Researches On The Microstructures And Mechanical Properties Of Mg-Dy-Nd-(Gd) Alloys

Magnesium alloys have not been widely used as a main structural material due to their low tensile strength, low deformation ability and bad corrosion resistance. Recent work has suggested that magnesium alloys with the addition of heavy rare earth elements (Gd, Dy, Sm, etc.) have obvious ageing strengthening response and better mechanical properties at room and elevated temperature than those of conventional magnesium alloys, thus maybe have better future application. In this study, the microstructures, mechanical properties and fracture behaviors of a new series of Mg-Dy-Nd-(Gd) alloys will be systemically researched, of which the morphology, microstructure, transformation law and strengthening mechanism of the precipitation phases are focused on. The aim of the present work is to provide experimental and theoretical results for the development and application of high performance magnesium rare earth alloys.Researches on the microstructures and mechanical properties of as cast Mg-Dy-Nd-(Gd) alloys indicate that the cast microstructures of DN73K, DN103K, DN123K and DGN443K alloys consist ofα-Mg phase, skeletal Mg5RE phase (f.c.c., a=2.24nm) along grain boundary and rectangular RE17Mg3 phase (f.c.c., a=0.526nm) at inner and fringe of the skeletal phases. The increasing amounts of eutectic phases in Mg-Dy-Nd alloy, with increasing content of Dy or replacing part of Dy by Gd, result in better tensile strength, lower elongation and obvious cleavage fracture.Researches on the microstructures and mechanical properties of solution treated Mg-Dy-Nd-(Gd) alloys indicate that the skeletal phases will be dispersed, and the grain size and rectangular RE17Mg3 phases will both grow up after being solution treated, which result in better mechanical property and toughness. The fracture fractographies are full of tearing ridge, indicating transgranular fracture.Researches on the age-hardening curves and ageing precipitation behaviors of Mg-Dy-Nd-(Gd) alloys indicate that these alloys have obvious age-hardening response, the precipitation sequence in DN73K alloy mainly involves formation ofβ″,β1 andβphases, the precipitation sequence in DN103K and DN123K alloys mainly involve formation ofβ″,β′,β1 andβphases, and the precipitation sequence in DGN443K alloy mainly involves formation ofβ″,β1 andβphases. Theβ" phases (DO19, a=2aMg=0.64nm, c=cMg=0.52nm), precipitated in { 112 0} planes, are fully coherent with matrix and the relationship betweenβ" and matrix is < 0001 >β″//< 0001 >αand { 11 00}β″//{ 11 00}α; the globularβ′phases (b.c.o., a=0.640nm, b=2.223nm, c=0.521nm) are fully coherent with matrix and the relationship betweenβ′and matrix is <001>β′//<0001>αand {100}β′//{ 112 0}α; the plane plateβ1 phases (f.c.c., a=0.74nm), precipitated in { 11 00} planes, may nucleate fromβ′andβ" phases, and the relationship betweenβ1 and matrix is <110>β1//<0001>αand { 11 1}β1//{ 112 0}α; the equilibriumβphase (f.c.c., a=2.223nm) can be nucleated fromβ1 phase by coherent transformation, and the relationship betweenβand matrix is identical to that ofβ1 and matrix.Researches on the mechanical properties and strengthening mechanisms of aged Mg-Dy-Nd-(Gd) alloys indicate that room temperature tensile strength of alloys increases with the increase of ageing time during earlier aging time, the strength decreases sharply with ageing time elongation after peak ageing time, and the high temperature strength don't vary obviously after longer ageing time at elevated temperature. Theβ″andβ′, precipitated in peak aged alloys, are the main strengthening phases, which lead to coherent strengthening for their coherence with matrix. The dislocation can extend acrossβ1, however, the movement of dislocation may be hindered byβphases, and then the strengthening mechanism will be Orowan strengthening mechanism. Deformation twins can be formed in peak aged alloys after being deformed at room temperature, and the decreasing numbers of twins with increasing experimental temperature result from decreasing capability of inducing twins by decreasing flow stress. The disappearance ofβ″and/orβ′can accelerate basal and non-basal planes slipping after being deformed at elevated temperature, which result in lower tensile strength and higher plasticity. The peak aged, early over aged and over aged fracture fractographies show cleavage fractures, and the peak aged fracture fractographies transform from cleavage fracture to ductile fracture with increasing experimental temperature.Researches on the microstructures and mechanical properties of extruded DN123K alloys indicate that the as-cast coarse skeletal phases has been disintegrated after being extruded at 350℃, and the tensile strength and toughness are improved. After being uniformly heat treated and extruded at 350℃and 450℃, the crystal structures of these elliptical particles, precipitated during extrusion process, are identical to those of skeletal phases, and the strength and plasticity can be improved obviously, but in the range of this experiments the extrusion temperature have less effect on the microstructure and tensile strength of extruded alloys. The fracture fractographies of extruded alloys show ductile fracture.