phase transformations during cooling have been discussed. Both diffusionless and diffusional transformations were found. The fcc-to-bcc transformation during solution treatment is discussed. It is shown that the growth of the bcc phase involves diffusion-controlled and solute drag-controlled boundary migration.

The formation of martensite-free zones in duplex Fe/1bMn/1bAl/1bC alloys

Abstract Needle- or martensite-free zones (MFZs) are usually found in duplex Fe/1bMn/1bAl/1bC alloys. Based on MFZs and the needle morphology, it was proposed that the needle structure was nucleated by a vacancy-induced (VIN) process and grown by a coherency and incoherency mechanism (CIM). It is shown that there are three kinds of locations of MFZs. In order to explain the formation of MFZs at different locations, it is shown that the needle structure is also controlled by an equal solute-sharing (ESS) effect during growth.

Laminate of 60 vol.% BeO/Be composite with 6061 A1 alloy

Abstract Several attempts were made to vacuum hotpress multi-layer laminates of a 60 vol.% BeO/Be composite with 6061 Al alloy. Even at 620°C under light pressure the creep of the Al layers applied enough tensile stress on the BeO/Be composite layers to cause the latter to fracture.